OpenGL 4 面向着色器语言食谱

In this chapter, we will cover the following recipes:

• Using a loading library to access the latest OpenGL functionality
• Using GLM for mathematics
• Determining the GLSL and OpenGL version
• Compiling a shader
• Linking a shader program
• Saving and loading a shader binary
• Loading a SPIR-V shader program

Introduction

The OpenGL Shading Language (GLSL) Version 4 brings unprecedented power and flexibility to programmers interested in creating modern, interactive, and graphical programs. It allows us to harness the power of modern Graphics Processing Units (GPUs) in a straightforward way by providing a simple yet powerful language and API. Of course, the first step toward using GLSL is to create a program that utilizes the OpenGL API. GLSL programs don't stand on their own; they must be a part of a larger OpenGL program. In this chapter, we will provide some tips and techniques for getting started. We'll cover how to load, compile, link, and export a GLSL shader program. First, let's start with some background.

The field of study called General Purpose Computing on Graphics Processing Units (GPGPU) is concerned with the utilization of GPUs (often using specialized APIs such as CUDA or OpenCL) to perform general-purpose computations such as fluid dynamics, molecular dynamics, and cryptography. With compute shaders, introduced in OpenGL 4.3, we can now do GPGPU within OpenGL. See Using Compute Shaders, for details about using compute shaders.

GLSL

The GLSL is a fundamental and integral part of the OpenGL API. Every program written using the OpenGL API will internally utilize one or several GLSL programs. These "mini-programs" are referred to as shader programs. A shader program usually consists of several components called shaders. Each shader executes within a different stage of the OpenGL pipeline. Each shader runs on the GPU, and as the name implies, they (typically) implement the algorithms related to lighting and shading effects. However, shaders are capable of doing much more than just shading. They can perform animation, generate additional geometry, tessellate geometry, or even perform generalized computation.

The field of study called General Purpose Computing on Graphics Processing Units (GPGPU) is concerned with the utilization of GPUs (often using specialized APIs such as CUDA or OpenCL) to perform general-purpose computations such as fluid dynamics, molecular dynamics, and cryptography. With compute shaders, introduced in OpenGL 4.3, we can now do GPGPU within OpenGL.

Shader programs are designed for direct execution on the GPU and are executed in parallel. For example, a fragment shader might be executed once for every pixel, with each execution running simultaneously. The number of processors on the graphics card determines how many can be executed at one time. This makes shader programs incredibly efficient, and provides the programmer with a simple API for implementing highly-parallel computation.

Shader programs form essential parts of the OpenGL pipeline. Prior to OpenGL Version 2.0, the shading algorithm was hardcoded into the pipeline and had only limited configurability. When we wanted to implement custom effects, we used various tricks to force the fixed-function pipeline into being more flexible than it really was. With the advent of GLSL, we now have the ability to replace this hardcoded functionality with our own programs written in GLSL, thus giving us a great deal of additional flexibility and power. For more details on this programmable pipeline, see the introduction to The Basics of GLSL Shaders.

In fact, OpenGL Version 3.2 and above not only provide this capability, but they require shader programs as part of every OpenGL program. The old fixed-function pipeline has been deprecated in favor of a new programmable pipeline, a key part of which is the shader program written in GLSL.

Profiles – core versus compatibility

OpenGL Version 3.0 introduced a deprecation model, which allowed for the gradual removal of functions from the OpenGL specification. Functions or features can be marked as deprecated, meaning that they are expected to be removed from a future version of OpenGL. For example, immediate mode-rendering using glBegin/glEnd was marked as deprecated in version 3.0 and removed in version 3.1.

In order to maintain backwards compatibility, compatibility profiles were introduced with OpenGL 3.2. A programmer that is writing code for a particular version of OpenGL (with older features removed) would use the core profile. Those who wanted to maintain compatibility with older functionality could use the compatibility profile.

It may be somewhat confusing that there is also the concept of a forward-compatible context, which is distinguished slightly from the concept of a core/compatibility profile. A context that is considered forward-compatible basically indicates that all deprecated functionality has been removed. In other words, if a context is forward-compatible, it only includes functions that are in the core, but not those that were marked as deprecated. Some Windows APIs provide the ability to select a forward-compatible status along with the profile.

The steps for selecting a core or compatibility profile depend on the Windows system's API. For example, with GLFW, one can select a forward-compatible, 4.6 core profile using the following code:

glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 4);
glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 6);
glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE); GLFWwindow *window = glfwCreateWindow(800, 600, "Title", nullptr, nullptr);

All programs in this book are designed to be compatible with a forward-compatible OpenGL 4.6 core profile. However, many of them can be used with older versions or even compatibility profiles.

Using a loading library to access the latest OpenGL functionality

The OpenGL application binary interface (ABI) is frozen to OpenGL Version 1.1 on Windows. Unfortunately for Windows developers, that means that it is not possible to link directly to functions that are provided in newer versions of OpenGL. Instead, one must gain access to the OpenGL functions by acquiring a function pointer at runtime. Getting access to the function pointers is not difficult, but requires somewhat tedious work, and has a tendency to clutter code. Additionally, Windows typically comes with a standard OpenGL gl.h file that also conforms to OpenGL 1.1.

The OpenGL wiki states that Microsoft has no plans to ever update the gl.h and opengl32.lib that come with their compilers. Thankfully, others have provided libraries that manage all of this for us by transparently providing the needed function pointers, while also exposing the needed functionality in header files. Such a library is called an OpenGL Loading Library (or OpenGL function loader), and there are several such libraries available. One of the oldest is OpenGL Extension Wrangler (GLEW). However, there are a few issues with GLEW. First, it provides one large header file that includes everything from all versions of OpenGL. It might be preferable to have a more streamlined header file that only includes functions that we might use. Second, GLEW is distributed as a library that needs to be compiled separately and linked into our project. I find it preferable to have a loader that can be included into a project simply by adding the source files and compiling them directly into our executable, avoiding the need to support another link-time dependency.

In this recipe, we'll use a loader generator named GLAD, available from https://github.com/Dav1dde/glad. This very flexible and efficient library can generate a header that includes only the needed functionality, and also generates just a few files (a source file and a few headers) that we can add directly into our project.

Getting ready

To use GLAD, you can either download and install it using pip (or from https://github.com/Dav1dde/glad), or you can use the web service available here: http://glad.dav1d.de/. If you choose to install it, you'll need Python. The install is simple and described in detail on the GitHub page.

How to do it...

The first step is to generate the header and source files for the OpenGL version and profile of your choice. For this example, we'll generate files for an OpenGL 4.6 core profile. We can then copy the files into our project and compile them directly alongside our code:

1. To generate the header and source files, run the following command:

    glad --generator=c --out-path=GL --profile=core --api=gl=4.6

2. The previous step will generate its output into a directory named GL. There will be two directories: GL/include and GL/src. You can move the GL directory into your project as is, or move the individual files into appropriate locations. Include GL/src/glad.c in your build, and put GL/include into your include path. Within your program code, include glad/glad.h whenever you need access to the OpenGL functions. Note that this fully replaces gl.h, so there is no need to include that.

3. In order to initialize the function pointers, you need to make sure to call a function that does so. The needed function is gladLoadGL(). Somewhere just after the GL context is created (typically in an initialization function), and before any OpenGL functions are called, use the following code:

    if (!gladLoadGL()) { std::cerr << "Unable to load OpenGL functions!" << std::endl; exit(EXIT_FAILURE);
    }

That's all there is to it!

How it works...

The command in step 1 generates a few header files and a source file. The header provides prototypes for all of the selected OpenGL functions, redefines them as function pointers, and defines all of the OpenGL constants as well. The source file provides initialization code for the function pointers as well as some other utility functions. We can include the glad/glad.h header file wherever we need prototypes for OpenGL functions, so all function entry points are available at compile time. At runtime, the gladLoadGL() call will initialize all available function pointers.

Some function pointers may not be successfully initialized. This might happen if your driver does not support the requested OpenGL version. If that happens, calling the functions will fail.

The command-line arguments available to GLAD are fully documented on the GitHub site and are available via glad -h. One can select any OpenGL version, select core/compatibility profiles, include desired extensions, and/or create debug callbacks.

There's more...

GLAD provides a web service at http://glad.dav1d.de/ that makes it easy to generate the loader source and header files without installing GLAD. Simply visit the URL, select the desired configuration, and the loader files will be generated and downloaded.

Using GLM for mathematics

Mathematics is the core to all of computer graphics. In earlier versions, OpenGL provided support for managing coordinate transformations and projections using the standard matrix stacks (GL_MODELVIEW and GL_PROJECTION). In modern versions of core OpenGL however, all of the functionality supporting the matrix stacks has been removed. Therefore, it is up to us to provide our own support for the usual transformation and projection matrices, and then pass them into our shaders. Of course, we could write our own matrix and vector classes to manage this, but some might prefer to use a ready-made, robust library.

One such library is OpenGL Mathematics (GLM), written by Christophe Riccio. Its design is based on the GLSL specification, so the syntax will be familiar to anyone using GLSL. Additionally, it provides extensions that include functionality similar to some of the much-missed OpenGL utility functions, such as glOrtho, glRotate, or gluLookAt.

Getting ready

Since GLM is a header-only library, the installation is simple. Download the latest GLM distribution from http://glm.g-truc.net. Then, unzip the archive file, and copy the glm directory contained inside to anywhere in your compiler's include path.

How to do it...

To use the GLM libraries, include the core header file, and headers for any extensions. For this example, we'll include the matrix transform extension:

#include <glm/glm.hpp>
#include <glm/gtc/matrix_transform.hpp>

The GLM classes are available in the glm namespace. The following is an example of how you might go about making use of some of them:

glm::vec4 position = glm::vec4(1.0f, 0.0f, 0.0f, 1.0f);
glm::mat4 view = glm::lookAt( glm::vec3(0.0f, 0.0f, 5.0f), glm::vec3(0.0f, 0.0f, 0.0f), glm::vec3(0.0f, 1.0f, 0.0f)
);
glm::mat4 model(1.0f); model = glm::rotate(model, 90.0f, glm::vec3(0.0f,1.0f,0.0));
glm::mat4 mv = view * model;
glm::vec4 transformed = mv * position;

How it works...

The GLM library is a header-only library. All of the implementation is included within the header files. It doesn't require separate compilation and you don't need to link your program to it. Just placing the header files in your include path is all that's required!

The previous example first creates vec4 (a four-component vector), which represents a position. Then, it creates a 4x4 view matrix by using the glm::lookAt function. This works in a similar fashion to the old gluLookAt function. Here, we set the camera's location at (0, 0, 5), looking toward the origin, with the up direction in the direction of the positive y axis. We then go on to create the model matrix by first storing the identity matrix in the model variable (via the single-argument constructor), and multiplying it by a rotation matrix using the glm::rotate function.

The multiplication here is implicitly done by the glm::rotate function. It multiplies its first parameter by the rotation matrix (on the right) that is generated by the function. The second parameter is the angle of rotation (in degrees), and the third parameter is the axis of rotation. Since before this statement, model is the identity matrix, the net result is that model becomes a rotation matrix of 90 degrees around the y axis.

Finally, we create our model-view matrix (mv) by multiplying the view and model variables, and then use the combined matrix to transform the position. Note that the multiplication operator has been overloaded to behave in the expected way.

The order is important here. Typically, the model matrix represents a transformation from object space to world space, and the view matrix is a transformation from world space to camera space. So to get a single matrix that transforms from object space to camera space, we want the model matrix to apply first. Therefore, the model matrix is multiplied on the right-hand side of the view matrix.

There's more...

It is not recommended to import all of the GLM namespaces using the following command:

using namespace glm;

This will most likely cause a number of namespace clashes. Instead, it is preferable to import symbols one at a time with the using statements as needed. For example:

#include <glm/glm.hpp> 
using glm::vec3; using glm::mat4;

Using the GLM types as input to OpenGL

GLM supports directly passing a GLM type to OpenGL using one of the OpenGL vector functions (with the v suffix). For example, to pass mat4 named proj to OpenGL, we can use the following code:

glm::mat4 proj = glm::perspective(viewAngle, aspect, nearDist, farDist);
glUniformMatrix4fv(location, 1, GL_FALSE, &proj[0][0]);

Alternatively, rather than using the ampersand operator, we can use the glm::value_ptr function to get a pointer to the content of the GLM type:

glUniformMatrix4fv(location, 1, GL_FALSE, glm::value_ptr(proj));

The latter version requires including the header file glm/gtc/type_ptr.hpp. The use of value_ptr is arguably cleaner, and works for any GLM type.

Determining the GLSL and OpenGL version

In order to support a wide range of systems, it is essential to be able to query for the supported OpenGL and GLSL version of the current driver. It is quite simple to do so, and there are two main functions involved: glGetString and glGetIntegerv.

Note that these functions must be called after the OpenGL context has been created.

How to do it...

The following code will print the version information to stdout:

const GLubyte *renderer = glGetString(GL_RENDERER); const GLubyte *vendor = glGetString(GL_VENDOR); const GLubyte *version = glGetString(GL_VERSION); const GLubyte *glslVersion = glGetString(GL_SHADING_LANGUAGE_VERSION); GLint major, minor;
glGetIntegerv(GL_MAJOR_VERSION, &major);
glGetIntegerv(GL_MINOR_VERSION, &minor); printf("GL Vendor : %s\n", vendor); printf("GL Renderer : %s\n", renderer); printf("GL Version (string) : %s\n", version); printf("GL Version (integer) : %d.%d\n", major, minor); printf("GLSL Version : %s\n", glslVersion);

How it works...

Note that there are two different ways to retrieve the OpenGL version: using glGetString and glGetIntegerv. The former can be useful for providing readable output, but may not be as convenient for programmatically checking the version because of the need to parse the string. The string provided by glGetString(GL_VERSION) should always begin with the major and minor versions separated by a dot, however, the minor version could be followed with a vendor-specific build number. Additionally, the rest of the string can contain additional vendor-specific information and may also include information about the selected profile. It is important to note that the use of glGetIntegerv to query for version information requires OpenGL 3.0 or greater.

The queries for GL_VENDOR and GL_RENDERER provide additional information about the OpenGL driver. The glGetString(GL_VENDOR) call returns the company responsible for the OpenGL implementation. The call to glGetString(GL_RENDERER) provides the name of the renderer, which is specific to a particular hardware platform (such as the ATI Radeon HD 5600 Series). Note that both of these do not vary from release to release, so they can be used to determine the current platform.

Of more importance to us in the context of this book is the call to glGetString(GL_SHADING_LANGUAGE_VERSION), which provides the supported GLSL version number. This string should begin with the major and minor version numbers separated by a period, but similar to the GL_VERSION query, may include other vendor-specific information.

There's more...

It is often useful to query for the supported extensions of the current OpenGL implementation. Extension names are indexed and can be individually queried by index. We use the glGetStringi variant for this. For example, to get the name of the extension stored at index i, we use glGetStringi(GL_EXTENSIONS, i). To print a list of all extensions, we could use the following code:

GLint nExtensions;
glGetIntegerv(GL_NUM_EXTENSIONS, &nExtensions); for (int i = 0; i < nExtensions; i++) printf("%s\n", glGetStringi(GL_EXTENSIONS, i));

Compiling a shader

To get started, we need to know how to compile our GLSL shaders. The GLSL compiler is built right into the OpenGL library, and shaders can only be compiled within the context of a running OpenGL program.

OpenGL 4.1 added the ability to save compiled shader programs to a file, enabling OpenGL programs to avoid the overhead of shader compilation by loading precompiled shader programs. OpenGL 4.6 added the ability to load shader programs compiled to (or written in) SPIR-V, an intermediate language for defining shaders.

Compiling a shader involves creating a shader object, providing the source code (as a string or set of strings) to the shader object, and asking the shader object to compile the code. The process is roughly represented by the following diagram:

Getting ready

To compile a shader, we'll need a basic example to work with. Let's start with the following simple vertex shader. Save it in a file named basic.vert.glsl:

#version 460 in vec3 VertexPosition; in vec3 VertexColor; out vec3 Color; void main() { Color = VertexColor; gl_Position = vec4(VertexPosition, 1.0);
}

In case you're curious about what this code does, it works as a "pass-through" shader. It takes the VertexPosition and VertexColor input attributes and passes them to the fragment shader via the gl_Position and Color output variables.

Next, we'll need to build a basic shell for an OpenGL program using a Window toolkit that supports OpenGL. Examples of cross-platform toolkits include GLFW, GLUT, FLTK, Qt, and wxWidgets. Throughout this text, I'll make the assumption that you can create a basic OpenGL program with your favorite toolkit. Virtually all toolkits have a hook for an initialization function, a resize callback (called upon resizing the window), and a drawing callback (called for each window refresh). For the purposes of this recipe, we need a program that creates and initializes an OpenGL context; it need not do anything other than display an empty OpenGL window. Note that you'll also need to load the OpenGL function pointers.

Finally, load the shader source code into std::string (or the char array). The following example assumes that the shaderCode variable is std::string containing the shader source code.

How to do it...

To compile a shader, use the following steps:

1. Create the shader object:

    GLuint vertShader = glCreateShader(GL_VERTEX_SHADER); if (0 == vertShader) { std::cerr << "Error creating vertex shader." << std::endl; exit(EXIT_FAILURE);
    }

2. Copy the source code into the shader object:

    std::string shaderCode = loadShaderAsString("basic.vert.glsl"); const GLchar * codeArray[] = { shaderCode.c_str() };
    glShaderSource(vertShader, 1, codeArray, NULL);

3. Compile the shader:

    glCompileShader(vertShader);

4. Verify the compilation status:

    GLint result;
    glGetShaderiv(vertShader, GL_COMPILE_STATUS, &result); if (GL_FALSE == result) { std::cerr << "Vertex shader compilation failed!" << std::endl; GLint logLen; glGetShaderiv(vertShader, GL_INFO_LOG_LENGTH, &logLen); if (logLen > 0) { std::string log(logLen, ' '); GLsizei written; glGetShaderInfoLog(vertShader, logLen, &written, &log[0]); std::cerr << "Shader log: " << std::endl << log; }
    }

How it works...

The first step is to create the shader object using the glCreateShader function. The argument is the type of shader, and can be one of the following: GL_VERTEX_SHADER, GL_FRAGMENT_SHADER, GL_GEOMETRY_SHADER, GL_TESS_EVALUATION_SHADER, GL_TESS_CONTROL_SHADER, or (as of version 4.3) GL_COMPUTE_SHADER. In this case, since we are compiling a vertex shader, we use GL_VERTEX_SHADER. This function returns the value used for referencing the vertex shader object, sometimes called the object handle. We store that value in the vertShader variable. If an error occurs while creating the shader object, this function will return 0, so we check for that and if it occurs, we print an appropriate message and terminate.

Following the creation of the shader object, we load the source code into the shader object using the glShaderSource function. This function is designed to accept an array of strings (as opposed to just a single one) in order to support the option of compiling multiple sources (files, strings) at once. So before we call glShaderSource, we place a pointer to our source code into an array named sourceArray.

The first argument to glShaderSource is the handle to the shader object. The second is the number of source code strings that are contained in the array. The third argument is a pointer to an array of source code strings. The final argument is an array of GLint values that contain the length of each source code string in the previous argument.

In the previous code, we pass a value of NULL, which indicates that each source code string is terminated by a null character. If our source code strings were not null terminated, then this argument must be a valid array. Note that once this function returns, the source code has been copied into the OpenGL internal memory, so the memory used to store the source code can be freed.

The next step is to compile the source code for the shader. We do this by simply calling glCompileShader, and passing the handle to the shader that is to be compiled. Of course, depending on the correctness of the source code, the compilation may fail, so the next step is to check whether the compilation was successful.

We can query for the compilation status by calling glGetShaderiv, which is a function for querying the attributes of a shader object. In this case, we are interested in the compilation status, so we use GL_COMPILE_STATUS as the second argument. The first argument is of course the handle to the shader object, and the third argument is a pointer to an integer where the status will be stored. The function provides a value of either GL_TRUE or GL_FALSE in the third argument, indicating whether the compilation was successful.

If the compile status is GL_FALSE, we can query for the shader log, which will provide additional details about the failure. We do so by first querying for the length of the log by calling glGetShaderiv again with a value of GL_INFO_LOG_LENGTH. This provides the length of the log in the logLen variable. Note that this includes the null termination character. We then allocate space for the log, and retrieve the log by calling glGetShaderInfoLog. The first parameter is the handle to the shader object, the second is the size of the character buffer for storing the log, the third argument is a pointer to an integer where the number of characters actually written (excluding the null terminator character) will be stored, and the fourth argument is a pointer to the character buffer for storing the log itself. Once the log is retrieved, we print it to stderr and free its memory space.

There's more...

The previous example only demonstrated how to compile a vertex shader. There are several other types of shaders, including fragment, geometry, and tessellation shaders. The technique for compiling is nearly identical for each shader type. The only significant difference is the argument to glCreateShader.

It is also important to note that shader compilation is only the first step. Similar to a language like C++, we need to link the program. While shader programs can consist of a single shader, for many use cases we have to compile two or more shaders, and then the shaders must be linked together into a shader program object. We'll see the steps involved in linking in the next recipe.

Deleting a shader object

Shader objects can be deleted when no longer needed by calling glDeleteShader. This frees the memory used by the shader and invalidates its handle. Note that if a shader object is already attached to a program object, it will not be immediately deleted, but flagged for deletion when it is detached from the program object.

Once we have compiled our shaders and before we can actually install them into the OpenGL pipeline, we need to link them together into a shader program. Among other things, the linking step involves making the connections between input variables from one shader to output variables of another, and making the connections between the input/output variables of a shader to appropriate locations in the OpenGL environment.

Linking involves steps that are similar to those involved in compiling a shader. We attach each shader object to a new shader program object and then tell the shader program object to link (making sure that the shader objects are compiled before linking):

Getting ready

For this recipe, we'll assume that you've already compiled two shader objects whose handles are stored in the vertShader and fragShader variables.

For this and a few other recipes in this chapter, we'll use the following source code for the fragment shader:

#version 460 in vec3 Color; out vec4 FragColor; void main() { FragColor = vec4(Color, 1.0);
}

For the vertex shader, we'll use the source code from the previous recipe.

How to do it...

In our OpenGL initialization function, and after the compilation of shader objects referred to by vertShader and fragShader, perform the following steps:

1. Create the program object using the following code:

    GLuint programHandle = glCreateProgram(); if (0 == programHandle) { std::cerr << "Error creating program object." << std::endl; exit(EXIT_FAILURE);
    }

2. Attach the shaders to the program object as follows:

    glAttachShader(programHandle, vertShader);
    glAttachShader(programHandle, fragShader);

3. Link the program:

    glLinkProgram(programHandle);

4. Verify the link status:

    GLint status;
    glGetProgramiv(programHandle, GL_LINK_STATUS, &status); if (GL_FALSE == status) { std::cerr << "Failed to link shader program!" << std::endl; GLint logLen; glGetProgramiv(programHandle, GL_INFO_LOG_LENGTH, &logLen); if (logLen > 0) { std::string log(logLen, ' '); GLsizei written; glGetProgramInfoLog(programHandle, logLen, &written, &log[0]); std::cerr << "Program log:" << std::endl << log; }
    }

5. If linking is successful, we can install the program into the OpenGL pipeline with glUseProgram:

    glDetachShader(programHandle, vertShader);
    glDetachShader(programHandle, fragShader);
    glDeleteShader(vertShader);
    glDeleteShader(fragShader);

How it works...

We start by calling glCreateProgram to create an empty program object. This function returns a handle to the program object, which we store in a variable named programHandle. If an error occurs with program creation, the function will return 0. We check for that, and if it occurs, we print an error message and exit.

Next, we attach each shader to the program object using glAttachShader. The first argument is the handle to the program object, and the second is the handle to the shader object to be attached.

Then, we link the program by calling glLinkProgram, providing the handle to the program object as the only argument. As with compilation, we check for the success or failure of the link, with the subsequent query.

We check the status of the link by calling glGetProgramiv. Similar to glGetShaderiv, glGetProgramiv allows us to query various attributes of the shader program. In this case, we ask for the status of the link by providing GL_LINK_STATUS as the second argument.The status is returned in the location pointed to by the third argument, in this case named status.

The link status is either GL_TRUE or GL_FALSE, indicating the success or failure of the link. If the value of the status is GL_FALSE, we retrieve and display the program information log, which should contain additional information and error messages. The program log is retrieved by the call to glGetProgramInfoLog. The first argument is the handle to the program object, the second is the size of the buffer to contain the log, the third is a pointer to a GLsizei variable where the number of bytes written to the buffer will be stored (excluding the null terminator), and the fourth is a pointer to the buffer that will store the log. The buffer can be allocated based on the size returned by the call to glGetProgramiv with the GL_INFO_LOG_LENGTH parameter. The string that is provided in log will be properly null terminated.

Finally, if the link is successful, we install the program into the OpenGL pipeline by calling glUseProgram, providing the handle to the program as the argument.

It is a good idea to detach and delete the shader object, regardless of whether the link is successful. However, if the shader objects might be needed to link another program, you should detach it from this program and skip deletion until later.

With the simple fragment shader from this recipe and the vertex shader from the previous recipe compiled, linked, and installed into the OpenGL pipeline, we have a complete OpenGL pipeline and are ready to begin rendering. Drawing a triangle and supplying different values for the Color attribute yields an image of a multi-colored triangle where the vertices are red, green, and blue, and inside the triangle, the three colors are interpolated, causing a blending of colors throughout:

For details on how to render the triangle, see Working with GLSL Programs.

There's more...

You can use multiple shader programs within a single OpenGL program. They can be swapped in and out of the OpenGL pipeline by calling glUseProgram to select the desired program.

Shader input/output variables

You may have noticed that the Color variable is used to send data from the vertex shader to the fragment shader. There is an output variable (out vec3) in the vertex shader and an input variable (in vec3) in the fragment shader, both with the same name. The value that the fragment shader receives is a value that is interpolated from the values of the corresponding output variable for each of the vertices (hence the blended colors in the earlier image). This interpolation is automatically done by hardware rasterizer before the execution of the fragment stage.

When linking a shader program, OpenGL makes the connections between input and output variables in the vertex and fragment shaders (among other things). If a vertex shader's output variable has the same name and type as a fragment shader's input variable, OpenGL will automatically link them together.

It is possible to connect (link) variables that do not have the same name or type by using layout qualifiers. With a layout qualifier, we can specify the location for each variable specifically. For example, suppose that I used this set of output variables in my vertex shader:

layout (location=0) out vec4 VertColor;
layout (location=1) out vec3 VertNormal;

I could use these variables in the fragment shader:

layout (location=0) in vec3 Color;
layout (location=1) in vec3 Normal;

Despite the fact that these have different names (and for Color, types), they will be connected by the linker when the program is linked due to the fact that they are assignedthe same locations. In this example, VertColor will be linked to Color, and VertNormal will be linked to Normal. This makes things more convenient. We're not required to use the same names for input/output variables, which gives us the flexibility to use names that might be more descriptive in each shader stage. More importantly, it is part of a larger framework, called separate shader objects.

In fact, this use of layout qualifiers to specify variable locations is required when compiling to SPIR-V. (see the Loading an SPIR-V shader program recipe).

Deleting a shader program

If a program is no longer needed, it can be deleted from OpenGL memory by calling glDeleteProgram, providing the program handle as the only argument. This invalidates the handle and frees the memory used by the program. Note that if the program object is currently in use, it will not be immediately deleted, but will be flagged for deletion when it is no longer in use.

Also, the deletion of a shader program detaches the shader objects that were attached to the program but does not delete them unless those shader objects have already been flagged for deletion by a previous call to glDeleteShader. Therefore, as mentioned before, it is a good idea to detach and delete them immediately, as soon as the program is linked, to avoid accidentally leaking shader objects.

Saving and loading a shader binary

OpenGL 4.1 introduced the glGetProgramBinary and glProgramBinary functions, which allow us to save and load compiled shader program binaries. Note that this functionality is still quite dependent on the OpenGL driver, and is not widely supported.

Unfortunately, the Intel drivers on macOS do not support any binary formats, and Apple has deprecated OpenGL in macOS Mojave.

In this recipe, we'll outline the steps involved in saving and loading a compiled shader program.

Getting ready

We'll begin assuming that a shader program has been successfully compiled, and its ID is in the program variable.

How to do it...

To save the shader binary, first verify that the driver supports at least one shader binary format:

GLint formats = 0;
glGetIntegerv(GL_NUM_PROGRAM_BINARY_FORMATS, &formats);
if (formats < 1) { std::cerr << "Driver does not support any binary formats." << std::endl; exit(EXIT_FAILURE);
}

Then, assuming at least one binary format is available, use glGetProgramBinary to retrieve the compiled shader code and write it to a file:

GLint length = 0;
glGetProgramiv(program, GL_PROGRAM_BINARY_LENGTH, &length); GLenum format = 0;
glGetProgramBinary(program, length, NULL, &format, buffer.data()); std::string fName("shader.bin");
std::cout << "Writing to " << fName << ", binary format = " << format << std::endl;
std::ofstream out(fName.c_str(), std::ios::binary);
out.write(reinterpret_cast<char *>(buffer.data()), length);
out.close();

To load and use a shader binary, retrieve the compiled program from storage, and use glProgramBinary to load it into the OpenGL context:

GLuint program = glCreateProgram(); std::ifstream inputStream("shader.bin", std::ios::binary);
std::istreambuf_iterator<char> startIt(inputStream), endIt;
std::vector<char> buffer(startIt, endIt); inputStream.close(); glProgramBinary(program, format, buffer.data(), buffer.size()); GLint status;
glGetprogramiv(program, GL_LINK_STATUS, &status);
if (GL_FALSE == status) { }

How it works...

Drivers can support zero or more binary formats. The call to glGetIntegerv with the GL_NUM_PROGRAM_BINARY_FORMATS constant queries the driver to see how many are available. If this number is zero, the OpenGL driver does not support reading or writing shader binaries. If the value is one or more, we're good to go.

If at least one binary format is available, we can use glGetProgramBinary to retrieve the compiled shader code shown earlier. The function will write the binary format used to the location pointed to by the fourth parameter. In the preceding example, the data is stored in the vector named buffer.

To load the shader binary, we can use glProgramBinary. This function will load a previously saved shader binary. It requires the binary format to be passed as the second parameter. We can then check GL_LINK_STATUS to verify that it was loaded without error.

Loading a SPIR-V shader program

Standard, Portable Intermediate Representation - V (SPIR-V) is an intermediate language designed and standardized by the Khronos Group for shaders. It is intended to be a compiler target for a number of different languages. In the Vulkan API, shaders are required to be compiled to SPIR-V before they can be loaded. SPIR-V is intended to provide developers with the freedom to develop their shaders in any language they want (as long as it can be compiled to SPIR-V), and avoid the need for an OpenGL (or Vulkan) implementation to provide compilers for multiple languages.

Support for SPIR-V shader binaries was added to OpenGL core with version 4.6, but is also available via the ARB_gl_spirv extension for earlier OpenGL versions.

Currently, the Khronos Group provides a reference compiler for compiling GLSL to SPIR-V. It is available on GitHub at https://github.com/KhronosGroup/glslang.

In this recipe, we'll go through the steps involved in precompiling a GLSL shader pair to SPIR-V, and then load it into an OpenGL program.

Getting ready

Download and compile the OpenGL shader validator from https://github.com/KhronosGroup/glslang. Make sure that the glslangValidator binary is available in your PATH command line. In this example, we'll use the shader pair located in the basic.vert.glsl and basic.frag.glsl files.

Note that you'll need to use explicit locations for all of your input/output variables in the shaders.

All variables used for input/output interfaces (in/out variables) must have a location assigned.

How to do it...

Start by compiling the shader pair into SPIR-V using the glslangValidator tool:

glslangValidator -G -o basic.vert.spv basic.vert.glsl
glslangValidator -G -o basic.frag.spv basic.frag.glsl

If successful, this produces the basic.vert.spv and basic.frag.spv SPIR-V output files.

To load your SPIR-V shaders into an OpenGL program, use glShaderBinary and glSpecializeShader. With glShaderBinary, use GL_SHADER_BINARY_FORMAT_SPIR_V as the binary format:

GLuint vertShader = glCreateShader(GL_VERTEX_SHADER); std::ifstream inStream("basic.vert.spv", std::ios::binary);
std::istreambuf_iterator<char> startIt(inStream), endIt;
std::vector<char> buffer(startIt, endIt);
inStream.close(); glShaderBinary(1, &vertShader, GL_SHADER_BINARY_FORMAT_SPIR_V, buffer.data(), buffer.size()); glSpecializeShader(vertShader, "main", 0, 0, 0); GLint status;
glGetShaderiv(vertShader, GL_COMPILE_STATUS, &status);
if (GL_FALSE == status) { }

The process is nearly exactly the same for the fragment shader; just use GL_FRAGMENT_SHADER instead of GL_VERTEX_SHADER on the first line.

Finally, we create the program object, attach the shaders, and link.

How it works...

The glShaderBinary function provides us with the ability to load shaders that have been compiled to the SPIR-V format. This part is fairly straightforward. The function that might be a bit more confusing is glSpecializeShader. We are required to call this function before the shader stage can be linked. This call is needed because a single SPIR-V file can have multiple entry points, and SPIR-V files can have specialization constants, which are parameters that the user can provide before it is compiled into native code.

At a minimum, we need to define the entry point for our shader. Since the source language is GLSL, the entry point is main. We specify the entry point(s) via the second argument. For GLSL, we simply use the main constant string. The last three parameters can be used to define the specialization constants. The first of the three is the number of constants, the next is a pointer to an array of constant indices, and the last is a pointer to an array of constant values.

The process of specializing an SPIR-V shader is similar to compiling a GLSL shader. Before calling glSpecializeShader, or if specialization fails, the compile status will be GL_FALSE. If specialization succeeds, the compile status will be GL_TRUE. As with GLSL shaders, we can query the shader info log to get detailed error messages (see the Compiling a shader recipe)..

There's more...

SPIR-V appears to be the future of shader programming in the Vulkan/OpenGL space. However, GLSL is not going away anytime soon. GLSL compilers still ship with OpenGL and there's currently no sign that they will be removed or deprecated. The OpenGL specification still considers GLSL to be the primary shading language.

However, if you're interested in getting on board with SPIR-V early, or you have an interest in moving toward Vulkan, it might be valuable to you to start working with SPIR-V now in OpenGL. Fortunately, that's possible, at least in recent versions of OpenGL.

The future of SPIR-V is very bright. There is already a (mostly complete) compiler for HLSL that targets SPIR-V, and it is likely that other languages will be developed soon. It's an exciting time for shader programming!

Working with GLSL Programs

In this chapter, we will cover the following recipes:

• Sending data to a shader using vertex attributes and vertex buffer objects
• Getting a list of active vertex input attributes and locations
• Sending data to a shader using uniform variables
• Getting a list of active uniform variables
• Using uniform blocks and uniform buffer objects
• Using program pipelines
• Getting debug messages
• Building a C++ shader program class

Introduction

In Chapter 1, Getting Started with GLSL, we covered the basics of compiling, linking, and exporting shader programs. In this chapter, we'll cover techniques for communication between shader programs and the host OpenGL program. To be more specific, we'll focus primarily on input. The input to shader programs is generally accomplished via attributes and uniform variables. In this chapter, we'll see several examples of the use of these types of variables. We'll also cover a recipe for mixing and matching shader program stages, and a recipe for creating a C++ shader program object.

We won't cover shader output in this chapter. Obviously, shader programs send their output to the default framebuffer, but there are several other techniques for receiving shader output. For example, the use of custom framebuffer objects allow us to store shader output to a texture or other buffer. A technique called transform feedback allows for the storage of vertex shader output into arbitrary buffers. You'll see many examples of these output techniques later in this book.

Sending data to a shader using vertex attributes and vertex buffer objects

The vertex shader is invoked once per vertex. Its main job is to process the data associated with the vertex, and pass it (and possibly other information) along to the next stage of the pipeline. In order to give our vertex shader something to work with, we must have some way of providing (per-vertex) input to the shader. Typically, this includes the vertex position, normal vector, and texture coordinates (among other things). In earlier versions of OpenGL (prior to 3.0), each piece of vertex information had a specific channel in the pipeline. It was provided to the shaders using functions such as glVertex, glTexCoord, and glNormal (or within client vertex arrays using glVertexPointer, glTexCoordPointer, or glNormalPointer). The shader would then access these values via built-in variables such as gl_Vertex and gl_Normal. This functionality was deprecated in OpenGL 3.0 and later removed. Instead, vertex information must now be provided using generic vertex attributes, usually in conjunction with (vertex) buffer objects. The programmer is now free to define an arbitrary set of per-vertex attributes to provide as input to the vertex shader. For example, in order to implement normal mapping, the programmer might decide that the position, normal vector, and tangent vector should be provided along with each vertex. With OpenGL 4, it's easy to define this as the set of input attributes. This gives us a great deal of flexibility to define our vertex information in any way that is appropriate for our application, but may require a bit of getting used to for those of us who are accustomed to the old way of doing things.

In the vertex shader, the per-vertex input attributes are defined by using the in GLSL qualifier. For example, to define a three-component vector input attribute named VertexColor, we use the following code:

in vec3 VertexColor;

Of course, the data for this attribute must be supplied by the OpenGL program. To do so, we make use of vertex buffer objects. The buffer object contains the values for the input attribute. In the main OpenGL program, we make the connection between the buffer and the input attribute and define how to step through the data. Then, when rendering, OpenGL pulls data for the input attribute from the buffer for each invocation of the vertex shader.

For this recipe, we'll draw a single triangle. Our vertex attributes will be position and color. We'll use a fragment shader to blend the colors of each vertex across the triangle to produce an image similar to the one shown as follows. The vertices of the triangle are red, green, and blue, and the interior of the triangle has those three colors blended together. The colors may not be visible in the printed text, but the variation in the shade should indicate the blending:

Getting ready

We'll start with an empty OpenGL program, and the following shaders:

The vertex shader (basic.vert.glsl):

#version 460 layout (location=0) in vec3 VertexPosition;
layout (location=1) in vec3 VertexColor; out vec3 Color; void main() { Color = VertexColor; gl_Position = vec4(VertexPosition, 1.0);
}

Attributes are the input variables to a vertex shader. In the previous code, there are two input attributes: VertexPosition and VertexColor. They are specified using the in GLSL keyword. Don't worry about the layout prefix, as we'll discuss that later. Our main OpenGL program needs to supply the data for these two attributes for each vertex. We will do so by mapping our polygon data to these variables.

It also has one output variable, named Color, which is sent to the fragment shader. In this case, Color is just an unchanged copy of VertexColor. Also, note that the VertexPosition attribute is simply expanded and passed along to the built-in gl_Position output variable for further processing.

The fragment shader (basic.frag.glsl):

#version 460 in vec3 Color; out vec4 FragColor; void main() { FragColor = vec4(Color, 1.0);
}

There is just one input variable for this shader, Color. This links to the corresponding output variable in the vertex shader, and will contain a value that has been interpolated across the triangle based on the values at the vertices. We simply expand and copy this color to the FragColor output variable.

Write code to compile and link these shaders into a shader program. In the following code, I'll assume that the handle to the shader program is programHandle.

How to do it...

Use the following steps to set up your buffer objects and render the triangle:

1. Create a global (or private instance) variable to hold our handle to the vertex array object:

    GLuint vaoHandle;

2. Within the initialization function, we create and populate the vertex buffer objects for each attribute:

    float positionData[] = { -0.8f, -0.8f, 0.0f, 0.8f, -0.8f, 0.0f, 0.0f, 0.8f, 0.0f };
    float colorData[] = { 1.0f, 0.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 0.0f, 1.0f }; GLuint vboHandles[2];
    glGenBuffers(2, vboHandles); GLuint positionBufferHandle = vboHandles[0];
    GLuint colorBufferHandle = vboHandles[1]; glBindBuffer(GL_ARRAY_BUFFER, positionBufferHandle);
    glBufferData(GL_ARRAY_BUFFER, 9 * sizeof(float), positionData, GL_STATIC_DRAW); glBindBuffer(GL_ARRAY_BUFFER, colorBufferHandle);
    glBufferData(GL_ARRAY_BUFFER, 9 * sizeof(float), colorData, GL_STATIC_DRAW);

3. Create and define a vertex array object, which defines the relationship between the buffers and the input attributes:

    glGenVertexArrays(1, &vaoHandle);
    glBindVertexArray(vaoHandle); glEnableVertexAttribArray(0); glEnableVertexAttribArray(1); glBindBuffer(GL_ARRAY_BUFFER, positionBufferHandle);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, NULL); glBindBuffer(GL_ARRAY_BUFFER, colorBufferHandle);
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 0, NULL);

4. In the render function, bind to the vertex array object and call glDrawArrays to initiate the rendering:

    glBindVertexArray(vaoHandle);
    glDrawArrays(GL_TRIANGLES, 0, 3);

How it works...

Vertex attributes are the input variables to our vertex shader. In the given vertex shader, our two attributes are VertexPosition and VertexColor. The main OpenGL program refers to vertex attributes by associating each (active) input variable with a generic attribute index. These generic indices are simply integers between 0 and GL_MAX_VERTEX_ATTRIBS - 1. We can specify the relationship between these indices and the attributes using the layout qualifier. For example, in our vertex shader, we use the layout qualifier to assign VertexPosition to attribute index 0 and VertexColor to attribute index 1:

layout (location = 0) in vec3 VertexPosition;
layout (location = 1) in vec3 VertexColor;

We refer to the vertex attributes in our OpenGL code by referring to the corresponding generic vertex attribute index.

It is not strictly necessary to explicitly specify the mappings between attribute variables and generic attribute indexes, because OpenGL will automatically map active vertex attributes to generic indexes when the program is linked. We could then query for the mappings and determine the indexes that correspond to the shader's input variables. It may be somewhat clearer, however, to explicitly specify the mapping, as we do in this example.

The first step involves setting up a pair of buffer objects to store our position and color data. As with most OpenGL objects, we start by creating the objects and acquiring handles to the two buffers by calling glGenBuffers. We then assign each handle to a separate descriptive variable to make the following code more clear.

For each buffer object, we first bind the buffer to the GL_ARRAY_BUFFER binding point by calling glBindBuffer. The first argument to glBindBuffer is the target binding point. In this case, since the data is essentially a generic array, we use GL_ARRAY_BUFFER. Examples of other kinds of targets (such as GL_UNIFORM_BUFFER or GL_ELEMENT_ARRAY_BUFFER) will be seen in later examples.

Once our buffer object is bound, we can populate the buffer with our vertex/color data by calling glBufferData. The second and third arguments to this function are the size of the array and a pointer to the array containing the data. Let's focus on the first and last arguments. The first argument indicates the target buffer object. The data provided in the third argument is copied into the buffer that is bound to this binding point. The last argument is one that gives OpenGL a hint about how the data will be used so that it can determine how best to manage the buffer internally. For full details about this argument, take a look into the OpenGL documentation. In our case, the data is specified once, will not be modified, and will be used many times for drawing operations, so this usage pattern best corresponds to the GL_STATIC_DRAW value.

Now that we have set up our buffer objects, we tie them together into a Vertex Array Object (VAO). The VAO contains information about the connections between the data in our buffers and the input vertex attributes. We create a VAO using the glGenVertexArrays function. This gives us a handle to our new object, which we store in the vaoHandle (global) variable. Then, we enable the generic vertex attribute indexes 0 and 1 by calling glEnableVertexAttribArray. Doing so indicates that that the values for the attributes will be accessed and used for rendering.

The next step makes the connection between the buffer objects and the generic vertex attribute indexes:

glBindBuffer(GL_ARRAY_BUFFER, positionBufferHandle);
glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, NULL );

First, we bind the buffer object to the GL_ARRAY_BUFFER binding point, then we call glVertexAttribPointer, which tells OpenGL which generic index the data should be used with, the format of the data stored in the buffer object, and where it is located within the buffer object that is bound to the GL_ARRAY_BUFFER binding point.

The first argument is the generic attribute index. The second is the number of components per vertex attribute (1, 2, 3, or 4). In this case, we are providing three-dimensional data, so we want three components per vertex. The third argument is the data type of each component in the buffer. The fourth is a Boolean that specifies whether the data should be automatically normalized (mapped to a range of [-1, 1] for signed integral values or [0, 1] for unsigned integral values). The fifth argument is the stride, which indicates the byte offset between consecutive attributes. Since our data is tightly packed, we can use a value of zero. The last argument is a pointer, which is not treated as a pointer! Instead, its value is interpreted as a byte offset from the beginning of the buffer to the first attribute in the buffer. In this case, there is no additional data in either buffer before the first element, so we use a value of zero.

The glVertexAttribPointer function stores (in the VAO's state) a pointer to the buffer currently bound to the GL_ARRAY_BUFFER binding point. When another buffer is bound to that binding point, it does not change the value of the pointer.

The VAO stores all of the OpenGL states related to the relationship between buffer objects and the generic vertex attributes, as well as the information about the format of the data in the buffer objects. This allows us to quickly return all of this state when rendering.

The VAO is an extremely important concept, but can be tricky to understand. It's important to remember that the VAO's state is primarily associated with the enabled attributes and their connection to buffer objects. It doesn't necessarily keep track of buffer bindings. For example, it doesn't remember what is bound to the GL_ARRAY_BUFFER binding point. We only bind to this point in order to set up the pointers via glVertexAttribPointer. Once we have the VAO set up (a one-time operation), we can issue a draw command to render our object. In our render function, we clear the color buffer using glClear, bind to the vertex array object, and call glDrawArrays to draw our triangle. The glDrawArrays function initiates rendering of primitives by stepping through the buffers for each enabled attribute array, and passing the data down the pipeline to our vertex shader. The first argument is the render mode (in this case, we are drawing triangles), the second is the starting index in the enabled arrays, and the third argument is the number of indices to be rendered (three vertices for a single triangle).

To summarize, we followed these steps:

1. Make sure to specify the generic vertex attribute indexes for each attribute in the vertex shader using the layout qualifier
2. Create and populate the buffer objects for each attribute
3. Create and define the vertex array object by calling glVertexAttribPointer while the appropriate buffer is bound
4. When rendering, bind to the vertex array object and call glDrawArrays, or an other appropriate rendering function (for example, glDrawElements)

There's more...

In the following section, we'll discuss some details, extensions, and alternatives to the previous technique.

Separate attribute format

With OpenGL 4.3, we have an alternate (arguably better) way of specifying the vertex array object state (attribute format, enabled attributes, and buffers). In the previous example, the glVertexAttribPointer function does two important things. First, it indirectly specifies which buffer contains the data for the attribute which is the buffer currently bound (at the time of the call) to GL_ARRAY_BUFFER. Secondly, it specifies the format of that data (type, offset, stride, and so on).

It is arguably clearer to separate these two concerns into their own functions. This is exactly what has been implemented in OpenGL 4.3. For example, to implement the same functionality as in step 3 of the previous How to do it... section, we would use the following code:

glGenVertexArrays(1, &vaoHandle);
glBindVertexArray(vaoHandle); glEnableVertexAttribArray(0);
glEnableVertexAttribArray(1); glBindVertexBuffer(0, positionBufferHandle, 0, sizeof(GLfloat) * 3);
glBindVertexBuffer(1, colorBufferHandle, 0, sizeof(GLfloat) * 3); glVertexAttribFormat(0, 3, GL_FLOAT, GL_FALSE, 0);
glVertexAttribBinding(0, 0); glVertexAttribFormat(1, 3, GL_FLOAT, GL_FALSE, 0);
glVertexAttribBinding(1, 1);

The first four lines of the previous code are exactly the same as in the first example. We create and bind to the VAO, then enable attributes 0 and 1. Next, we bind our two buffers to two different indexes within the vertex buffer binding point using glBindVertexBuffer. Note that we're no longer using the GL_ARRAY_BUFFER binding point. Instead, we now have a new binding point specifically for vertex buffers. This binding point has several indexes (usually from 0 - 15), so we can bind multiple buffers to this point. The first argument to glBindVertexBuffer specifies the index within the vertex buffer binding point. Here, we bind our position buffer to index 0 and our color buffer to index 1.

The indexes within the vertex buffer binding point need not be the same as the attribute locations.

The other arguments to glBindVertexBuffer are as follows. The second argument is the buffer to be bound, the third is the offset from the beginning of the buffer to where the data begins, and the fourth is the stride, which is the distance between successive elements within the buffer. Unlike glVertexAttribPointer, we can't use a 0 value here for tightly packed data, because OpenGL can't determine the size of the data without more information, so we need to specify it explicitly here.

Next, we call glVertexAttribFormat to specify the format of the data for the attribute. Note that this time, this is decoupled from the buffer that stores the data. Instead, we're just specifying the format to expect for this attribute. The arguments are the same as the first four arguments to glVertexAttribPointer.

The glVertexAttribBinding function specifies the relationship between buffers that are bound to the vertex buffer binding point and attributes. The first argument is the attribute location, and the second is the index within the vertex buffer binding point. In this example, they are the same, but they don't need to be. Also note that the buffer bindings of the vertex buffer binding point (specified by glBindVertexBuffer) are part of the VAO state, unlike the binding to GL_ARRAY_BUFFER.

This version is arguably clearer and easier to understand. It removes the confusing aspects of the invisible pointers that are managed in the VAO, and makes the relationship between attributes and buffers much clearer with glVertexAttribBinding. Additionally, it separates concerns that really don't need to be combined.

Fragment shader output

You may have noticed that I've neglected to say anything about the FragColor output variable in the fragment shader. This variable receives the final output color for each fragment (pixel). Like vertex input variables, this variable needs to be associated with a proper location. Of course, we would typically like this to be linked to the back color buffer, which by default (in double buffered systems) is "color number" zero. (The relationship of the color numbers to render buffers can be changed by using glDrawBuffers). In this program, we are relying on the fact that the linker will automatically link our only fragment output variable to color number zero. To explicitly do so, we could (and probably should) have used a layout qualifier in the fragment shader:

layout (location = 0) out vec4 FragColor;

We are free to define multiple output variables for a fragment shader, thereby enabling us to render to multiple output buffers. This can be quite useful for specialized algorithms such as deferred rendering.

Specifying attribute indexes without using layout qualifiers

If you'd rather not clutter up your vertex shader code with the layout qualifiers (or you're using a version of OpenGL that doesn't support them), you can define the attribute indexes within the OpenGL program. We can do so by calling glBindAttribLocation just prior to linking the shader program. For example, we'd add the following code to the main OpenGL program just before the link step:

glBindAttribLocation(programHandle, 0, "VertexPosition");
glBindAttribLocation(programHandle, 1, "VertexColor");

This would indicate to the linker that VertexPosition should correspond to generic attribute index 0 and VertexColor to index 1.

Similarly, we can specify the color number for the fragment shader output variables without using the layout qualifier. We do so by calling glBindFragDataLocation prior to linking the shader program:

glBindFragDataLocation(programHandle, 0, "FragColor");

This would tell the linker to bind the FragColor output variable to color number 0.

Using element arrays

It is often the case that we need to step through our vertex arrays in a non-linear fashion. In other words, we may want to jump around the data rather than just move through it from beginning to end. For example, we might want to draw a cube where the vertex data consists of only eight positions (the corners of the cube). In order to draw the cube, we would need to draw 12 triangles (two for each face), each of which consists of three vertices. All of the needed position data is in the original eight positions, but to draw all the triangles, we'll need to jump around and use each position for at least three different triangles.

To jump around in our vertex arrays, we can make use of element arrays. The element array is another buffer that defines the indices used when stepping through the vertex arrays. For details on using element arrays, take a look at the glDrawElements function in the OpenGL documentation (http://www.opengl.org/sdk/docs/man).

Interleaved arrays

In this example, we used two buffers (one for color and one for position). Instead, we could have used just a single buffer and combined all of the data. In general, it is possible to combine the data for multiple attributes into a single buffer. The data for multiple attributes can be interleaved within an array, so that all of the data for a given vertex is grouped together within the buffer. Doing so just requires careful use of the stride argument to glVertexAttribPointer or glBindVertexBuffer. Take a look at the documentation for full details (http://www.opengl.org/sdk/docs/man).

The decision about when to use interleaved arrays and when to use separate arrays is highly dependent on the situation. Interleaved arrays may bring better results due to the fact that data is accessed together and resides closer in memory (so-called locality of reference), resulting in better caching performance.

Getting a list of active vertex input attributes and locations

As covered in the previous recipe, the input variables within a vertex shader are linked to generic vertex attribute indices at the time the program is linked. If we need to specify the relationship, we can either use layout qualifiers within the shader, or we could call glBindAttribLocation before linking.

However, it may be preferable to let the linker create the mappings automatically and query for them after program linking is complete. In this recipe, we'll see a simple example that prints all the active attributes and their indices.

Getting ready

Start with an OpenGL program that compiles and links a shader pair. You could use the shaders from the previous recipe.

As in previous recipes, we'll assume that the handle to the shader program is stored in a variable named programHandle.

How to do it...

After linking and enabling the shader program, use the following code to display the list of active attributes:

1. Start by querying for the number of active attributes:

    GLint numAttribs;
    glGetProgramInterfaceiv(programHandle, GL_PROGRAM_INPUT, GL_ACTIVE_RESOURCES, &numAttribs);

2. Loop through each attribute, query for the length of the name, the type, and the attribute location, and print the results to standard out:

    GLenum properties[] = {GL_NAME_LENGTH, GL_TYPE, GL_LOCATION}; std::cout << "Active attributes" << std::endl;
    for (int i = 0; i < numAttribs; ++i) { GLint results[3]; glGetProgramResourceiv(programHhandle, GL_PROGRAM_INPUT, i, 3, properties, 3, NULL, results); GLint nameBufSize = results[0] + 1; char * name = new char[nameBufSize]; glGetProgramResourceName(programHandle, GL_PROGRAM_INPUT, i, nameBufSize, NULL, name); printf("%-5d %s (%s)n", results[2], name, getTypeString(results[1])); delete [] name;
    }

How it works...

In step 1, we query for the number of active attributes by calling glGetProgramInterfaceiv. The first argument is the handle to the program object, and the second (GL_PROGRAM_INPUT) indicates that we are querying for information about the program input variables (the vertex attributes). The third argument (GL_ACTIVE_RESOURCES) indicates that we want the number of active resources. The result is stored in the location pointed to by the last argument, numAttribs.

Now that we have the number of attributes, we query for information about each one. The indices of the attributes run from 0 to numAttribs-1. We loop over those indices and for each we call glGetProgramResourceiv to get the length of the name, the type, and the location. We specify what information we would like to receive by means of an array of GLenum values called properties. The first argument is the handle to the program object, the second is the resource that we are querying (GL_PROGRAM_INPUT). The third is the index of the attribute and the fourth is the number of values in the properties array, which is the fifth argument. The properties array contains GLenum values, which specify the specific properties we would like to receive. In this example, the array contains GL_NAME_LENGTH, GL_TYPE, and GL_LOCATION, which indicates that we want the length of the attribute's name, the data type of the attribute, and its location. The sixth argument is the size of the buffer that will receive the results; the seventh argument is a pointer to an integer that would receive the number of results that were written. If that argument is NULL, then no information is provided. Finally, the last argument is a pointer to a GLint array that will receive the results. Each item in the properties array corresponds to the same index in the results array.

Next, we retrieve the name of the attribute by allocating a buffer to store the name and calling glGetProgramResourceName. The results array contains the length of the name in the first element, so we allocate an array of that size with an extra character just for good measure. The OpenGL documentation says that the size returned from glGetProgramResourceiv includes the null terminator, but it doesn't hurt to make sure by making a bit of additional space. In my tests, I've found this to be necessary on the latest NVIDIA drivers.

Finally, we get the name by calling glGetProgramResourceName, and then print the information to the screen. We print the attribute's location, name, and type. The location is available in the third element of the results array, and the type is in the second. Note the use of the getTypeString function. This is a simple custom function that just returns a string representation of the data type. The data type is represented by one of the OpenGL defined constants, such as GL_FLOAT, GL_FLOAT_VEC2, or GL_FLOAT_VEC3.

The getTypeString function consists of just one big switch statement returning a human-readable string corresponding to the value of the parameter.

The output of the previous code looks like this when it is run on the shaders from the previous recipes:

Active attributes:
1 VertexColor (vec3)
0 VertexPosition (vec3)

There's more...

It should be noted that in order for a vertex shader input variable to be considered active, it must be used within the vertex shader. In other words, a variable is considered active if it is determined by the GLSL linker that it may be accessed during program execution. If a variable is declared within a shader, but not used, the previous code will not display the variable because it is not considered active and is effectively ignored by OpenGL.

Sending data to a shader using uniform variables

Vertex attributes offer one avenue for providing input to shaders; a second technique is uniform variables. Uniform variables are intended to be used for data that may change relatively infrequently compared to per-vertex attributes. In fact, it is simply not possible to set per-vertex attributes with uniform variables. For example, uniform variables are well-suited to the matrices used for modeling, viewing, and projective transformations.

Within a shader, uniform variables are read-only. Their values can only be changed from outside the shader, via the OpenGL API. However, they can be initialized within the shader by assigning them to a constant value along with the declaration.

Uniform variables can appear in any shader within a shader program, and are always used as input variables. They can be declared in one or more shaders within a program, but if a variable with a given name is declared in more than one shader, its type must be the same in all shaders. In other words, the uniform variables are held in a shared uniform namespace for the entire shader program.

In this recipe, we'll draw the same triangle as in previous recipes in this chapter; however, this time, we'll rotate the triangle using a uniform matrix variable:

Getting ready

We'll use the following vertex shader:

#version 430  layout (location = 0) in vec3 VertexPosition; layout (location = 1) in vec3 VertexColor; out vec3 Color; uniform mat4 RotationMatrix; void main() { Color = VertexColor; gl_Position = RotationMatrix * vec4(VertexPosition, 1.0);
}

Note that the RotationMatrix variable is declared using the uniform qualifier. We'll provide the data for this variable via the OpenGL program.

The RotationMatrix variable is also used to transform VertexPosition before assigning it to the default output position variable, gl_Position.

We'll use the same fragment shader as in the previous recipes:

#version 460  in vec3 Color; layout (location = 0) out vec4 FragColor; void main() { FragColor = vec4(Color, 1.0);
}

Within the main OpenGL code, we determine the rotation matrix and send it to the shader's uniform variable. To create our rotation matrix, we'll use the GLM library. Within the main OpenGL code, add the following include statements:

#include <glm/glm.hpp>
#include <glm/gtc/matrix_transform.hpp>

We'll also assume that code has been written to compile and link the shaders, and to create the vertex array object for the color triangle. We'll assume that the handle to the vertex array object is vaoHandle, and the handle to the program object is programHandle.

How to do it...

Within the render method, use the following code:

glClear(GL_COLOR_BUFFER_BIT); glm::mat4 rotationMatrix = glm::rotate(glm::mat4(1.0f), angle, glm::vec3(0.0f,0.0f,1.0f));
GLuint location = glGetUniformLocation(programHandle, "RotationMatrix");
if (location >= 0) { glUniformMatrix4fv(location, 1, GL_FALSE, glm::value_ptr(rotationMatrix));
}
glBindVertexArray(vaoHandle);
glDrawArrays(GL_TRIANGLES, 0, 3);

How it works...

The steps involved in setting the value of a uniform variable include finding the location of the variable, and assigning a value to that location using one of the glUniform functions.

In this example, we start by clearing the color buffer, then creating a rotation matrix using GLM. Next, we query for the location of the uniform variable by calling glGetUniformLocation. This function takes the handle to the shader program object and the name of the uniform variable, and returns its location. If the uniform variable is not an active uniform variable, the function returns -1.

It is inefficient to query the location of a uniform variable in each frame. A more efficient approach would be to cache the location during the shader compilation stage, and use it here.

We then assign a value to the uniform variable's location using glUniformMatrix4fv. The first argument is the uniform variable's location. The second is the number of matrices that are being assigned (note that the uniform variable could be an array). The third is a Boolean value indicating whether the matrix should be transposed when loaded into the uniform variable. With GLM matrices, a transpose is not required, so we use GL_FALSE here. If you were implementing the matrix using an array, and the data was in row-major order, you might need to use GL_TRUE for this argument. The last argument is a pointer to the data for the uniform variable.

There's more...

Of course, uniform variables can be any valid GLSL type, including complex types such as arrays or structures. OpenGL provides a glUniform function with the usual suffixes, appropriate for each type. For example, to assign to a variable of the vec3 type, one would use glUniform3f or glUniform3fv.

For arrays, one can use the functions ending in v to initialize multiple values within the array. Note that if it is desired, one can query for the location of a particular element of the uniform array using the [] operator. For example, to query for the location of the second element of MyArray:

GLuint location = glGetUniformLocation(programHandle, "MyArray[1]");

For structures, the members of the structure must be initialized individually. As with arrays, one can query for the location of a member of a structure using something like the following:

GLuint location = glGetUniformLocation(programHandle, "MyMatrices.Rotation");

Where the structure variable is MyMatrices and the member of the structure is Rotation.

Getting a list of active uniform variables

While it is a simple process to query for the location of an individual uniform variable, there may be instances where it can be useful to generate a list of all active uniform variables. For example, one might choose to create a set of variables to store the location of each uniform and assign their values after the program is linked. This would avoid the need to query for uniform locations when setting the value of the uniform variables, creating slightly more efficient code.

The process for listing uniform variables is very similar to the process for listing attributes, so this recipe will refer the reader back to the previous recipe for a detailed explanation.

Getting ready

Start with a basic OpenGL program that compiles and links a shader program. In the following recipes, we'll assume that the handle to the program is in a variable named programHandle.

How to do it...

After linking and enabling the shader program, use the following code to display the list of active uniforms:

1. Start by querying for the number of active uniform variables:

    GLint numUniforms = 0; glGetProgramInterfaceiv( handle, GL_UNIFORM, GL_ACTIVE_RESOURCES, & numUniforms);

2. Loop through each uniform index and query for the length of the name, the type, the location, and the block index:

    GLenum properties[] = {GL_NAME_LENGTH, GL_TYPE, GL_LOCATION, GL_BLOCK_INDEX}; std::cout << "Active uniforms" << std::endl; for( int i = 0; i < numUniforms; ++i ) { GLint results[4]; glGetProgramResourceiv(handle, GL_UNIFORM, i, 4, properties, 4, NULL, results); if (results[3] != -1) continue; GLint nameBufSize = results[0] + 1; char * name = new char[nameBufSize]; glGetProgramResourceName(handle, GL_UNIFORM, i, nameBufSize, NULL, name); printf("%-5d %s (%s)n", results[2], name, getTypeString(results[1])); delete [] name;
    }

How it works...

The first and most obvious is that we use GL_UNIFORM instead of GL_PROGRAM_INPUT as the interface that we are querying in glGetProgramResourceiv and glGetProgramInterfaceiv. Second, we query for the block index (using GL_BLOCK_INDEX in the properties array). The reason for this is that some uniform variables are contained within a uniform block. For this example, we only want information about uniforms that are not within blocks. The block index will be -1 if the uniform variable is not within a block, so we skip any uniform variables that do not have a block index of -1.

Again, we use the getTypeString function to convert the type value into a human-readable string.

When this is run on the shader program from the previous recipe, we see the following output:

Active uniforms:
0 RotationMatrix (mat4)

There's more...

As with vertex attributes, a uniform variable is not considered active unless it is determined by the GLSL linker that it will be used within the shader.

The previous code is only valid for OpenGL 4.3 and later. Alternatively, you can achieve similar results using the glGetProgramiv, glGetActiveUniform, glGetUniformLocation, and glGetActiveUniformName functions.

Using uniform blocks and uniform buffer objects

If your program involves multiple shader programs that use the same uniform variables, one has to manage the variables separately for each program. Uniform locations are generated when a program is linked, so the locations of the uniforms may change from one program to the next. The data for those uniforms may have to be regenerated and applied to the new locations.

Uniform blocks were designed to ease the sharing of uniform data between programs. With uniform blocks, one can create a buffer object for storing the values of all the uniform variables, and bind the buffer to the uniform block. When changing programs, the same buffer object need only be rebound to the corresponding block in the new program.

A uniform block is simply a group of uniform variables defined within a syntactical structure known as a uniform block. For example, in this recipe, we'll use the following uniform block:

uniform BlobSettings { vec4 InnerColor; vec4 OuterColor; float RadiusInner; float RadiusOuter;
};

This defines a block with the name BlobSettings that contains four uniform variables.
With this type of block definition, the variables within the block are still part of the global scope and do not need to be qualified with the block name.

The buffer object used to store the data for the uniforms is often referred to as a uniform buffer object. We'll see that a uniform buffer object is simply a buffer object that is bound to a certain location.

For this recipe, we'll use a simple example to demonstrate the use of uniform buffer objects and uniform blocks. We'll draw a quad (two triangles) with texture coordinates, and use our fragment shader to fill the quad with a fuzzy circle. The circle is a solid color in the center, but at its edge, it gradually fades to the background color, as shown in the following image:

Getting ready

Start with an OpenGL program that draws two triangles to form a quad. Provide the position at vertex attribute location 0, and the texture coordinate (0 to 1 in each direction) at vertex attribute location 1.

We'll use the following vertex shader:

#version 430  layout (location = 0) in vec3 VertexPosition; layout (location = 1) in vec3 VertexTexCoord; out vec3 TexCoord; void main() { TexCoord = VertexTexCoord; gl_Position = vec4(VertexPosition, 1.0);
}

The fragment shader contains the uniform block, and is responsible for drawing our fuzzy circle:

#version 430  in vec3 TexCoord; layout (location = 0) out vec4 FragColor; layout (binding = 0) uniform BlobSettings { vec4 InnerColor; vec4 OuterColor; float RadiusInner; float RadiusOuter;
}; void main() { float dx = TexCoord.x - 0.5; float dy = TexCoord.y - 0.5; float dist = sqrt(dx * dx + dy * dy); FragColor = mix(InnerColor, OuterColor, smoothstep(RadiusInner, RadiusOuter, dist));
}

Note the uniform block named BlobSettings. The variables within this block define the parameters of our fuzzy circle. The OuterColor variable defines the color outside of the circle. InnerColor is the color inside of the circle. RadiusInner is the radius that defines the part of the circle that is a solid color (inside the fuzzy edge), and the distance from the center of the circle to the inner edge of the fuzzy boundary. RadiusOuter is the outer edge of the fuzzy boundary of the circle (when the color is equal to OuterColor).

The code within the main function computes the distance of the texture coordinate to the center of the quad located at (0.5, 0.5). It then uses that distance to compute the color by using the smoothstep function. This function provides a value that smoothly varies between 0.0 and 1.0 when the value of the third argument is between the values of the first two arguments. Otherwise, it returns 0.0 or 1.0, depending on whether dist is less than the first or greater than the second, respectively. The mix function is then used to linearly interpolate between InnerColor and OuterColor based on the value returned by the smoothstep function.

How to do it...

In the OpenGL program, after linking the shader program, use the following steps to assign data to the uniform block in the fragment shader:

1. Get the index of the uniform block using glGetUniformBlockIndex:

    GLuint blockIndex = glGetUniformBlockIndex(programHandle, "BlobSettings");

2. Allocate space for the buffer to contain the data for the uniform block. We get the size using glGetActiveUniformBlockiv:

    GLint blockSize;
    glGetActiveUniformBlockiv(programHandle, blockIndex, GL_UNIFORM_BLOCK_DATA_SIZE, &blockSize); GLubyte *blockBuffer;
    blockBuffer = (GLubyte *) malloc(blockSize);

3. Query for the offset of each variable within the block. To do so, we first find the index of each variable within the block:

    const GLchar *names[] = { "InnerColor", "OuterColor", "RadiusInner", "RadiusOuter" }; GLuint indices[4];
    glGetUniformIndices(programHandle, 4, names, indices); GLint offset[4];
    glGetActiveUniformsiv(programHandle, 4, indices, GL_UNIFORM_OFFSET, offset);

4. Place the data into the buffer at the appropriate offsets:

    GLfloat outerColor[] = {0.0f, 0.0f, 0.0f, 0.0f};
    GLfloat innerColor[] = {1.0f, 1.0f, 0.75f, 1.0f}; GLfloat innerRadius = 0.25f, outerRadius = 0.45f; memcpy(blockBuffer + offset[0], innerColor, 4 * sizeof(GLfloat)); memcpy(blockBuffer + offset[1], outerColor, 4 * sizeof(GLfloat)); memcpy(blockBuffer + offset[2], &innerRadius, sizeof(GLfloat)); memcpy(blockBuffer + offset[3], &outerRadius, sizeof(GLfloat));

5. Create the buffer object and copy the data into it:

    GLuint uboHandle;
    glGenBuffers(1, &uboHandle);
    glBindBuffer(GL_UNIFORM_BUFFER, uboHandle);
    glBufferData(GL_UNIFORM_BUFFER, blockSize, blockBuffer, GL_DYNAMIC_DRAW);

6. Bind the buffer object to the uniform buffer-binding point at the index specified by the binding-layout qualifier in the fragment shader (0):

    glBindBufferBase(GL_UNIFORM_BUFFER, 0, uboHandle);

How it works...

Phew! This seems like a lot of work! However, the real advantage comes when using multiple programs where the same buffer object can be used for each program. Let's take a look at each step individually.

First, we get the index of the uniform block by calling glGetUniformBlockIndex, then we query for the size of the block by calling glGetActiveUniformBlockiv. After getting the size, we allocate a temporary buffer named blockBuffer to hold the data for our block.

The layout of data within a uniform block is implementation-dependent, and implementations may use different padding and/or byte alignment. So in order to accurately lay out our data, we need to query for the offset of each variable within the block. This is done in two steps. First, we query for the index of each variable within the block by calling glGetUniformIndices. This accepts an array of the names variable (third argument) and returns the indices of the variables in the indices array (fourth argument).

Then, we use the indices to query for the offsets by calling glGetActiveUniformsiv. When the fourth argument is GL_UNIFORM_OFFSET, this returns the offset of each variable in the array pointed to by the fifth argument. This function can also be used to query for the size and type, however, in this case we choose not to do so in order to keep the code simple (albeit less general).

The next step involves filling our temporary buffer, blockBuffer, with the data for the uniforms at the appropriate offsets. Here, we use the standard library function, memcpy, to accomplish this.

Now that the temporary buffer is populated with the data with the appropriate layout, we can create our buffer object and copy the data into the buffer object. We call glGenBuffers to generate a buffer handle, and then bind that buffer to the GL_UNIFORM_BUFFER binding point by calling glBindBuffer. The space is allocated within the buffer object and the data is copied when glBufferData is called. We use GL_DYNAMIC_DRAW as the usage hint here because uniform data may be changed somewhat often during rendering. Of course, this is entirely dependent on the situation.

Finally, we associate the buffer object with the uniform block by calling glBindBufferBase. This function binds to an index within a buffer binding point. Certain binding points are also called indexed buffer targets. This means that the target is actually an array of targets, and glBindBufferBase allows us to bind to one index within the array. In this case, we bind it to the index that we specified in the layout qualifier in the fragment shader: layout (binding = 0). These two indices must match.

You might be wondering why we use glBindBuffer and glBindBufferBase with GL_UNIFORM_BUFFER. Aren't these the same binding points used in two different contexts? The answer is that the GL_UNIFORM_BUFFER point can be used in each function with a slightly different meaning. With glBindBuffer, we bind to a point that can be used for filling or modifying a buffer, but can't be used as a source of data for the shader. When we use glBindBufferBase, we are binding to an index within a location that can be directly sourced by the shader. Granted, that's a bit confusing.

There's more...

If the data for a uniform block needs to be changed at some later time, one can call glBufferSubData to replace all or part of the data within the buffer. If you do so, don't forget to first bind the buffer to the generic binding point, GL_UNIFORM_BUFFER.

Using an instance name with a uniform block

A uniform block can have an optional instance name. For example, with our BlobSettings block, we could have used the instance name Blob, as shown here:

uniform BlobSettings { vec4 InnerColor; vec4 OuterColor; float RadiusInner; float RadiusOuter;
} Blob;

In this case, the variables within the block are placed within a namespace qualified by the instance name. Therefore, our shader code needs to refer to them prefixed with the instance name. For example:

FragColor = mix(Blob.InnerColor, Blob.OuterColor, smoothstep(Blob.RadiusInner, Blob.RadiusOuter, dist)
);

Additionally, we need to qualify the variable names (with the BlobSettings block name) within the OpenGL code when querying for variable indices:

const GLchar *names[] = { "BlobSettings.InnerColor", "BlobSettings.OuterColor", "BlobSettings.RadiusInner", "BlobSettings.RadiusOuter" }; GLuint indices[4];
glGetUniformIndices(programHandle, 4, names, indices);

Using layout qualifiers with uniform blocks

Since the layout of the data within a uniform buffer object is implementation-dependent, it required us to query for the variable offsets. However, one can avoid this by asking OpenGL to use the standard layout, std140. This is accomplished by using a layout qualifier when declaring the uniform block. For example:

layout( std140 ) uniform BlobSettings {
};

The std140 layout is described in detail within the OpenGL specification document (available at http://www.opengl.org).

Other options for the layout qualifier that apply to uniform block layouts include packed and shared. The packed qualifier simply states that the implementation is free to optimize memory in whatever way it finds necessary (based on variable usage or other criteria). With the packed qualifier, we still need to query for the offsets of each variable. The shared qualifier guarantees that the layout will be consistent between multiple programs and program stages provided that the uniform block declaration does not change. If you are planning to use the same buffer object between multiple programs and/or program stages, it is a good idea to use the shared option.

There are two other layout qualifiers that are worth mentioning: row_major and column_major. These define the ordering of data within the matrix type variables within the uniform block.

One can use multiple (non-conflicting) qualifiers for a block. For example, to define a block with both the row_major and shared qualifiers, we would use the following syntax:

layout (row_major, ) uniform BlobSettings { };

Using program pipelines

Program pipeline objects were introduced as part of the separable shader objects extension, and moved into core OpenGL with version 4.1. They allow programmers to mix and match shader stages from multiple separable shader programs. To understand how this works and why it may be useful, let's go through a hypothetical example.

Suppose we have one vertex shader and two fragment shaders. Suppose that the code in the vertex shader will function correctly with both fragment shaders. I could simply create two different shader programs, reusing the OpenGL shader object containing the vertex shader. However, if the vertex shader has a lot of uniform variables, then every time that I switch between the two shader programs, I would (potentially) need to reset some or all of those uniform variables. This is because the uniform variables are part of the shader program's state, so changes to the uniforms in one program would not carry over to the other program, even when the two share a shader object. The uniform information is stored in the shader program object, not the shader object.

Shader program objects include the values of active uniform variables; this information is not stored within a shader object.

With separable shader objects, we can create shader programs that include one or more shader stages. Prior to this extension, we were required to include at least a vertex and fragment shader. Such programs are called separable because they are not necessarily linked to specific other stages. They can be separated and linked to different stages at different times. Separable programs can contain just a single stage (or more if desired).

With program pipelines, we can create pipelines that mix and match the stages of separable programs. This enables us to avoid losing the state of uniform variables in a given shader stage when switching other stages. For example, in the preceding scenario, where we have one vertex shader (shader A) and two fragment shaders (B and C), we could create three shader programs, each containing a single stage. Then, we could create two pipelines. The first pipeline would use the vertex shader from program A and the fragment shader from program B, and the second would use the vertex shader from program A and the fragment shader from program C. We could switch between the two pipelines without losing any of the uniform variable state in shader stage A, because we haven't actually switched shader programs—we're using the same shader program (containing stage A) in both pipelines.

Getting ready

For this recipe, we'll follow through with the earlier example. We'll need a single vertex shader and two compatible fragment shaders. Let's assume that the file names are separable.vert.glsl, separable1.frag.glsl, and separable2.frag.glsl.

Separable shaders require you to re-declare the built-in gl_PerVertex output block if you use any of its members. Since you will nearly always use one of its members(gl_Position), you'll very likely need to add the following to your vertex shader:

out gl_PerVertex { vec4 gl_Position; float gl_PointSize; float gl_ClipDistance[];
};

How to do it...

Start by loading the shader files into std::string:

std::string vertCode = loadShaderCode("separable.vert.glsl"); std::string fragCode1 = loadShaderCode("separable1.frag.glsl"); std::string fragCode2 = loadShaderCode("separable2.frag.glsl");

Next, we'll create one shader program for each using glCreateShaderProgramv:

GLuint programs[3]; const GLchar *codePtrs = { vertCode.c_str(), fragCode1.c_str(), fragCode2.c_str() }; programs[0] = glCreateShaderProgramv(GL_VERTEX_SHADER, 1, codePtrs); programs[1] = glCreateShaderProgramv(GL_FRAGMENT_SHADER, 1, codePtrs + 1); programs[2] = glCreateShaderProgramv(GL_FRAGMENT_SHADER, 1, codePtrs + 2);

Now, we'll create two program pipelines. The first will use the vertex shader and the first fragment shader, and the second will use the vertex shader and the second fragment shader:

GLuint pipelines[2];
glCreateProgramPipelines(2, pipelines); glUseProgramStages(pipelines[0], GL_VERTEX_SHADER_BIT, programs[0]);
glUseProgramStages(pipelines[0], GL_FRAGMENT_SHADER_BIT, programs[1]); glUseProgramStages(pipelines[1], GL_VERTEX_SHADER_BIT, programs[0]);
glUseProgramStages(pipelines[1], GL_FRAGMENT_SHADER_BIT, programs[2]);

To set uniform variables in separable shaders, the recommended technique is to use glProgramUniform rather than glUniform. With separable shaders and program pipelines, it can be a bit tedious and tricky to determine which shader stage is affected by the glUniform functions. The glProgramUniform functions allow us to specify the target program directly. For example, here, we'll set a uniform in the vertex shader program(shared by the two pipelines):

GLint location = glGetUniformLocation(programs[0], uniformName);
glProgramUniform3f(programs[0], location, 0, 1, 0);

To render, we first need to make sure that no programs are currently bound. If there is a program bound via glUseProgram, it will ignore any program pipelines:

glUseProgram(0);

Now, we can use the pipelines that we set up earlier:

glBindProgramPipeline(pipelines[0]); glBindProgramPipeline(pipelines[1]);

How it works...

The glCreateShaderProgramv function provides a simple way to create a separable shader program consisting of a single stage. We pass the shader stage and the code string to function which creates a shader object, compiles it, creates a separable program, attaches the shader, and links the program, and returns the name of the new shader program. We should check for errors immediately after each call. All errors will be in the program info log.

Once we have the shader programs, we create the pipelines. We create two pipeline objects using glCreateProgramPipelines. Then, we set the stages for each pipeline using glUseProgramStages. The first argument to glUseProgramStages is the pipeline name, the second is a bit string indicating the stages that are to be used from the program, and the last argument is the program name. The bit string for the second argument can be composed of some combination of GL_VERTEX_SHADER_BIT, GL_FRAGMENT_SHADER_BIT, and so on. Use the bitwise OR operator (|) to combine bits.

As mentioned earlier, when using program pipelines, it is a good idea to use glProgramUniform rather than glUniform to set uniform variables. It can be difficult to determine the program that is being affected when using glUniform due to the fact that a pipeline can and usually does involve multiple programs. There is a function called glActiveShaderProgram that can be used to specify the program affected by glUniform calls, or you can simply use glUseProgram. However, there's no need to bother with any of that, because glProgramUniform makes it clear and simple. With glProgramUniform, we specify the target program directly as the first argument.

Before rendering with pipelines, it is important to make sure that there is no program that is currently bound to the GL context via glUseProgram. If there is, it will be used instead of the pipeline. Therefore, you might want to call glUseProgram(0) before rendering, just to be sure.

Finally, we use glBindProgramPipeline to enable one of our pipelines before rendering. In this example, the first draw will use the vertex shader and the first fragment shader. The second draw will use the vertex shader and the second fragment shader.

There's more...

In the preceding example, we used glCreateShaderProgramv to create each single-stage program. However, you can also use the more familiar glCreateProgram to do the same thing. In fact, if you want to create a program with more than one stage (say, a vertex shader and a geometry shader), you need to use glCreateProgram. However, since we want to use it with shader pipelines, it is important to use glProgramParameteri to designate it as a separable program. Here's an example of creating a single stage program using glCreateProgram, assuming that vertShader is the name of a previously-compiled vertex shader object:

GLuint program = glCreateProgram();
glProgramParameteri(program, GL_PROGRAM_SEPARABLE, GL_TRUE);
glAttachShader(program, vertShader);
glLinkProgram(program);

You could attach more than one shader before linking.

Program pipelines make it easy to mix and match shader stages, while maintaining uniform state. However, the added complexity may not be worth it for many situations. If your shaders are complex, with lots of uniform states, and you need to switch portions of the pipeline often, it might be a good alternative.

Getting debug messages

Prior to recent versions of OpenGL, the traditional way to get debug information was to call glGetError. Unfortunately, that is an exceedingly tedious method to debug a program. The glGetError function returns an error code if an error has occurred at some point before the function was called.

This means that if we're chasing down a bug, we essentially need to call glGetError after every function call to an OpenGL function, or do a binary search-like process where we call it before and after a block of code, and then move the two calls closer to each other until we determine the source of the error. What a pain!

Thankfully, as of OpenGL 4.3, we now have support for a more modern method for debugging. Now, we can register a debug callback function that will be executed whenever an error occurs, or other informational message is generated. Not only that, but we can send our own custom messages to be handled by the same callback, and we can filter the messages using a variety of criteria.

Getting ready

Create an OpenGL program with a debug context. While it is not strictly necessary to acquire a debug context, we might not get messages that are as informative as when we are using a debug context. To create an OpenGL context using GLFW with debugging enabled, use the following function call prior to creating the window:

glfwWindowHint(GLFW_OPENGL_DEBUG_CONTEXT, GL_TRUE);

An OpenGL debug context will have debug messages enabled by default. If, however, you need to enable debug messages explicitly, use the following call:

glEnable(GL_DEBUG_OUTPUT);

How to do it...

Use the following steps:

1. Create a callback function to receive the debug messages. The function must conform to a specific prototype described in the OpenGL documentation. For this example, we'll use the following one:

    void debugCallback(GLenum source, GLenum type, GLuint id, GLenum severity, GLsizei length, const GLchar * message, const void * param) { printf("%s:%s[%s](%d): %sn", sourceStr, typeStr, severityStr, id, message);
    }

2. Register our callback with OpenGL using glDebugMessageCallback:

    glDebugMessageCallback( debugCallback, nullptr );

3. Enable all messages, all sources, all levels, and all IDs:

    glDebugMessageControl(GL_DONT_CARE, GL_DONT_CARE, GL_DONT_CARE, 0, NULL, GL_TRUE);

How it works...

The debugCallback callback function has several parameters, the most important of which is the debug message itself (the sixth parameter, message). For this example, we simply print the message to the standard output, but we could send it to a log file or some other destination.

The first four parameters to debugCallback describe the source, type, ID number, and severity of the message. The ID number is an unsigned integer specific to the message. The possible values for the source, type, and severity parameters are described in the following tables.

The source parameter can have any of the following values:

Source

Generated by

GL_DEBUG_SOURCE_API

Calls to the OpenGL API

GL_DEBUG_SOURCE_WINDOW_SYSTEM

Calls to a Windows system API

GL_DEBUG_SOURCE_THIRD_PARTY

An application associated with OpenGL

GL_DEBUG_SOURCE_APPLICATION

The application itself

GL_DEBUG_SOURCE_OTHER

Some other source

The type parameter can have any of the following values:

Type

Description

GL_DEBUG_TYPE_ERROR

An error from the OpenGL API

GL_DEBUG_TYPE_DEPRECATED_BEHAVIOR

Behavior that has been deprecated

GL_DEBUG_TYPE_UNDEFINED_BEHAVIOR

Undefined behavior

GL_DEBUG_TYPE_PORTABILITY

Some functionality is not portable

GL_DEBUG_TYPE_PERFORMANCE

Possible performance issues

GL_DEBUG_TYPE_MARKER

An annotation

GL_DEBUG_TYPE_PUSH_GROUP

Messages related to debug group push

GL_DEBUG_TYPE_POP_GROUP

Messages related to debug group pop

GL_DEBUG_TYPE_OTHER

Other messages

The severity parameter can have the following values:

Severity

Meaning

GL_DEBUG_SEVERITY_HIGH

Errors or dangerous behavior

GL_DEBUG_SEVERITY_MEDIUM

Major performance warnings, other warnings, or use of deprecated functionality

GL_DEBUG_SEVERITY_LOW

Redundant state changes, unimportant undefined behavior

GL_DEBUG_SEVERITY_NOTIFICATION

A notification, not an error, or performance issue

The length parameter is the length of the message string, excluding the null terminator. The last parameter, param, is a user-defined pointer. We can use this to point to some custom objects that might be helpful to the callback function. This parameter can be set using the second parameter to glDebugMessageCallback.

Within debugCallback, we convert each GLenum parameter into a string. Due to space constraints, I don't show all of that code here, but it can be found in the example code for this book. We then print all of the information to the standard output.

The call to glDebugMessageCallback registers our callback function with the OpenGL debug system. The first parameter is a pointer to our callback function, and the second parameter (nullptr, in this example) can be a pointer to any object that we would like to pass into the callback. This pointer is passed as the last parameter with every call to debugCallback.

Finally, the call to glDebugMessageControl determines our message filters. This function can be used to selectively turn on or off any combination of message source, type, ID, or severity. In this example, we turn everything on.

There's more...

OpenGL also provides support for stacks of named debug groups. This means that we can remember all of our debug message filter settings on a stack and return to them after some changes have been made. This might be useful, for example, if there are sections of code where we need to filter some kinds of messages and other sections where we want a different set of messages.

The functions involved are glPushDebugGroup and glPopDebugGroup. A call to glPushDebugGroup generates a debug message with the GL_DEBUG_TYPE_PUSH_GROUP type, and retains the current state of our debug filters on a stack. We can then change our filters using glDebugMessageControl, and later return to the original state using glPopDebugGroup. Similarly, the glPopDebugGroup function generates a debug message with the GL_DEBUG_TYPE_POP_GROUP type.

Building a C++ shader program class

If you are using C++, it can be very convenient to create classes to encapsulate some of the OpenGL objects. A prime example is the shader program object. In this recipe, we'll look at a design for a C++ class that can be used to manage a shader program.

Getting ready

There's not much to prepare for with this one; you just need a build environment that supports C++. Also, I'll assume that you are using GLM for matrix and vector support; if not, just leave out the functions involving the GLM classes.

How to do it...

First, we'll use a custom exception class for errors that might occur during compilation or linking:

class GLSLProgramException : public std::runtime_error { public: GLSLProgramException( const string & msg ) : std::runtime_error(msg) { }
};

We'll use enum for the various shader types:

namespace GLSLShader { enum GLSLShaderType { VERTEX = GL_VERTEX_SHADER, FRAGMENT = GL_FRAGMENT_SHADER, GEOMETRY = GL_GEOMETRY_SHADER, TESS_CONTROL = GL_TESS_CONTROL_SHADER, TESS_EVALUATION = GL_TESS_EVALUATION_SHADER, COMPUTE = GL_COMPUTE_SHADER };
};

The program class itself has the following interface:

class GLSLProgram { private: int handle; bool linked; std::map<string, int> uniformLocations; GLint getUniformLocation(const char *); public: GLSLProgram(); ~GLSLProgram(); GLSLProgram(const GLSLProgram &) = delete; GLSLProgram & operator=(const GLSLProgram &) = delete; void compileShader(const char * filename); void compileShader(const char * filename, GLSLShader::GLSLShaderType type ); void compileShader(const string & source, GLSLShader::GLSLShaderType type, const char * filename = nullptr); void link(); void use(); void validate(); int getHandle(); bool isLinked(); void bindAttribLocation(GLuint location, const char * name); void bindFragDataLocation(GLuint location, const char * name); void setUniform(const char *name, float x, float y, float z); void setUniform(const char *name, const glm::vec3 & v); void setUniform(const char *name, const glm::vec4 & v); void setUniform(const char *name, const glm::mat4 & m); void setUniform(const char *name, const glm::mat3 & m); void setUniform(const char *name, float val); void setUniform(const char *name, int val); void setUniform(const char *name, bool val); void findUniformLocations(); };

The techniques involved in the implementation of these functions are covered in previous recipes in this chapter. Due to space limitations, I won't include the code here, but we'll discuss some of the design decisions in the next section.

How it works...

The state stored within a GLSLProgram object includes the handle to the OpenGL shader program object (handle), a bool variable indicating whether or not the program has been successfully linked (linked), and map, which is used to store uniform locations as they are discovered (uniformLocations).

The compileShader overloads will throw GLSLProgramException if the compilation fails. The first version determines the type of shader based on the filename extension. In the second version, the caller provides the shader type, and the third version is used to compile a shader, taking the shader's source code from a string. The filename can be provided as a third argument if the string was taken from a file, which is helpful for providing better error messages.

The GLSLProgramException error message will contain the contents of the shader log or program log when an error occurs. The getUniformLocation private function is used by the setUniform functions to find the location of a uniform variable. It checks the uniformLocations map first, and if the location is not found, queries OpenGL for the location, and stores the result in the map before returning. The fileExists function is used by compileShaderFromFile to check for file existence.

The constructor simply initializes linked to false and handle to 0. The handle variable will be initialized by calling glCreateProgram when the first shader is compiled.

The link function simply attempts to link the program by calling glLinkProgram. It then checks the link status, and if successful, sets the linked variable to true and returns true. Otherwise, it gets the program log (by calling glGetProgramInfoLog), stores the result in GLSLProgramException, and throws it. If the link is successful, it calls findUniformLocations, which gets a list of all active uniform variables and stores their locations in the map named uniformLocations, keyed by their names. Regardless of whether the link is successful, it detaches and deletes all shader objects before returning or throwing an exception. After all of this, it detaches and deletes the shader objects, because they are no longer needed.

The use function simply calls glUseProgram if the program has already been successfully linked, otherwise it does nothing.

The getHandle and isLinked functions are simply getter functions that return handle to the OpenGL program object and the value of the linked variable.

The bindAttribLocation and bindFragDataLocation functions are wrappers around glBindAttribLocation and glBindFragDataLocation. Note that these functions should only be called prior to linking the program.

The setUniform overloaded functions are straightforward wrappers around the appropriate glUniform functions. As mentioned previously, the uniform locations are queried and stored when the program is linked, so each setUniform function checks the map to get the cached uniform location.

The destructor takes care of deleting the program object.

Finally, the printActiveUniforms, printActiveUniformBlocks, and printActiveAttribs functions are useful for debugging purposes. They simply display a list of the active uniforms/attributes to the standard output.

The following is a simple example of the use of the GLSLProgram class:

GLSLProgram prog; try { prog.compileShader("myshader.vert.glsl"); prog.compileShader("myshader.frag.glsl"); prog.link(); prog.validate(); prog.use();
} catch( GLSLProgramException &e ) { cerr << e.what() << endl; exit(EXIT_FAILURE);
} prog.setUniform("ModelViewMatrix", matrix); prog.setUniform("LightPosition", 1.0f, 1.0f, 1.0f);

The Basics of GLSL Shaders

In this chapter, we will cover the following recipes:

• Diffuse and per-vertex shading with a single point light source
• Implementing the Phong reflection model
• Using functions in shaders
• Implementing two-sided shading
• Implementing flat shading
• Using subroutines to select shader functionality
• Discarding fragments to create a perforated look

Introduction

Shaders were first added into OpenGL in version 2.0, introducing programmability into the formerly fixed-function OpenGL pipeline. Shaders give us the power to implement custom rendering algorithms and provide us with a greater degree of flexibility in the implementation of those techniques. With shaders, we can run custom code directly on the GPU, providing us with the opportunity to leverage the high degree of parallelism available with modern GPUs.

Shaders are implemented using the OpenGL Shading Language (GLSL). GLSL is syntactically similar to C, which should make it easier for experienced OpenGL programmers to learn. Due to the nature of this text, I won't present a thorough introduction to GLSL here. Instead, if you're new to GLSL, reading through these recipes should help you to learn the language through example. If you are already comfortable with GLSL, but don't have experience with version 4.x, you'll see how to implement these techniques by utilizing the newer API. However, before we jump into GLSL programming, let's take a quick look at how vertex and fragment shaders fit within the OpenGL pipeline.

Vertex and fragment shaders

In OpenGL Version 4.3 and above, there are six shader stages/types: vertex, geometry, tessellation control, tessellation evaluation, fragment, and compute. In this chapter, we'll focus only on the vertex and fragment stages.

Shaders are fundamental parts of the modern OpenGL pipeline. The following block diagram shows a simplified view of the OpenGL pipeline with only the vertex and fragment shaders installed:

Vertex data is sent down the pipeline and arrives at the vertex shader via shader input variables. The vertex shader's input variables correspond to the vertex attributes. In general, a shader receives its input via programmer-defined input variables, and the data for those variables comes either from the main OpenGL application or previous pipeline stages (other shaders). For example, a fragment shader's input variables might be fed from the output variables of the vertex shader. Data can also be provided to any shader stage using uniform variables. These are used for information that changes less often than vertex attributes (for example, matrices, light position, and other settings). The following diagram shows a simplified view of the relationships between input and output variables when there are two shaders active (vertex and fragment):

The vertex shader is executed once for each vertex, in parallel. The data corresponding to the position of the vertex must be transformed into clip space coordinates and assigned to the output variable gl_Position before the vertex shader finishes execution. The vertex shader can send other information down the pipeline using shader output variables. For example, the vertex shader might also compute the color associated with the vertex. That color would be passed to later stages via an appropriate output variable.

Between the vertex and fragment shader, vertices are assembled into primitives, clipping takes place, and the viewport transformation is applied (among other operations). The rasterization process then takes place and the polygon is filled (if necessary). The fragment shader is executed once for each fragment of the polygon being rendered (typically in parallel). Data provided from the vertex shader is (by default) interpolated in a perspective correct manner, and provided to the fragment shader via shader input variables. The fragment shader determines the appropriate color for the pixel and sends it to the frame buffer using output variables. The depth information is handled automatically, but can be modified by the fragment shader if desired.

Learning the basics first

Programmable shaders give us tremendous power and flexibility. A good place to start is to learn how to implement a simple, common reflection model known as the Phong reflection model. It is a good basis for building upon.

In this chapter, we'll look at the basic techniques for implementing the Phong model. We'll modify it in a few simple ways, including two-sided rendering and flat shading. Along the way, we'll also see some examples of other GLSL features such as functions, subroutines, and the discard keyword.

Diffuse and per-vertex shading with a single point light source

Before learning the full Phong reflection model, we'll start with just one part: diffuse reflection. It is a simple reflection model that makes the assumption that the surface exhibits purely diffuse reflection. That is to say that the surface appears to scatter light in all directions equally, regardless of direction.

Incoming light strikes the surface and penetrates slightly before being reradiated in all directions. Of course, the incoming light interacts with the surface before it is scattered, causing some wavelengths to be fully or partially absorbed and others to be scattered. A typical example of a diffuse surface is a surface that has been painted with a matte paint. The surface has a dull look with no shine at all.

The following image shows a torus rendered with diffuse shading:

The mathematical model for diffuse reflection involves two vectors: the direction from the surface point to the light source (s\begin{aligned}s \end{aligned}s​), and the normal vector at the surface point (nnn). The vectors are represented in the following diagram:

The amount of incoming light (or radiance) per unit area that strikes a surface is dependent on the orientation of the surface with respect to the light source. The physics of the situation tells us that the amount of radiation per unit area is maximal when the light arrives along the direction of the normal vector, and zero when the light is perpendicular to the normal vector. In between, it is proportional to the cosine of the angle between the direction towards the light source and the normal vector. So, since the dot product is proportional to the cosine of the angle between two vectors, we can express the amount of radiation striking the surface as the product of the light intensity and the dot product of sss and nnn, as follows:

Ld(s⋅n)L_d(s\cdot n)Ld​(s⋅n)

LdL_dLd​ is the intensity of the light source, and the vectors sss and nnn are assumed to be normalized.

The dot product of two unit vectors is equal to the cosine of the angle between them.

As stated previously, some of the incoming light is absorbed before it is reemitted. We can model this interaction by using a reflection coefficient (KdK_dKd​), which represents the fraction of the incoming light that is scattered. This is sometimes called the diffuse reflectivity, or the diffuse reflection coefficient. The diffuse reflectivity becomes a scaling factor, so the intensity of the outgoing light can be expressed as follows:

L=KdLd(s⋅n)L=K_dL_d(s\cdot n)L=Kd​Ld​(s⋅n)

Because this model depends only on the direction towards the light source and the normal to the surface, not on the direction towards the viewer, we have a model that represents uniform (omnidirectional) scattering.

In this recipe, we'll evaluate this equation at each vertex in the vertex shader and interpolate the resulting color across the face. We'll use uniform variables for the KdK_dKd​ and LdL_dLd​ terms as well as the light position.

In this and the following recipes, light intensities and material reflectivity coefficients are represented by three-component (RGB) vectors. Therefore, the equations should be treated as component-wise operations, applied to each of the three components separately. Luckily, the GLSL will make this nearly transparent because the necessary operators operate component-wise on vector variables.

Getting ready

Start with an OpenGL application that provides the vertex position in attribute location 0, and the vertex normal in attribute location 1. The OpenGL application should also provide the standard transformation matrices (projection, modelview, and normal) via uniform variables.

The light position (in camera coordinates), Kd, and Ld should also be provided by the OpenGL application via uniform variables. Note that Kd and Ld are of type vec3. We can use vec3 to store an RGB color as well as a vector or point.

How to do it...

To create a shader pair that implements diffuse shading, take the following steps:

1. The vertex shader computes the diffuse reflection equation and sends the result to the fragment shader via the output variable LightIntensity:

    layout (location = 0) in vec3 VertexPosition; layout (location = 1) in vec3 VertexNormal; out vec3 LightIntensity; uniform vec4 LightPosition;
    uniform vec3 Kd; uniform vec3 Ld; uniform mat4 ModelViewMatrix; uniform mat3 NormalMatrix; uniform mat4 ProjectionMatrix; uniform mat4 MVP; void main() { vec3 tnorm = normalize( NormalMatrix * VertexNormal); vec4 camCoords = ModelViewMatrix * vec4(VertexPosition,1.0); vec3 s = normalize(vec3(LightPosition - camCoords)); LightIntensity = Ld * Kd * max(dot(s, tnorm), 0.0); gl_Position = MVP * vec4(VertexPosition, 1.0);
    }

2. The fragment shader simply applies the color to the fragment:

    in vec3 LightIntensity; layout( location = 0 ) out vec4 FragColor; void main() { FragColor = vec4(LightIntensity, 1.0);
    }

3. Compile and link both shaders within the OpenGL application and install the shader program prior to rendering.

How it works...

The vertex shader does all of the work in this example. The diffuse reflection is computed in camera coordinates by first transforming the normal vector using the normal matrix, normalizing, and storing the result in tnorm. Note that the normalization here may not be necessary if your normal vectors are already normalized and the normal matrix does not do any scaling.

The normal matrix is the inverse transpose of the upper-left 3x3 portion of the model-view matrix. We use the inverse transpose because normal vectors transform differently than the vertex position. For a more thorough discussion of the normal matrix, and the reasons why, see any introductory computer graphics textbook. If your model-view matrix does not include any nonuniform scaling, then one can use the upper-left 3x3 of the model-view matrix in place of the normal matrix to transform your normal vectors. However, if your model-view matrix does include (uniform) scaling, you'll still need to (re)normalize your normal vectors after transforming them.

The next step converts the vertex position to camera coordinates by transforming it with the model-view matrix. Then, we compute the direction toward the light source by subtracting the vertex position from the light position and storing the result in s.

Next, we compute the scattered light intensity using the equation described previously and store the result in the output variable LightIntensity. Note the use of the max function here. If the dot product is less than zero, then the angle between the normal vector and the light direction is greater than 90 degrees. This means that the incoming light is coming from inside the surface. Since such a situation would mean that no radiation reaches the surface, and the dot product would produce negative values, we use a value of 0.0. However, you may decide that you want to properly light both sides of your surface, in which case the normal vector needs to be reversed for those situations where the light is striking the back side of the surface.

Finally, we convert the vertex position to clip space coordinates by multiplying it with the model-view projection matrix, (which is projection * view * model) and store the result in the built-in output variable gl_Position.

The subsequent stage of the OpenGL pipeline expects that the vertex position will be provided in clip space coordinates in the output variable gl_Position. This variable does not directly correspond to any input variable in the fragment shader, but is used by the OpenGL pipeline in the primitive assembly, clipping, and rasterization stages that follow the vertex shader. It is important that we always provide a valid value for this variable.

Since LightIntensity is an output variable from the vertex shader, its value is interpolated across the face and passed into the fragment shader. The fragment shader then simply assigns the value to the output fragment.

There's more...

Diffuse shading is a technique that models only a very limited range of surfaces. It is best used for surfaces that have a matte appearance. Additionally, with the technique used previously, the dark areas may look a bit too dark. In fact, those areas that are not directly illuminated are completely black. In real scenes, there is typically some light that has been reflected about the room that brightens these surfaces. In the following recipes, we'll look at ways to model more surface types, as well as providing some light for those dark parts of the surface.

Implementing the Phong reflection model

In this recipe, we'll implement the well-known Phong reflection model. The OpenGL fixed-function pipeline's default shading technique was very similar to the one presented here. It models the light-surface interaction as a combination of three components: ambient, diffuse, and specular. The ambient component is intended to model light that has been reflected so many times that it appears to be emanating uniformly from all directions. The diffuse component was discussed in the previous recipe, and represents omnidirectional reflection. The specular component models the shininess of the surface and represents glossy reflection around a preferred direction. Combining these three components together can model a nice (but limited) variety of surface types. This shading model is called the Phong reflection model (or Phong shading model), after graphics researcher Bui Tuong Phong.

An example of a torus rendered with the Phong shading model is shown in the following image:

The Phong model is implemented as the sum of three components: ambient, diffuse, and specular. The ambient component represents light that illuminates all surfaces equally and reflects equally in all directions. It is used to help brighten some of the darker areas within a scene. Since it does not depend on the incoming or outgoing directions of the light, it can be modeled simply by multiplying the light source intensity (LaL_aLa​) by the surface reflectivity (KaK_aKa​):

Ia=LaKaI_a=L_aK_aIa​=La​Ka​

The diffuse component models a rough surface that scatters light in all directions (see Diffuse and per-vertex shading with a single point light source recipe in this chapter). The diffuse contribution is given by the following equation:

Id=Ldkd(s⋅n)I_d=L_dk_d(s\cdot n)Id​=Ld​kd​(s⋅n)

The specular component is used for modeling the shininess of a surface. When a surface has a glossy shine to it, the light is reflected off of the surface, scattered around some preferred direction. We model this so that the reflected light is strongest in the direction of perfect(mirror-like) reflection. The physics of the situation tells us that for perfect reflection, the angle of incidence is the same as the angle of reflection and that the vectors are coplanar with the surface normal, as shown in the following diagram:

In the preceding diagram, rrr represents the direction of pure reflection corresponding to the incoming light vector (−s-s−s), and nnn is the surface normal. We can compute rrr by using the following equation:

r=−s+2(s⋅n)nr=-s+2(s\cdot n)nr=−s+2(s⋅n)n

To model specular reflection, we need to compute the following (normalized) vectors: the direction toward the light source (sss), the vector of perfect reflection (rrr), the vector toward the viewer (vvv), and the surface normal (nnn). These vectors are represented in the following diagram:

We would like the reflection to be maximal when the viewer is aligned with the vector rrr, and to fall off quickly as the viewer moves farther away from alignment with rrr. This can be modeled using the cosine of the angle between vvv and rrr raised to some power (fff):

Is=LsKs(r⋅v)fI_s=L_sK_s(r\cdot v)^fIs​=Ls​Ks​(r⋅v)f

(Recall that the dot product is proportional to the cosine of the angle between the vectors involved.) The larger the power, the faster the value drops toward zero as the angle between vvv and rrr increases. Again, similar to the other components, we also introduce a specular light intensity term (LsL_sLs​) and reflectivity term (KsK_sKs​). It is common to set the KsK_sKs​ term to some grayscale value (for example, (0.8, 0.8, 0.8)), since glossy reflection is not (generally) wavelength dependent.

The specular component creates specular highlights (bright spots) that are typical of glossy surfaces. The larger the power of fff in the equation, the smaller the specular highlight and the shinier the surface. The value for fff is typically chosen to be somewhere between 1 and 200.

Putting all of this together by simply summing the three terms, we have the following shading equation:

I=Ia+Id+Is=LaKa+LdKd(s⋅n)+LsKs(r⋅v)fI=I_a+I_d+I_s=L_aK_a+L_dK_d(s\cdot n)+L_sK_s(r\cdot v)^fI=Ia​+Id​+Is​=La​Ka​+Ld​Kd​(s⋅n)+Ls​Ks​(r⋅v)f

In the following code, we'll evaluate this equation in the vertex shader, and interpolate the color across the polygon.

Getting ready

In the OpenGL application, provide the vertex position in location 0 and the vertex normal in location 1. The light position and the other configurable terms for our lighting equation are uniform variables in the vertex shader and their values must be set from the OpenGL application.

How to do it...

To create a shader pair that implements the Phong reflection model, take the following steps:

1. The vertex shader computes the Phong reflection model at the vertex position and sends the result to the fragment shader:

    layout (location = 0) in vec3 VertexPosition; layout (location = 1) in vec3 VertexNormal; out vec3 LightIntensity; uniform struct LightInfo { vec4 Position; vec3 La; vec3 Ld; vec3 Ls; } Light; uniform struct MaterialInfo { vec3 Ka; vec3 Kd; vec3 Ks; float Shininess; } Material; uniform mat4 ModelViewMatrix; uniform mat3 NormalMatrix; uniform mat4 ProjectionMatrix; uniform mat4 MVP; void main() { vec3 n = normalize(NormalMatrix * VertexNormal); vec4 camCoords = ModelViewMatrix * vec4(VertexPosition,1.0); vec3 ambient = Light.La * Material.Ka; vec3 s = normalize(vec3(Light.Position - camCoords)); float sDotN = max(dot(s, n), 0.0); vec3 diffuse = Light.Ld * Material.Kd * sDotN; vec3 spec = vec3(0.0); if (sDotN > 0.0) { vec3 v = normalize(-camCoords.xyz); vec3 r = reflect(-s, n); spec = Light.Ls * Material.Ks * pow(max( dot(r,v), 0.0 ), Material.Shininess); } LightIntensity = ambient + diffuse + spec; gl_Position = MVP * vec4(VertexPosition, 1.0);
    }

2. The fragment shader simply applies the color to the fragment:

    in vec3 LightIntensity; layout( location = 0 ) out vec4 FragColor; void main() { FragColor = vec4(LightIntensity, 1.0);
    }

3. Compile and link both shaders within the OpenGL application, and install the shader program prior to rendering.

How it works...

The vertex shader computes the shading equation in eye coordinates. It begins by transforming the vertex normal into camera coordinates and normalizing, then storing the result in n. The vertex position is also transformed into camera coordinates and stored in camCoords.

The ambient component is computed and stored in the variable ambient.

Next, we compute the normalized direction towards the light source (s). This is done by subtracting the vertex position in camera coordinates from the light position and normalizing the result.

The dot product of s and n is computed next. As in the preceding recipe, we use the built-in function max to limit the range of values to between zero and one. The result is stored in the variable named sDotN, and is used to compute the diffuse component. The resulting value for the diffuse component is stored in the variable diffuse.

Before computing the specular component, we check the value of sDotN. If sDotN is zero, then there is no light reaching the surface, so there is no point in computing the specular component, as its value must be zero. Otherwise, if sDotN is greater than zero, we compute the specular component using the equation presented earlier.

If we did not check sDotN before computing the specular component, it is possible that some specular highlights could appear on faces that are facing away from the light source. This is clearly an unrealistic and undesirable result. Another way to solve this problem is to multiply both the specular and diffuse components by sDotN (instead of only the diffuse component as we are doing now). This is actually somewhat more physically accurate, but is not part of the traditional Phong model.

The direction toward the viewer (v) is the negation of the position (normalized) because in camera coordinates the viewer is at the origin.

We compute the direction of pure reflection by calling the GLSL built-in function reflect, which reflects the first argument about the second. We don't need to normalize the result because the two vectors involved are already normalized. When computing the specular component, we use the built-in function max to limit the range of values of the dot product to between zero and one, and the function pow raises the dot product to the power of the Shininess exponent (corresponding to fff in our lighting equation).

The sum of the three components is then stored in the output variable LightIntensity. This value will be associated with the vertex and passed down the pipeline. Before reaching the fragment shader, its value will be interpolated in a perspective correct manner across the face of the polygon.

Finally, the vertex shader transforms the position into clip coordinates, and assigns the result to the built-in output variable gl_Position.

The fragment shader simply applies the interpolated value of LightIntensity to the output fragment by storing it in the shader output variable FragColor.

There's more...

The Phong reflection model works quite well, but has some drawbacks. A slight change to the model, introduced by James Blinn, is more commonly used in practice. The Blinn-Phong model replaces the vector of pure reflection with the so-called halfway vector, and produces specular highlights that have been shown to be more realistic.

Using a nonlocal viewer

We can avoid the extra normalization needed to compute the vector towards the viewer (vvv) by using a so-called nonlocal viewer. Instead of computing the direction toward the origin, we simply use the constant vector (0, 0, 1) for all vertices. Of course, it is not accurate, but in practice the visual results are very similar, often visually indistinguishable, saving us one normalization.

Per-vertex versus per-fragment

Since the shading equation is computed within the vertex shader, we refer to this as per-vertex shading. Per-vertex shading is also called Gouraud shading. One of the disadvantages of this is that specular highlights can be warped or lost, due to the fact that the shading equation is not evaluated at each point across the face.

For example, a specular highlight that should appear in the middle of a polygon might not appear at all when per-vertex shading is used, because of the fact that the shading equation is only computed at the vertices where the specular component is near zero.

Directional lights

We can also avoid the need to compute a light direction (sss) for each vertex if we assume a directional light. A directional light source is one that has no position, only a direction. Instead of computing the direction towards the source for each vertex, a constant vector is used, which represents the direction towards the remote light source. This is a good way to model lighting from distant sources such as sunlight. We'll look at an example of this in the Shading with a directional light source recipe of Chapter 4, Lighting and Shading.

Light attenuation with distance

You might think that this shading model is missing one important component. It doesn't take into account the effect of the distance to the light source. In fact, it is known that the intensity of radiation from a source falls off in proportion to the inverse square of the distance from the source. So why not include this in our model?

It would be fairly simple to do so, however, the visual results are often less than appealing. It tends to exaggerate the distance effects and create unrealistic-looking images. Remember, our equation is just an approximation of the physics involved and is not a truly realistic model, so it is not surprising that adding a term based on a strict physical law produces unrealistic results.

In the OpenGL fixed-function pipeline, it was possible to turn on distance attenuation using the glLight function. If desired, it would be straightforward to add a few uniform variables to our shader to produce the same effect.

Using functions in shaders

The GLSL supports functions that are syntactically similar to C functions. However, the calling conventions are somewhat different. In the following example, we'll revisit the Phong shader using functions to help provide abstractions for the major steps.

Getting ready

As with previous recipes, provide the vertex position at attribute location 0 and the vertex normal at attribute location 1. Uniform variables for all of the Phong coefficients should be set from the OpenGL side, as well as the light position and the standard matrices.

How to do it...

The vertex shader is nearly identical to the one from the previous recipe, except that the Phong model is evaluated within a function, and we add another function to convert the position and the normal to camera coordinates:

void getCamSpace(out vec3 norm, out vec3 position) { norm = normalize( NormalMatrix * VertexNormal); position = (ModelViewMatrix * vec4(VertexPosition,1.0)).xyz;
} vec3 phongModel(vec3 position, vec3 n) { vec3 ambient = Light.La * Material.Ka; vec3 s = normalize(Light.Position.xyz - position); float sDotN = max(dot(s,n), 0.0); vec3 diffuse = Light.Ld * Material.Kd * sDotN; vec3 spec = vec3(0.0); if (sDotN > 0.0) { vec3 v = normalize(-position.xyz); vec3 r = reflect(-s, n); spec = Light.Ls * Material.Ks * pow(max(dot(r, v), 0.0), Material.Shininess); } return ambient + diffuse + spec;
} void main() { getCamSpace(camNorm, camPosition); LightIntensity = phongModel(camPosition, camNorm); gl_Position = MVP * vec4(VertexPosition,1.0);
}

The fragment shader has no changes from the previous recipe.

How it works...

In GLSL functions, the parameter evaluation strategy is call by value-return (also called call by copy-restore or call by value-result). Parameter variables can be qualified with in, out, or inout. Arguments corresponding to input parameters (those qualified with in or inout) are copied into the parameter variable at call time, and output parameters (those qualified with out or inout) are copied back to the corresponding argument before the function returns. If a parameter variable does not have any of the three qualifiers, the default qualifier is in.

We've created two functions in the vertex shader. The first, named getCamSpace, transforms the vertex position and vertex normal into camera coordinates, and returns them via output parameters. In the main function, we create two uninitialized variables(camNorm and camPosition) to store the results, and then call the function with the variables as the function's arguments. The function stores the results into the parameter variables (n and position) which are copied into the arguments before the function returns.

The second function, phongModel, uses only input parameters. The function receives the eye-space position and normal, and computes the result of the Phong reflection model. The result is returned by the function and stored in the shader output variable LightIntensity.

There's more...

Since it makes no sense to read from an output parameter variable, output parameters should only be written to within the function. Their value is undefined.

Within a function, writing to an input-only parameter (qualified with in) is allowed. The function's copy of the argument is modified, and changes are not reflected in the argument.

The const qualifier

The additional qualifier const can be used with input-only parameters (not with out or inout). This qualifier makes the input parameter read-only, so it cannot be written to within the function.

Function overloading

Functions can be overloaded by creating multiple functions with the same name, but with a different number and/or type of parameters. As with many languages, two overloaded functions may not differ in return type only.

Passing arrays or structures to a function

It should be noted that when passing arrays or structures to functions, they are passed by value. If a large array or structure is passed, it can incur a large copy operation, which may not be desired. It would be a better choice to declare these variables in the global scope.

When rendering a mesh that is completely closed, the back faces of polygons are hidden. However, if a mesh contains holes, it might be the case that the back faces would become visible. In this case, the polygons may be shaded incorrectly due to the fact that the normal vector is pointing in the wrong direction. To properly shade those back faces, one needs to invert the normal vector and compute the lighting equations based on the inverted normal.

The following image shows a teapot with the lid removed. On the left, the Phong model is used. On the right, the Phong model is augmented with the two-sided rendering technique discussed in this recipe:

In this recipe, we'll look at an example that uses the Phong model discussed in the previous recipes, augmented with the ability to correctly shade back faces.

Getting ready

The vertex position should be provided in attribute location 0 and the vertex normal in attribute location 1. As in the previous examples, the lighting parameters must be provided to the shader via uniform variables.

How to do it...

To implement a shader pair that uses the Phong reflection model with two-sided lighting, take the following steps:

1. The vertex shader is similar to the one in the previous recipe, except that it computes the Phong equation twice. First, without any change to the normal vector, and again with the normal inverted. The results are stored in output variables FrontColor and BackColor, respectively:

    out vec3 FrontColor; out vec3 BackColor; vec3 phongModel(vec3 position, vec3 n) { } void main() { vec3 tnorm = normalize(NormalMatrix * VertexNormal); vec3 camCoords = (ModelViewMatrix * vec4(VertexPosition, 1.0)).xyz; FrontColor = phongModel(camCoords, tnorm); BackColor = phongModel(camCoords, -tnorm); gl_Position = MVP * vec4(VertexPosition, 1.0);
    }

2. The fragment shader chooses which color to use based on the value of the built-in gl_FrontFacing variable:

    in vec3 FrontColor; in vec3 BackColor; layout( location = 0 ) out vec4 FragColor; void main() { if (gl_FrontFacing) { FragColor = vec4(FrontColor, 1.0); } else { FragColor = vec4(BackColor, 1.0); }
    }

How it works...

In the vertex shader, we compute the lighting equation using both the vertex normal and the inverted version, and pass each color to the fragment shader. The fragment shader chooses and applies the appropriate color depending on the orientation of the face.

The evaluation of the reflection model is placed within a function named phongModel. The function is called twice, first using the normal vector (transformed into camera coordinates), and second using the inverted normal vector. The combined results are stored in FrontColor and BackColor, respectively.

There are a few aspects of the shading model that are independent of the orientation of the normal vector (such as the ambient component). One could optimize this code by rewriting it so that the redundant calculations are only done once. However, in this recipe, we compute the entire shading model twice in the interest of making things clear and readable.

In the fragment shader, we determine which color to apply based on the value of the built-in variable gl_FrontFacing. This is a Boolean value that indicates whether the fragment is part of a front- or back-facing polygon. Note that this determination is based on the winding of the polygon, and not the normal vector. (A polygon is said to have counterclockwise winding if the vertices are specified in counterclockwise order, as viewed from the front side of the polygon.) By default, when rendering, if the order of the vertices appear on the screen in a counterclockwise order, it indicates a front-facing polygon, however, we can change this by calling glFrontFace from the OpenGL program.

There's more...

In the vertex shader, we determine the front side of the polygon by the direction of the normal vector, and in the fragment shader, the determination is based on the polygon's winding. For this to work properly, the normal vector must be defined appropriately for the face determined by the setting of glFrontFace.

An alternative choice for this recipe would be to determine whether the face being shaded is a front or back face first in the vertex shader, and send only a single result to the fragment shader. One way to do this would be to compute the dot product between a vector pointing towards the camera (the origin in camera coordinates), and the normal. If the dot product is negative, then the normal must be pointing away from the viewer, meaning that the viewer is seeing the back side of the face.

In which case, we invert the normal. Specifically, we could change the main function in the vertex shader as follows:

void main() { vec3 tnorm = normalize( NormalMatrix * VertexNormal); vec3 camCoords = (ModelViewMatrix * vec4(VertexPosition, 1.0)).xyz; vec3 v = normalize(-camCoords.xyz); float vDotN = dot(v, tnorm); if (vDotN >= 0) { Color = phongModel(camCoords, tnorm); } else { Color = phongModel(camCoords, -tnorm); } gl_Position = MVP * vec4(VertexPosition,1.0);
}

In this case, we only need a single output variable to send to the fragment shader (Color, in the preceding code), and the fragment shader simply applies the color to the fragment. In this version, there's no need to check the value of gl_FrontFacing in the fragment shader.

In this version, the only thing that is used to determine whether or not it is a front face is the normal vector. The polygon winding is not used. If the normals at the vertices of a polygon are not parallel (which is often the case for curved shapes), then it may be the case that some vertices are treated as front and others are treated as back. This has the potential of producing unwanted artifacts as the color is blended across the face. It would be better to compute all of the reflection model in the fragment shader, as is common practice these days.

It can sometimes be useful to visually determine which faces are front-facing and which are back-facing (based on winding). For example, when working with arbitrary meshes, polygons may not be specified using the appropriate winding. As another example, when developing a mesh procedurally, it can sometimes be helpful to determine which faces have proper winding in order to help with debugging. We can easily tweak our fragment shader to help us solve these kinds of problems by mixing a solid color with all back (or front) faces. For example, we could change the else clause within our fragment shader to the following:

FragColor = mix(vec4(BackColor, 1.0), vec4(1.0,0.0,0.0,1.0), 0.7);

This would mix a solid red color with all back faces, helping them stand out, as shown in the following image. In the image, back faces are mixed with 70 percent red, as shown in the preceding code:

Implementing flat shading

Per-vertex shading involves computation of the shading model at each vertex and associating the result (a color) with that vertex. The colors are then interpolated across the face of the polygon to produce a smooth shading effect. This is also referred to as Gouraud shading. In earlier versions of OpenGL, this per-vertex shading with color interpolation was the default shading technique.

It is sometimes desirable to use a single color for each polygon so that there is no variation of color across the face of the polygon, causing each polygon to have a flat appearance. This can be useful in situations where the shape of the object warrants such a technique, perhaps because the faces really are intended to look flat, or to help visualize the locations of the polygons in a complex mesh. Using a single color for each polygon is commonly called flat shading.

The following image shows a mesh rendered with the Phong reflection model. On the left, Gouraud shading is used. On the right, flat shading is used:

In earlier versions of OpenGL, flat shading was enabled by calling the function
glShadeModel with the argument GL_FLAT, in which case the computed color of the last vertex of each polygon was used across the entire face.

In OpenGL 4, flat shading is facilitated by the interpolation qualifiers available for shader input/output variables.

How to do it...

To modify the Phong reflection model to use flat shading, take the following steps:

1. Use the same vertex shader as in the Phong example provided earlier. Change the output variable LightIntensity as follows:

    flat out vec3 LightIntensity;

2. Change the corresponding variable in the fragment shader to use the flat qualifier:

    flat in vec3 LightIntensity;

3. Compile and link both shaders within the OpenGL application, and install the shader program prior to rendering.

How it works...

Flat shading is enabled by qualifying the vertex output variable (and its corresponding fragment input variable) with the flat qualifier. This qualifier indicates that no interpolation of the value is to be done before it reaches the fragment shader. The value presented to the fragment shader will be the one corresponding to the result of the invocation of the vertex shader for either the first or last vertex of the polygon. This vertex is called the provoking vertex, and can be configured using the OpenGL function glProvokingVertex. For example, the following call:

glProvokingVertex(GL_FIRST_VERTEX_CONVENTION);

Indicates that the first vertex should be used as the value for the flat-shaded variable. The GL_LAST_VERTEX_CONVENTION argument indicates that the last vertex should be used. The default value is GL_LAST_VERTEX_CONVENTION.

In GLSL, a subroutine is a mechanism for binding a function call to one of a set of possible function definitions based on the value of a variable. In many ways, it is similar to function pointers in C. A uniform variable serves as the pointer and is used to invoke the function. The value of this variable can be set from the OpenGL side, thereby binding it to one of a few possible definitions. The subroutine's function definitions need not have the same name, but must have the same number and type of parameters and the same return type.

Subroutines therefore provide a way to select alternative implementations at runtime without swapping shader programs and/or recompiling, or using the if statements along with a uniform variable. For example, a single shader could be written to provide several shading algorithms intended for use on different objects within the scene. When rendering the scene, rather than swapping shader programs or using a conditional statement, we can simply change the subroutine's uniform variable to choose the appropriate shading algorithm as each object is rendered.

Since performance is crucial in shader programs, avoiding a conditional statement or a shader swap may be valuable. With subroutines, we can implement the functionality of a conditional statement or shader swap without the computational overhead. However, modern drivers do a good job of handling conditionals, so the benefits of subroutines over conditionals is not always clear-cut. Depending on the condition, conditional statements based on uniform variables can be as efficient as subroutines.

In this example, we'll demonstrate the use of subroutines by rendering a teapot twice. The first teapot will be rendered with the full Phong reflection model described earlier. The second teapot will be rendered with diffuse shading only. A subroutine uniform will be used to choose between the two shading techniques.

Subroutines are not supported in SPIR-V. Therefore, their use should probably be avoided. Since SPIR-V is evidently the future of shaders in OpenGL, subroutines should be considered deprecated.

In the following image, we can see an example of a rendering that was created using subroutines. The teapot on the left is rendered with the full Phong reflection model, and the teapot on the right is rendered with diffuse shading only. A subroutine is used to switch between shader functionality:

Getting ready

As with the previous recipes, provide the vertex position at attribute location 0 and the vertex normal at attribute location 1. Uniform variables for all of the Phong coefficients should be set from the OpenGL side, as well as the light position and the standard matrices.

We'll assume that, in the OpenGL application, the programHandle variable contains the handle to the shader program object.

How to do it...

To create a shader program that uses a subroutine to switch between pure-diffuse and Phong, take the following steps:

1. Set up the vertex shader with a subroutine uniform variable, and two functions of the subroutine type:

    subroutine vec3 shadeModelType(vec3 position, vec3 normal); subroutine uniform shadeModelType shadeModel; out vec3 LightIntensity; subroutine( shadeModelType ) vec3 phongModel(vec3 position, vec3 norm) { } subroutine( shadeModelType ) vec3 diffuseOnly(vec3 position, vec3 norm) { } void main() { LightIntensity = shadeModel(camPosition, camNorm); gl_Position = MVP * vec4(VertexPosition, 1.0);
    }

2. The fragment shader is the same as the one in The Phong reflection model recipe.

3. In the OpenGL application, compile and link the shaders into a shader program, and install the program into the OpenGL pipeline.

4. Within the render function of the OpenGL application, use the following code:

    GLuint phongIndex = glGetSubroutineIndex(programHandle, GL_VERTEX_SHADER,"phongModel");
    GLuint diffuseIndex = glGetSubroutineIndex(programHandle, GL_VERTEX_SHADER, "diffuseOnly"); glUniformSubroutinesuiv( GL_VERTEX_SHADER, 1, &phongIndex);
    ... glUniformSubroutinesuiv( GL_VERTEX_SHADER, 1, &diffuseIndex);
    ... 

How it works...

In this example, the subroutine is defined within the vertex shader. The first step involves declaring the subroutine type, as follows:

subroutine vec3 shadeModelType(vec3 position, vec3 normal);

This defines a new subroutine type with the name shadeModelType. The syntax is very similar to a function prototype, in that it defines a name, a parameter list, and a return type. As with function prototypes, the parameter names are optional.

After creating the new subroutine type, we declare a uniform variable of that type named shadeModel:

subroutine uniform shadeModelType shadeModel;

This variable serves as our function pointer and will be assigned to one of the two possible functions in the OpenGL application.

We declare two functions to be part of the subroutine by prefixing their definition with the subroutine qualifier:

subroutine (shadeModelType)

This indicates that the function matches the subroutine type, and therefore its header must match the one in the subroutine type definition. We use this prefix for the definition of the functions phongModel and diffuseOnly. The diffuseOnly function computes the diffuse shading equation, and the phongModel function computes the complete Phong reflection equation.

We call one of the two subroutine functions by utilizing the subroutine uniform shadeModel within the main function:

LightIntensity = shadeModel( eyePosition, eyeNorm );

Again, this call will be bound to one of the two functions depending on the value of the subroutine uniform shadeModel, which we will set within the OpenGL application.

Within the render function of the OpenGL application, we assign a value to the subroutine uniform with the following two steps:

1. First, we query for the index of each subroutine function using glGetSubroutineIndex. The first argument is the program handle. The second is the shader stage. In this case, the subroutine is defined within the vertex shader, so we use GL_VERTEX_SHADER here. The third argument is the name of the subroutine. We query for each function individually and store the indexes in the variables phongIndex and diffuseIndex.

2. Second, we select the appropriate subroutine function. To do so, we need to set the value of the subroutine uniform shadeModel by calling glUniformSubroutinesuiv. This function is designed for setting multiple subroutine uniforms at once. In our case, of course, we are setting only a single uniform. The first argument is the shader stage (GL_VERTEX_SHADER), the second is the number of uniforms being set, and the third is a pointer to an array of subroutine function indexes. Since we are setting a single uniform, we simply provide the address of the GLuint variable containing the index, rather than a true array of values. Of course, we would use an array if multiple uniforms were being set. In general, the array of values provided as the third argument is assigned to subroutine uniform variables in the following way. The i element of the array is assigned to the subroutine uniform variable with index i. Since we have provided only a single value, we are setting the subroutine uniform at index zero.

You may be wondering, "How do we know that our subroutine uniform is located at index zero? We didn't query for the index before calling glUniformSubroutinesuiv!" The reason that this code works is that we are relying on the fact that OpenGL will always number the indexes of the subroutines consecutively starting at zero. If we had multiple subroutine uniforms, we could (and should) query for their indexes using glGetSubroutineUniformLocation, and then order our array appropriately.

glUniformSubroutinesuiv requires us to set all subroutine uniform variables at once, in a single call. This is so that they can be validated by OpenGL in a single burst.

There's more...

Unfortunately, subroutine bindings get reset when a shader program is unbound (switched out) from the pipeline by calling glUseProgram or another technique. This requires us to call glUniformSubroutinsuiv each time that we activate a shader program.

A subroutine function defined in a shader can match more than one subroutine type. The subroutine qualifier can contain a comma-separated list of subroutine types. For example, if a subroutine matched the types type1 and type2, we could use the following qualifier:

subroutine(type1, type2)

This would allow us to use subroutine uniforms of differing types to refer to the same subroutine function.

Discarding fragments to create a perforated look

Fragment shaders can make use of the discard keyword to throw away fragments. Use of this keyword causes the fragment shader to stop execution, without writing anything (including depth) to the output buffer. This provides a way to create holes in polygons without using blending. In fact, since fragments are completely discarded, there is no dependence on the order in which objects are drawn, saving us the trouble of doing any depth sorting that might have been necessary if blending was used.

In this recipe, we'll draw a teapot, and use the discard keyword to remove fragments selectively, based on texture coordinates. The result will look like the following image:

Getting ready

The vertex position, normal, and texture coordinates must be provided to the vertex shader from the OpenGL application. The position should be provided at location 0, the normal at location 1, and the texture coordinates at location 2. As in the previous examples, the lighting parameters must be set from the OpenGL application via the appropriate uniform variables.

How to do it...

To create a shader program that discards fragments based on a square lattice (as in the preceding image):

1. In the vertex shader, we use two-sided lighting, and include the texture coordinate:

    layout (location = 0) in vec3 VertexPosition; layout (location = 1) in vec3 VertexNormal; layout (location = 2) in vec2 VertexTexCoord; out vec3 FrontColor; out vec3 BackColor; out vec2 TexCoord; void main() { TexCoord = VertexTexCoord; vec3 camNorm, camPosition; getCamSpace(camNorm, camPosition); FrontColor = phongModel(camPosition, eyeNorm); BackColor = phongModel(camPosition, -eyeNorm); gl_Position = MVP * vec4(VertexPosition,1.0);
    }

2. In the fragment shader, discard the fragment based on a certain condition by using the discard keyword:

    in vec3 FrontColor; in vec3 BackColor; in vec2 TexCoord; layout( location = 0 ) out vec4 FragColor; void main() { const float scale = 15.0; bvec2 toDiscard = greaterThan(fract(TexCoord * scale), vec2(0.2,0.2)); if (all(toDiscard)) discard; if (gl_FrontFacing) FragColor = vec4(FrontColor, 1.0); else FragColor = vec4(BackColor, 1.0);
    }

3. Compile and link both shaders within the OpenGL application, and install the shader program prior to rendering.

How it works...

Since we will be discarding some parts of the teapot, we will be able to see through the teapot to the other side. This will cause the back sides of some polygons to become visible. Therefore, we need to compute the lighting equation appropriately for both sides of each face. We'll use the same technique presented earlier in the two-sided shading recipe.

The vertex shader is essentially the same as in the two-sided shading recipe, with the main difference being the addition of the texture coordinate. To manage the texture coordinate, we have an additional input variable, VertexTexCoord, that corresponds to attribute location 2. The value of this input variable is passed directly on to the fragment shader unchanged via the output variable TexCoord. The Phong reflection model is calculated twice, once using the given normal vector, storing the result in FrontColor, and again using the reversed normal, storing that result in BackColor.

In the fragment shader, we calculate whether or not the fragment should be discarded based on a simple technique designed to produce the lattice-like pattern shown in the preceding image. We first scale the texture coordinate by the arbitrary scaling factor scale. This corresponds to the number of lattice rectangles per unit (scaled) texture coordinate. We then compute the fractional part of each component of the scaled texture coordinate using the built-in function fract. Each component is compared to 0.2 using the built-in the greaterThan function, and the result is stored in the Boolean vector toDiscard. The greaterThan function compares the two vectors component-wise, and stores the Boolean results in the corresponding components of the return value.

If both components of the vector toDiscard are true, then the fragment lies within the inside of each lattice frame, and therefore we wish to discard this fragment. We can use the built-in function all to help with this check. The function all will return true if all of the components of the parameter vector are true. If the function returns true, we execute the discard statement to reject the fragment.

In the else branch, we color the fragment based on the orientation of the polygon, as in the Implementing two-sided shading recipe presented earlier.

Lighting and Shading

In this chapter, we will cover the following recipes:

• Shading with multiple positional lights
• Shading with a directional light source
• Using per-fragment shading for improved realism
• The Blinn-Phong reflection model
• Simulating a spotlight
• Creating a cartoon shading effect
• Simulating fog
• A physically-based reflection model

Introduction

In Chapter 3, The Basics of GLSL Shaders, we covered a number of techniques for implementing some of the shading effects that were produced by the former fixed-function pipeline. We also looked at some basic features of GLSL such as functions and subroutines. In this chapter, we'll move beyond those introductory features, and see how to produce shading effects such as spotlights, fog, and cartoon style shading. We'll cover how to use multiple light sources and how to improve the realism of the results with a technique called per-fragment shading.

We'll also cover the very popular and important Blinn-Phong reflection model and directional light sources.
Finally, we'll cover how to fine-tune the depth test by configuring the early depth test optimization.

Shading with multiple positional lights

When shading with multiple light sources, we need to evaluate the reflection model for each light, and sum the results to determine the total light intensity reflected by a surface location. The natural choice is to create uniform arrays to store the position and intensity of each light. We'll use an array of structures so that we can store the values for multiple lights within a single uniform variable.

The following image shows a "pig" mesh rendered with five light sources of different colors. Note the multiple specular highlights:

Getting ready

Set up your OpenGL program with the vertex position in attribute location zero, and the normal in location one.

How to do it...

To create a shader program that renders using the Blinn-Phong reflection model with multiple light sources, use the following steps:

In the vertex shader, we'll use a similar structure as in the previous recipes, except we will use an array of structures for the lights. In addition, we just store two intensities for each light. The first is the ambient intensity and the second is used for both diffuse and specular. The phongModel function is updated to use light information from one of the values in the array:

layout (location = 0) in vec3 VertexPosition; layout (location = 1) in vec3 VertexNormal; out vec3 Color; uniform struct LightInfo { vec4 Position; vec3 La; vec3 L; } lights[5]; vec3 phongModel(int light, vec3 position, vec3 n) { vec3 ambient = lights[light].La * Material.Ka; vec3 s = normalize(lights[light].Position.xyz - position); float sDotN = max(dot(s,n), 0.0); vec3 diffuse = Material.Kd * sDotN; vec3 spec = vec3(0.0); if (sDotN > 0.0) { vec3 v = normalize(-position.xyz); vec3 r = reflect( -s, n ); spec = Material.Ks * pow(max(dot(r,v), 0.0), Material.Shininess); } return ambient + lights[light].L * (diffuse + spec);
} void main() { vec3 camNorm = normalize(NormalMatrix * VertexNormal); vec3 camPosition = (ModelViewMatrix * vec4(VertexPosition, 1.0)).xyz; Color = vec3(0.0); for( int i = 0; i < 5; i++ ) Color += phongModel(i, camPosition, camNorm); gl_Position = MVP * vec4(VertexPosition, 1.0);
}

The fragment shader simply applies the color to the fragment, as in previous recipes.

In the OpenGL application, set the values for the lights array in the vertex shader. For each light, use something similar to the following code. This example uses the C++ shader program class described in Chapter 2, Working with GLSL Programs (prog is a GLSLProgram object):

prog.setUniform("lights[0].L", glm::vec3(0.0f,0.8f,0.8f)); prog.setUniform("lights[0].La", glm::vec3(0.0f,0.2f,0.2f)); prog.setUniform("lights[0].Position", position);

How it works...

Within the vertex shader, the lighting parameters are stored in the uniform array lights. Each element of the array is a struct of type LightInfo. This example uses five lights. The diffuse/specular light intensity is stored in the L field, the ambient intensity is stored in the La field, and the position in camera coordinates is stored in the Position field.

The rest of the uniform variables are essentially the same as in the Phong model shader presented in Chapter 3, The Basics of GLSL Shaders.

The phongModel function is responsible for computing the shading equation for a given light source. The index of the light is provided as the first parameter, light. The equation is computed based on the values in the lights array in that index. In this example, we don't use a separate light intensity for the diffuse and specular components.

In the main function, a for loop is used to compute the shading equation for each light, and the results are summed into the output variable Color.

The fragment shader simply applies the interpolated color to the fragment.

Shading with a directional light source

A core component of a shading equation is the vector that points from the surface location toward the light source (sss, in previous examples). For lights that are extremely far away, there is very little variation in this vector over the surface of an object. In fact, for very distant light sources, the vector is essentially the same for all points on a surface (another way of thinking about this is that the light rays are nearly parallel). Such a model would be appropriate for a distant, but powerful, light source such as the sun. Such a light source is commonly called a directional light source because it does not have a specific position, only a direction.

Of course, we are ignoring the fact that, in reality, the intensity of the light decreases with the square of the distance from the source. However, it is not uncommon to ignore this aspect for directional light sources.

If we are using a directional light source, the direction toward the source is the same for all points in the scene. Therefore, we can increase the efficiency of our shading calculations because we no longer need to recompute the direction toward the light source for each location on the surface.

Of course, there is a visual difference between a positional light source and a directional one. The following images show a torus rendered with a positional light (left) and a directional light (right). In the left image, the light is located somewhat close to the torus. The directional light covers more of the surface of the torus due to the fact that all of the rays are parallel:

In previous versions of OpenGL, the fourth component of the light position was used to determine whether or not a light was considered directional. A zero in the fourth component indicated that the light source was directional and the position was to be treated as a direction toward the source (a vector). Otherwise, the position was treated as the actual location of the light source. In this example, we'll emulate the same functionality.

Getting ready

Set up your OpenGL program with the vertex position in attribute location zero, and the vertex normal in location one.

How to do it...

To create a shader program that implements the Phong reflection model using a directional light source, we'll use the same vertex shader as in the previous recipe, except with a single light source. Within the phongModel function, replace the calculation of the s vector with the following:

vec3 s; if (Light.Position.w == 0.0) s = normalize(Light.Position.xyz); else s = normalize(Light.Position.xyz - position);

How it works...

Within the vertex shader, the fourth coordinate of the uniform variable Light.Position is used to determine whether or not the light is to be treated as a directional light. Inside the phongModel function, which is responsible for computing the shading equation, the value of the vector s is determined based on whether or not the fourth coordinate of Light.Position is zero. If the value is zero, Light.Position is normalized and used as the direction toward the light source. Otherwise, Light.Position is treated as a location in eye coordinates, and we compute the direction toward the light source by subtracting the vertex position from Light.Position and normalizing the result.

There's more...

There is a slight efficiency gain when using directional lights, due to the fact that there is no need to recompute the light direction for each vertex. This saves a subtraction operation, which is a small gain but could accumulate when there are several lights or when the lighting is computed per-fragment.

Using per-fragment shading for improved realism

When the shading equation is evaluated within the vertex shader (as we have done in previous recipes), we end up with a color associated with each vertex. That color is then interpolated across the face, and the fragment shader assigns that interpolated color to the output fragment. As mentioned previously, this technique is called Gouraud shading. Gouraud shading (like all shading techniques) is an approximation, and can lead to some less than desirable results when, for example, the reflection characteristics at the vertices have little resemblance to those in the center of the polygon. For example, a bright specular highlight may reside in the center of a polygon but not at its vertices. Simply evaluating the shading equation at the vertices would prevent the specular highlight from appearing in the rendered result. Other undesirable artifacts, such as edges of polygons, may also appear when Gouraud shading is used, due to the fact that color interpolation may not match the value of the reflection model across the face.

To improve the accuracy of our results, we can move the computation of the shading equation from the vertex shader to the fragment shader. Instead of interpolating color across the polygon, we interpolate the position and normal vector, and use these values to evaluate the shading equation at each fragment. This technique is called Phong shading or Phong interpolation. The results from Phong shading are much more accurate and provide more pleasing results, but some undesirable artifacts may still appear.

The following image shows the difference between Gouraud and Phong shading. The scene on the left is rendered with Gouraud (per-vertex) shading, and on the right is the same scene rendered using Phong (per-fragment) shading. Underneath the teapot is a partial plane, drawn with a single quad. Note the difference in the specular highlight on the teapot, as well as the variation in the color of the plane beneath the teapot:

In this example, we'll implement Phong shading by passing the position and normal from the vertex shader to the fragment shader, and then evaluating the Phong reflection model within the fragment shader.

Getting ready

Set up your program with the vertex position in attribute location zero, and the normal in location one. Your OpenGL application must also provide the values for the uniform variables, as in the previous recipes.

How to do it...

To create a shader program that can be used for implementing per-fragment (or Phong) shading using the Phong reflection model, use the following steps:

1. The vertex shader simply converts the position and normal to camera coordinates and passes them to the fragment shader:

    layout (location = 0) in vec3 VertexPosition; layout (location = 1) in vec3 VertexNormal; out vec3 Position; out vec3 Normal; uniform mat4 ModelViewMatrix, NormalMatrix, ProjectionMatrix, MVP; void main() { Normal = normalize(NormalMatrix * VertexNormal); Position = (ModelViewMatrix * vec4(VertexPosition,1.0) ).xyz; gl_Position = MVP * vec4(VertexPosition,1.0);
    }

2. The fragment shader evaluates the Phong reflection model using the values passed from the vertex shader:

    in vec3 Position; in vec3 Normal; layout( location = 0 ) out vec4 FragColor; vec3 phongModel(vec3 position, vec3 n) { } void main() { FragColor = vec4(phongModel(Position, normalize(Normal)), 1);
    }

How it works...

The vertex shader has two output variables: Position and Normal. In the main function, we convert the vertex normal to camera coordinates by transforming with the normal matrix, and then store the converted value in Normal. Similarly, the vertex position is converted to eye coordinates by transforming it by the model-view matrix, and the converted value is stored in Position. The values of Position and Normal are automatically interpolated and provided to the fragment shader via the corresponding input variables. The fragment shader then computes the Phong reflection model using the values provided. Here, we re-normalize the Normal vector because the interpolation process can create vectors that are not of unit length.

Finally, the result is then stored in the output variable FragColor.

There's more...

Evaluating the shading equation within the fragment shader produces more accurate renderings. However, the price we pay is in the evaluation of the shading model for each pixel of the polygon, rather than at each vertex. The good news is that with modern graphics cards, there may be enough processing power to evaluate all of the fragments for a polygon in parallel. This can essentially provide nearly equivalent performance for either per-fragment or per-vertex shading.

The Blinn-Phong reflection model

As covered in the Implementing the Phong reflection model recipe in Chapter 3, The Basics of GLSL Shaders, the specular term in the equation involves the dot product of the vector of pure reflection (rrr), and the direction toward the viewer (vvv):

In order to evaluate the preceding equation, we need to find the vector of pure reflection (rrr), which is the reflection of the vector toward the light source (sss) about the normal vector (nnn):

r=−s+2(s⋅n)nr=-s+2(s\cdot n)nr=−s+2(s⋅n)n

This equation is implemented using the GLSL function reflect.

We can avoid calculating rrr by making use of the following observation. When vvv is aligned with rrr, the normal vector (nnn) must be halfway between vvv and sss. Let's define the halfway vector (hhh) as the vector that is halfway between vvv and sss, where hhh is normalized after the addition:

h=v+sh=v+sh=v+s

The following diagram shows the relative positions of the halfway vector and the others:

We can then replace the dot product in the equation for the specular component, with the dot product of hhh and nnn:

Is=LsKs(h⋅n)fI_s=L_sK_s(h\cdot n)^fIs​=Ls​Ks​(h⋅n)f

Computing hhh requires fewer operations than it takes to compute rrr, so we should expect some efficiency gain by using the halfway vector. The angle between the halfway vector and the normal vector is proportional to the angle between the vector of pure reflection (rrr) and the vector toward the viewer (vvv) when all vectors are co-planar. Therefore, we expect that the visual results will be similar, although not exactly the same.

This small modification to the Phong reflection model was proposed by James Blinn, a researcher who worked at NASA's Jet Propulsion Laboratory (JPL). His modified version, which uses the halfway vector in the specular term is, therefore called the Blinn-Phong model.

It is interesting to note that the Blinn-Phong model, despite appearing to be somewhat ad hoc, produces results that match physical measurements more closely than the Phong model.

Getting ready

Start by utilizing the same shader program that was presented in the recipe Using per-fragment shading for improved realism, and set up your OpenGL program as described there.

How to do it...

Using the same shader pair as in the recipe Using per-fragment shading for improved realism, replace the phongModel function in the fragment shader with the following:

vec3 blinnPhong(vec3 position, vec3 n) { vec3 ambient = Light.La * Material.Ka; vec3 s = normalize( Light.Position.xyz - position ); float sDotN = max( dot(s,n), 0.0 ); vec3 diffuse = Material.Kd * sDotN; vec3 spec = vec3(0.0); if (sDotN > 0.0) { vec3 v = normalize(-position.xyz); vec3 h = normalize( v + s ); spec = Material.Ks * pow(max(dot(h,n), 0.0), Material.Shininess); } return ambient + Light.L * (diffuse + spec);
}

How it works...

We compute the halfway vector by summing the direction toward the viewer (v), and the direction toward the light source (s), and normalizing the result. The value for the halfway vector is then stored in h. The specular calculation is then modified to use the dot product between h and the normal vector (n). The rest of the calculation is unchanged.

There's more...

The following screenshot shows the teapot rendered using the Blinn-Phong model (right), versus the same rendering using the equation provided in the Implementing the Phong reflection model recipe in Chapter 3, The Basics of GLSL Shaders (left). The halfway vector produces a larger specular highlight. If desired, we could compensate for the difference in the size of the specular highlight by increasing the value of the exponent Material.Shininess:

Simulating a spotlight

The fixed function pipeline had the ability to define light sources as spotlights. In such a configuration, the light source was considered to be one that only radiated light within a cone, the apex of which was located at the light source. Additionally, the light was attenuated so that it was maximal along the axis of the cone and decreased toward the outside edges. This allowed us to create light sources that had a similar visual effect to a real spotlight.

The following screenshot shows a teapot and a torus rendered with a single spotlight. Note the slight decrease in the intensity of the spotlight from the center toward the outside edge:

In this recipe, we'll use a shader to implement a spotlight effect, similar to that produced by the fixed-function pipeline:

The spotlight's cone is defined by a spotlight direction (ddd, in the preceding image), a cutoff angle (ccc, in the preceding image), and a position (PPP, in the preceding image). The intensity of the spotlight is considered to be the strongest along the axis of the cone, and decreases as you move toward the edges.

Getting ready

Start with the same vertex shader from the recipe, Using per-fragment shading for improved realism. Your OpenGL program must set the values for all uniform variables defined in that vertex shader as well as the fragment shader shown next.

How to do it...

To create a shader program that uses the ADS shading model with a spotlight, use the following code for the fragment shader:

in vec3 Position; in vec3 Normal; uniform struct SpotLightInfo { vec3 Position; vec3 L; vec3 La; vec3 Direction; float Exponent; float Cutoff; } Spot; layout( location = 0 ) out vec4 FragColor; vec3 blinnPhongSpot(vec3 position, vec3 n) { vec3 ambient = Spot.La * Material.Ka, diffuse = vec3(0), spec = vec3(0); vec3 s = normalize( Spot.Position - position ); float cosAng = dot(-s, normalize(Spot.Direction)); float angle = acos( cosAng ); float spotScale = 0.0; if(angle < Spot.Cutoff ) { spotScale = pow( cosAng, Spot.Exponent ); float sDotN = max( dot(s,n), 0.0 ); diffuse = Material.Kd * sDotN; if( sDotN > 0.0 ) { vec3 v = normalize(-position.xyz); vec3 h = normalize( v + s ); spec = Material.Ks * pow( max( dot(h,n), 0.0 ), Material.Shininess ); } } return ambient + spotScale * Spot.L * (diffuse + spec);
} void main() { FragColor = vec4(blinnPhongSpot(Position, normalize(Normal)), 1);
}

How it works...

The SpotLightInfo structure defines all of the configuration options for the spotlight. We declare a single uniform variable named Spot to store the data for our spotlight. The Position field defines the location of the spotlight in eye coordinates. The L field is the intensity (diffuse and specular) of the spotlight, and La is the ambient intensity. The Direction field will contain the direction that the spotlight is pointing, which defines the center axis of the spotlight's cone. This vector should be specified in camera coordinates. Within the OpenGL program, it should be transformed by the normal matrix in the same way that normal vectors would be transformed. We could do so within the shader; however, within the shader, the normal matrix would be specified for the object being rendered. This may not be the appropriate transform for the spotlight's direction.

The Exponent field defines the exponent that is used when calculating the angular attenuation of the spotlight. The intensity of the spotlight is decreased in proportion to the cosine of the angle between the vector from the light to the surface location (the negation of the variable s) and the direction of the spotlight. That cosine term is then raised to the power of the variable Exponent. The larger the value of this variable, the faster the intensity of the spotlight is decreased. This is similar to the exponent in the specular shading term.

The Cutoff field defines the angle between the central axis and the outer edge of the spotlight's cone of light. We specify this angle in radians.

The blinnPhongSpot function computes the Blinn-Phong reflection model, using a spotlight as the light source. The first line computes the ambient lighting component and stores it in the ambient variable. The second line computes the vector from the surface location to the spotlight's position (sss). Next, we compute the dot product between the direction from the spotlight to the surface point (−s-s−s) and the direction of the spotlight and store the result in cosAng. The angle between them is then computed and stored in the variable angle. The variable spotScale will be used to scale the value of the spotlight's diffuse/specular intensity. It is initially set to zero.

We then compare the value of the angle variable with that of the Spot.Cutoff variable. If angle is greater than zero and less than Spot.Cutoff, then the surface point is within the spotlight's cone. Otherwise, the surface point only receives ambient light, so we skip the rest and return only the ambient component.

If angle is less than Spot.Cutoff, we compute the spotScale value by raising the dot product of -s and spotDir to the power of Spot.Exponent. The value of spotScale is used to scale the intensity of the light so that the light is maximal in the center of the cone, and decreases as you move toward the edges. Finally, the Blinn-Phong reflection model is computed as usual.

Creating a cartoon shading effect

Toon shading (also called cel shading) is a non-photorealistic rendering technique that is intended to mimic the style of shading often used in hand-drawn animation. There are many different techniques that are used to produce this effect. In this recipe, we'll use a very simple technique that involves a slight modification to the ambient and diffuse shading model.

The basic effect is to have large areas of constant color with sharp transitions between them. This simulates the way that an artist might shade an object using strokes of a pen or brush. The following image shows an example of a teapot and torus rendered with toon shading:

The technique presented here involves computing only the ambient and diffuse components of the typical ADS shading model, and quantizing the cosine term of the diffuse component. In other words, the value of the dot product normally used in the diffuse term is restricted to a fixed number of possible values. The following table illustrates the concept for four levels:

Cosine of the angle between s and n

Value used

Between 1 and 0.75

0.75

Between 0.75 and 0.5

0.5

Between 0.5 and 0.25

0.25

Between 0.25 and 0.0

0.0

In the preceding table, sss is the vector toward the light source and nnn is the normal vector at the surface. By restricting the value of the cosine term in this way, the shading displays strong discontinuities from one level to another (see the preceding image), simulating the pen strokes of hand-drawn cel animation.

Getting ready

Start with the same vertex shader from the Using per-fragment shading for improved realism recipe. Your OpenGL program must set the values for all uniform variables defined in that vertex shader as well as the fragment shader code described here.

How to do it...

To create a shader program that produces a toon shading effect, use the following fragment shader:

in vec3 Position; in vec3 Normal; uniform struct LightInfo { vec4 Position; vec3 La; vec3 L; } Light; uniform struct MaterialInfo { vec3 Ka; vec3 Kd; } Material; const int levels = 3; const float scaleFactor = 1.0 / levels; layout( location = 0 ) out vec4 FragColor; vec3 toonShade() { vec3 n = normalize( Normal ); vec3 s = normalize( Light.Position.xyz - Position ); vec3 ambient = Light.La * Material.Ka; float sDotN = max( dot( s, n ), 0.0 ); vec3 diffuse = Material.Kd * floor( sDotN * levels ) * scaleFactor; return ambient + Light.L * diffuse;
} void main() { FragColor = vec4(toonShade(), 1.0);
}

How it works...

The constant variable, levels, defines how many distinct values will be used in the diffuse calculation. This could also be defined as a uniform variable to allow for configuration from the main OpenGL application. We will use this variable to quantize the value of the cosine term in the diffuse calculation.

The toonShade function is the most significant part of this shader. We start by computing s, the vector toward the light source. Next, we compute the cosine term of the diffuse component by evaluating the dot product of s and Normal. The next line quantizes that value in the following way. Since the two vectors are normalized, and we have removed negative values with the max function, we are sure that the value of cosine is between zero and one. By multiplying this value by levels and taking the floor, the result will be an integer between 0 and levels-1. When we divide that value by levels (by multiplying by scaleFactor), we scale these integral values to be between zero and one again. The result is a value that can be one of levels possible values spaced between zero and one. This result is then multiplied by Material.Kd, the diffuse reflectivity term.

Finally, we combine the diffuse and ambient components together to get the final color for the fragment.

There's more...

When quantizing the cosine term, we could have used ceil instead of floor. Doing so would have simply shifted each of the possible values up by one level. This would make the levels of shading slightly brighter.

The typical cartoon style seen in most cel animation includes black outlines around the silhouettes and along other edges of a shape. The shading model presented here does not produce those black outlines. There are several techniques for producing them, and we'll look at one later on in this book.

Simulating fog

A simple fog effect can be achieved by mixing the color of each fragment with a constant fog color. The amount of influence of the fog color is determined by the distance from the camera. We could use either a linear relationship between the distance and the amount of fog color, or we could use a non-linear relationship such as an exponential one.

The following image shows four teapots rendered with a fog effect produced by mixing the fog color in a linear relationship with distance:

To define this linear relationship, we can use the following equation:

f=dmax−∣z∣dmax−dminf=\frac{d_{max}-\begin{vmatrix}z \end{vmatrix}}{d_{max}-d_{min}}f=dmax​−dmin​dmax​−∣z​∣​

In the preceding equation, dmind_{min}dmin​ is the distance from the eye where the fog is minimal (no fog contribution), and dmaxd_{max}dmax​ is the distance where the fog color obscures all other colors in the scene. The variable zzz represents the distance from the eye. The value fff is the fog factor. A fog factor of zero represents 100% fog, and a factor of one represents no fog. Since fog typically looks thickest at longer distances, the fog factor is minimal when ∣z∣\begin{vmatrix}z \end{vmatrix}​z​​ is equal to dmaxd_{max}dmax​, and maximal when ∣z∣\begin{vmatrix}z \end{vmatrix}​z​​ is equal to dmind_{min}dmin​.

Since the fog is applied by the fragment shader, the effect will only be visible on the objects that are rendered. It will not appear on any empty space in the scene (the background). To help make the fog effect consistent, you should use a background color that matches the maximum fog color.

Getting ready

Start with the same vertex shader from the Using per-fragment shading for improved realism recipe. Your OpenGL program must set the values for all uniform variables defined in that vertex shader as well as the fragment shader shown in the following section.

How to do it...

To create a shader that produces a fog-like effect, use the following code for the fragment shader:

in vec3 Position; in vec3 Normal; uniform struct FogInfo { float MaxDist; float MinDist; vec3 Color;
} Fog; layout( location = 0 ) out vec4 FragColor; vec3 blinnPhong(vec3 position, vec3 n) { } void main() { float dist = abs( Position.z ); float fogFactor = (Fog.MaxDist - dist) / (Fog.MaxDist - Fog.MinDist); fogFactor = clamp( fogFactor, 0.0, 1.0 ); vec3 shadeColor = blinnPhong(Position, normalize(Normal)); vec3 color = mix( Fog.Color, shadeColor, fogFactor ); FragColor = vec4(color, 1.0);
}

How it works...

In this shader, the blinnPhong function is exactly the same as the one used in The Blinn-Phong reflection model recipe. The part of this shader that deals with the fog effect lies within the main function.

The uniform variable Fog contains the parameters that define the extent and color of the fog. The MinDist field is the distance from the eye to the fog's starting point, and MaxDist is the distance to the point where the fog is maximal. The Color field is the color of the fog.

The dist variable is used to store the distance from the surface point to the eye position.
The z coordinate of the position is used as an estimate of the actual distance. The fogFactor variable is computed using the preceding equation. Since dist may not be between Fog.MinDist and Fog.MaxDist, we clamp the value of fogFactor to be between zero and one.

We then call the blinnPhong function to evaluate the reflection model. The result of this is stored in the shadeColor variable.

Finally, we mix shadeColor and Fog.Color together based on the value of fogFactor, and the result is used as the fragment color.

There's more...

In this recipe, we used a linear relationship between the amount of fog color and the distance from the eye. Another choice would be to use an exponential relationship. For example, the following equation could be used:

f=e−d∣z∣f=e^{-d\begin{vmatrix}z \end{vmatrix}}f=e−d∣z​∣

In the above equation, ddd represents the density of the fog. Larger values would create thicker fog. We could also square the exponent to create a slightly different relationship (a faster increase in the fog with distance).

f=e−(dz)2f=e^{-(dz)^2}f=e−(dz)2

Computing distance from the eye

In the preceding code, we used the absolute value of the z coordinate as the distance from the camera. This may cause the fog to look a bit unrealistic in certain situations. To compute a more precise distance, we could replace the line:

float dist = abs( Position.z );

With the following:

float dist = length( Position.xyz );

Of course, the latter version requires a square root, and therefore would be a bit slower in practice.

A physically-based reflection model

Physically-based rendering or PBR is an umbrella term that encompasses tools and techniques that make use of physically-based models of light and reflection. The term itself is somewhat loosely defined, but can generally be described as a shading/reflection model that tries to model the physics of light interacting with matter as accurately as possible. The term may mean slightly different things to different people, but for our purposes, we are interested primarily in how it differs from the Phong and the Blinn-Phong reflection models.

The Blinn-Phong model is an empirical model of reflection based on observation. A PBR model could also be considered an empirical model, but in general, it is more detailed and accurate with regards to the physics of the interaction being represented. The Blinn-Phong model uses a few parameters which are not physically based but produce effective results. For example, separating the light intensity into three (or two) separate values is not physically accurate (there's only one light). However, it provides many "tuneable" parameters to the artist to work with, giving them the flexibility to achieve the desired look.

In recent years, PBR techniques have gained favor due to the fact that they reduce the number of tuneable parameters, and provide more consistent results across a wide variety of materials. Artists have found that previous models (non-PBR) have a tendency to be tricky to "get right." When a scene consists of a wide variety of materials, the parameters may require a significant amount of "tweaking" to make consistent. With PBR-based techniques, the reflection models try to represent the physics more accurately, which tends to make things look more consistent under a wide variety of lighting settings, reducing the amount of fine-tuning required by artists.

In this recipe, we'll implement a basic PBR-based reflection model using point light sources. Before we get started, however, let's go over the math. In the following equations, we'll use the vectors nnn, lll, vvv, and hhh, defined as follows:

• nnn: The surface normal
• lll: The vector representing the direction of incoming light
• vvv: The direction toward the viewer (camera)
• hhh: The vector halfway between lll and vvv (as in the Blinn-Phong model)

A popular mathematical model for describing how light scatters from a surface is called the reflectance equation (a special case of the rendering equation), and has the following form:

L0(v)=∫Ωf(l,v)Li(l)(n⋅l)dωiL_0(v)=\int_{\Omega}f(l,v)L_i(l)(n\cdot l)d\omega_iL0​(v)=∫Ω​f(l,v)Li​(l)(n⋅l)dωi​

This integral might look a bit scary, but basically, it means the following. The amount of outgoing radiance from a surface (LoL_oLo​) toward the viewer (vvv), is equal to the integral (think weighted-sum) of the BRDF (fff) times the amount of incoming radiance (LiL_iLi​). The integral is over the hemisphere above the surface, for all incoming light directions (lll) within that hemisphere weighted by a cosine factor (n⋅ln\cdot ln⋅l). This cosine factor is a weight that essentially represents the geometry of the situation. The more directly that the light hits the surface, the higher it is weighted. A more complete derivation of this equation is available in several texts. In this recipe, we'll simplify this integral to a simple sum, assuming that the only sources of incoming radiance are point light sources. This is of course a huge simplification, we'll consider some techniques to evaluate the integral more accurately later.

The most significant term for us is the BRDF term (fff), which stands for bidirectional reflectance distribution function. It represents the fraction of radiance that is reflected from a surface point, given the incoming direction (lll) and the outgoing direction (vvv). Its value is a spectral value (R,G,B), with components ranging from 0 to 1. In this recipe, we'll model the BRDF as a sum of two parts: the diffuse BRDF and the specular BRDF:

f(l,v)=fd+fsf(l,v)=f_d+f_sf(l,v)=fd​+fs​

The diffuse BRDF represents light that is absorbed into the surface slightly and then is re-radiated. It is common to model this term so that the radiated light has no preferred direction. It is radiated equally in all outgoing directions. This is also called Lambertian reflectance. Since it has no dependence on the incoming or outgoing directions, the Lambertian BRDF is simply a constant value:

fd=cdiffπf_d=\frac{c_{diff}}{\pi}fd​=πcdiff​​

The cdiffc_{diff}cdiff​ term represents the fraction of light that is diffusely radiated. It is commonly considered the diffuse color of the object.

The specular term represents surface reflectance. Light that is reflected directly off the surface of the object without being absorbed. This is also sometimes called glossy reflectance. A common way to model this reflectance is based on microfacet theory. This theory was developed to describe reflection from general, non-optically flat surfaces. It models the surface as consisting of small facets that are optically flat (mirrors) and are oriented in various directions. Only those that are oriented correctly to reflect toward the viewer can contribute to the BRDF.

We represent this BRDF as a product of three terms and a correction factor (the denominator):

fs=F(l,h)G(l,v,h)D(h)4(n⋅l)(n⋅v)f_s=\frac{F(l,h)G(l,v,h)D(h)}{4(n\cdot l)(n\cdot v)}fs​=4(n⋅l)(n⋅v)F(l,h)G(l,v,h)D(h)​

The FFF term represents Fresnel reflection, the fraction of light reflected from an optically flat surface. The Fresnel reflectance depends on the angle between the normal and the direction of incoming light (angle of incidence). However, since we are using microfacet theory, the microfacet surfaces that contribute are the ones that have their normal vector parallel to the halfway vector (hhh). Therefore, we use the angle between lll and hhh instead of the angle between lll and nnn.

The Fresnel reflectance also depends on the index of refraction of the surface. However, we'll use an approximation that instead uses a different parameter. It is known as the Schlick approximation:

F(l,h)=F0+(1−F0)(1−(l⋅h))5F(l,h)=F_0+(1-F_0)(1-(l\cdot h))^5F(l,h)=F0​+(1−F0​)(1−(l⋅h))5

Rather than using the index of refraction, this approximation uses F0F_0F0​, the characteristic specular reflectance of the material. Or in other words, the reflectance when the angle of incidence is zero degrees. This term is useful in that it can be used as a specular "color", which is somewhat more intuitive and natural for artists.

To further understand this F0F_0F0​ term, let's consider values for common materials. It turns out that a material's optical properties are closely tied to its electrical properties. It is therefore helpful to divide materials into three categories: dielectrics (insulators), metals (conductors), and semiconductors. Our model will ignore the third category and focus on the first two. Metals generally do not exhibit any diffuse reflection, because any light that is refracted into the surface is completely absorbed by the free electrons. The value for F0F_0F0​ is much larger for metals than for dielectrics. In fact, dielectrics have very low values for F0F_0F0​, usually in the range of 0.05 (for all RGB components). This leads us to the following technique.

We'll associate a color with a material. If the material is a metal, there's no diffuse reflection, so we set cdiffc_{diff}cdiff​ to (0,0,0), and use the color as the value for F0F_0F0​ in the Fresnel term. If the material is a dielectric, we set F0F_0F0​ to some small value (we'll use (0.04, 0.04, 0.04)), and use the color as the value for cdiffc_{diff}cdiff​. Essentially, we use two slightly different models for metals and dielectrics, switching between the two as needed. Rather than using the same model for both metals and non-metals and tweaking parameters to represent each, we separate them into two different categories each with a slightly different BRDF model. The popular term for this is metalness workflow.

Next, let's consider the DDD term in the specular BRDF. This is the microgeometry normal distribution function (or microfacet distribution function). It describes the statistical distribution of microsurface orientations. It has a scalar value, and gives the relative concentration of microfacet normals in the direction h. This term has a strong effect on the size and shape of the specular highlight. There are many choices for this function, and several have been developed in recent years based on physical measurements. We'll use a popular one from graphics researchers Trowbridge and Reitz, which was also given the name GGX by a separate research team:

D(h)=α2π((n⋅h)2(α2−1)+1)2D(h)=\frac{\alpha^2}{\pi((n\cdot h)^2(\alpha^2-1) + 1)^2}D(h)=π((n⋅h)2(α2−1)+1)2α2​

In this equation, α\alphaα is a term that represents the roughness of the surface. Following the lead of others, we'll use a roughness parameter rrr, and set α\alphaα to r2r^2r2.

Finally, we'll consider the GGG term in the specular BRDF. This is the geometry function and describes the probability that microsurfaces with a given normal will be visible from both the light direction (lll) and the view direction (vvv). Its value is a scalar between 0 and 1. It is essential for energy conservation. We'll use the following model for GGG:

G(l,v,h)=Gl(l)Gl(v)G(l,v,h)=G_l(l)G_l(v)G(l,v,h)=Gl​(l)Gl​(v)

Where:

Gl(v)=n⋅v(n⋅v)(1−k)+kG_l(v)=\frac{n\cdot v}{(n\cdot v)(1-k)+k}Gl​(v)=(n⋅v)(1−k)+kn⋅v​

The constant kkk is a value that is proportional to the roughness. Again, following the lead of others (see the following See also), we'll use the following for kkk:

k=(r+1)28k=\frac{(r+1)^2}{8}k=8(r+1)2​

Putting all of this together, we now have a complete representation for our BRDF. Before jumping into the code, let's revisit the reflectance equation. This is the first equation we discussed, containing an integral over all directions over the hemisphere above the surface. It would be too costly to try to evaluate this integral in an interactive application, so we'll simplify it by making the assumption that all incoming light comes directly from point light sources. If we do so, the integral reduces to the following sum:

L0(v)=π∑i=1NLif(li,v)(n⋅li)L_0(v)=\pi\sum_{i=1}^{N}L_if(l_i,v)(n\cdot l_i)L0​(v)=π∑i=1N​Li​f(li​,v)(n⋅li​)

Where NNN is the number of point light sources, LiL_iLi​ is the illumination received at the surface due to ithi^{th}ith light source and lil_ili​ is the direction toward the ithi^{th}ith light source. Since the intensity of light decreases with distance, we'll use an inverse-square relationship. However, other models could be used here:

Li=Iidi2L_i=\frac{I_i}{d_i^2}Li​=di2​Ii​​

IiI_iIi​ is the intensity of the source and did_idi​ is the distance from the surface point to the light source.

We now have a complete microfacet-based model that can be applied for metallic surfaces and dielectrics. As we covered earlier, we'll modify the BRDF slightly depending on whether we are working with a metal or a dielectric. The number of parameters to this BRDF is relatively small. The following parameters will define a material:

• The surface roughness (rrr), a value between 0 and 1
• Whether or not the material is metallic (Boolean)
• A color which is interpreted as the diffuse color for dielectrics, or the characteristic specular reflectance (F0F_0F0​) for metals

These parameters are quite intuitive and understandable, as opposed to the many parameters in the Blinn-Phong model from the previous recipe. There's just one color, and roughness is a more intuitive concept than the specular exponent.

Getting ready

We'll set up our shader by starting with the shader pair from the Blinn-Phong recipe, but we'll change the fragment shader. Let's set up some uniforms for the light and material information.

For the light sources, we just need a position and an intensity:

uniform struct LightInfo { vec4 Position; vec3 L; } Light[3];

For materials, we need the three values mentioned previously:

uniform struct MaterialInfo { float Rough; bool Metal; vec3 Color; } Material;

How to do it...

We'll define a function for each of the three terms in the specular BRDF. Use the following steps:

1. Define a function for the Fresnel term using the Schlick approximation:

    vec3 schlickFresnel(float lDotH) { vec3 f0 = vec3(0.04); if (Material.Metal) { f0 = Material.Color; } return f0 + (1 - f0) * pow(1.0 - lDotH, 5);
    }

2. Define a function for the geometry term GGG:

    float geomSmith(float dotProd) { float k = (Material.Rough + 1.0) * (Material.Rough + 1.0) / 8.0; float denom = dotProd * (1 - k) + k; return 1.0 / denom;
    }

3. The normal distribution function DDD, based on GGX/Trowbridge-Reitz:

    float ggxDistribution(float nDotH) { float alpha2 = Material.Rough * Material.Rough * Material.Rough * Material.Rough; float d = (nDotH * nDotH) * (alpha2 - 1) + 1; return alpha2 / (PI * d * d);
    }

4. We'll now define a function that computes the entire model for a single light source:

    vec3 microfacetModel(int lightIdx, vec3 position, vec3 n) { vec3 diffuseBrdf = vec3(0.0); if (!Material.Metal) { diffuseBrdf = Material.Color; } vec3 l = vec3(0.0), lightI = Light[lightIdx].L; if (Light[lightIdx].Position.w == 0.0) { l = normalize(Light[lightIdx].Position.xyz); } else { l = Light[lightIdx].Position.xyz - position; float dist = length(l); l = normalize(l); lightI /= (dist * dist); } vec3 v = normalize( -position ); vec3 h = normalize( v + l ); float nDotH = dot( n, h ); float lDotH = dot( l, h ); float nDotL = max( dot( n, l ), 0.0 ); float nDotV = dot( n, v ); vec3 specBrdf = 0.25 * ggxDistribution(nDotH) * schlickFresnel(lDotH) * geomSmith(nDotL) * geomSmith(nDotV); return (diffuseBrdf + PI * specBrdf) * lightI * nDotL;
    }

5. We put this all together by summing over the light sources, applying Gamma correction, and writing out the result:

    void main() { vec3 sum = vec3(0), n = normalize(Normal); for (int i = 0; i < 3; i++) { sum += microfacetModel(i, Position, n); } FragColor = vec4(sum, 1);
    }

How it works...

The schlickFresnel function computes the value of FFF. If the material is a metal, the value for F0F_0F0​ is taken from the value of Material.Color. Otherwise, we simply use (0.04, 0.04, 0.04). Since most dielectrics have similar small values for F0, this is a relatively good approximation for common dielectrics.

The geomSmith and ggxDistribution functions are straightforward implementations of the equations described previously. However, in geomSmith, we omit the numerator. This is due to the fact that it will cancel with the denominator of the overall specular BRDF.

The microfacetModel function computes the BRDF. The diffuse term is set to 0 if the material is metallic, otherwise, it is set to the value of the material's color. Note that we omit the factor of π here. This is due to the fact that it will cancel with the π\piπ term in the overall sum (the last summation equation), so no need to include it here.

Next, we determine the Li term (lightI) and the vector lll, depending on whether it is a directional light or a positional one. If it is directional, lightI is just the value of Light[lightIdx].L, otherwise, it is scaled by the inverse square of the distance to the light source.

Then, we calculate the specular BRDF (specBrdf), using the functions we defined previously. Note that (as mentioned previously) we omit the denominator of the BRDF (except for the factor of 0.25) due to the fact that those two dot products cancel with the numerators of the G1G_1G1​ functions.

The final result of this function is the total BRDF times the light intensity, times the dot product of nnn and lll. We only multiply the specular BRDF times π\piπ due to the fact that we omitted the π\piπ term from the diffuse BRDF.

Results for some simple materials are shown in the following image:

The back row shows dielectric (non-metal) materials with increasing roughness from left to right. The front row shows five metallic materials with various values for F0F_0F0​.

There's more...

Rather than making Material.Metal a Boolean value, one could choose to make it a continuous value between 0 and 1. Indeed, this is exactly what some implementations do. The value would then be used to interpolate between the two models (metallic and dielectric). However, that makes the parameter somewhat less intuitive for artists and you may find that the extra configurability may not be all that useful.

Chapter 1. Using Textures

In this chapter, we will cover the following recipes:

• Applying a 2D texture
• Applying multiple textures
• Using alpha maps to discard pixels
• Using normal maps
• Parallax mapping
• Steep parallax mapping with self shadowing
• Simulating reflection with cube maps
• Simulating refraction with cube maps
• Applying a projected texture
• Rendering to a texture
• Using sampler objects
• Diffuse image-based lighting

Introduction

Textures are an important and fundamental aspect of real-time rendering in general, and OpenGL in particular. The use of textures within a shader opens up a huge range of possibilities. Beyond just using textures as sources of color information, they can be used for things like depth information, shading parameters, displacement maps, normal vectors, and other vertex data. The list is virtually endless. Textures are among the most widely used tools for advanced effects in OpenGL programs, and that isn't likely to change any time soon.

In OpenGL 4, we now have the ability to read and write to memory via buffer textures, shader storage buffer objects, and image textures (image load/store). This further muddies the waters of what exactly defines a texture. In general, we might just think of it as a buffer of data that may or may not contain an image.

OpenGL 4.2 introduced immutable storage textures. Despite what the term may imply, immutable storage textures are not textures that can't change. Instead, the term immutable refers to the fact that, once the texture is allocated, the storage cannot be changed. That is, the size, format, and number of layers are fixed, but the texture content itself can be modified. The word immutable refers to the allocation of the memory, not the content of the memory. Immutable storage textures are preferable in the vast majority of cases because of the fact that many runtime (draw-time) consistency checks can be avoided, and you include a certain degree of "type safety" since we can't accidentally change the allocation of a texture. Throughout this book, we'll use immutable storage textures exclusively.

Immutable storage textures are allocated using the glTexStorage* functions. If you're experienced with textures, you might be accustomed to using glTexImage* functions, which are still supported but create mutable storage textures.

In this chapter, we'll look at some basic and advanced texturing techniques. We'll start with the basics by just applying color textures and move on to using textures as normal maps and environment maps. With environment maps, we can simulate things such as reflection and refraction. We'll see an example of projecting a texture onto objects in a scene similar to the way that a slide projector projects an image. Finally, we'll wrap up with an example of rendering directly to a texture using framebuffer objects (FBOs) and then applying that texture to an object.

Applying a 2D texture

In GLSL, applying a texture to a surface involves accessing texture memory to retrieve a color associated with a texture coordinate, and then applying that color to the output fragment. The application of the color to the output fragment could involve mixing the color with the color produced by a shading model, simply applying the color directly, using the color in the reflection model, or some other mixing process. In GLSL, textures are accessed via sampler variables. A sampler variable is a handle to a texture unit. It is typically declared as a uniform variable within the shader and initialized within the main OpenGL application to point to the appropriate texture unit.

In this recipe, we'll look at a simple example involving the application of a 2D texture to a surface, as shown in the following image. We'll use the texture color as the diffuse (and ambient) reflectivity term in the Blinn-Phong reflection model. The following image shows the results of a brick texture applied to a cube. The texture is shown on the right and the rendered result is on the left:

Getting ready

Set up your OpenGL application to provide the vertex position in attribute location 0, the vertex normal in attribute location 1, and the texture coordinate in attribute location 2. The parameters for the Blinn-Phong reflection model are declared again as uniform variables within the shader, and must be initialized from the OpenGL program. Make the handle to the shader available in a variable named programHandle.

How to do it...

To render a simple shape with a 2D texture, use the following steps:

1. We'll define a simple (static) function for loading and initializing textures:

    GLuint Texture::loadTexture( const std::string & fName ) { int width, height; unsigned char * data = Texture::loadPixels(fName, width, height); GLuint tex = 0; if( data != nullptr ) { glGenTextures(1, &tex); glBindTexture(GL_TEXTURE_2D, tex); glTexStorage2D(GL_TEXTURE_2D, 1, GL_RGBA8, width, height); glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, data); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST); Texture::deletePixels(data); } return tex;
    }

2. In the initialization of the OpenGL application, use the following code to load the texture, bind it to texture unit 0, and set the uniform variable Tex1 to that texture unit:

    GLuint tid = Texture::loadTexture("brick1.png");
    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, tid); int loc = glGetUniformLocation(programHandle, "Tex1");
    glUniform1i(loc, 0);

3. The vertex shader passes the texture coordinate to the fragment shader:

    layout (location = 0) in vec3 VertexPosition; layout (location = 1) in vec3 VertexNormal; layout (location = 2) in vec2 VertexTexCoord; out vec3 Position; out vec3 Normal; out vec2 TexCoord; void main()
    { TexCoord = VertexTexCoord; }

4. The fragment shader looks up the texture value and applies it to the diffuse reflectivity in the Blinn-Phong model:

    in vec3 Position; in vec3 Normal; in vec2 TexCoord; uniform sampler2D Tex1; void blinnPhong( vec3 pos, vec3 n ) { vec3 texColor = texture(Tex1, TexCoord).rgb; vec3 ambient = Light.La * texColor; vec3 diffuse = texColor * sDotN; return ambient + Light.L * (diffuse + spec);
    } void main() { FragColor = vec4( blinnPhong(Position, normalize(Normal) ), 1 );
    }

How it works...

The first code segment defines a simple function that loads the texture from a file, copies the texture data to OpenGL memory, and sets up the mag and min filters. It returns the texture ID. The first step, loading the texture image file, is accomplished by calling another method (Texture::loadPixels), which uses an image loader that is provided along with the example code. The loader comes from a header file stb_image.h, available on GitHub (https://github.com/nothings/stb). It reads the image and stores the pixel data into an array of unsigned bytes in RGBA order. The width and height of the image are returned via the last two parameters. We keep a pointer to the image data, simply named data.

The next two lines involve creating a new texture object by calling glGenTextures. The handle for the new texture object is stored in the tex variable.

To load and configure the texture object, we do the following.

1. We call glBindTexture to bind the new texture object to the GL_TEXTURE_2D target.
2. Once the texture is bound to that target, we allocate immutable storage for the texture with glTexStorage2D.
3. After that, we copy the data for that texture into the texture object using glTexSubImage2D. The last argument to this function is a pointer to the raw data for the image.
4. The next steps involve setting the magnification and minification filters for the texture object using glTexParameteri. For this example, we'll use GL_LINEAR for the former and GL_NEAREST for the latter.

The texture filter setting determines whether any interpolation will be done prior to returning the color from the texture. This setting can have a strong effect on the quality of the results. In this example, GL_LINEAR indicates that it will return a weighted average of the four texels that are nearest to the texture coordinates. For details on the other filtering options, see the OpenGL documentation for glTexParameteri: http://www.opengl.org/wiki/GLAPI/glTexParam eter.

Next, we delete the texture data pointed to by data. There's no need to hang on to this because it was copied into texture memory via glTexSubImage2D. To do so, we call the Texture::deletePixels function. (Internally, that calls the function provided by the stb_image library stbi_image_free.) Then, we return the ID of the new texture object.

In the next code segment, we call our Texture::loadTexture function to load the texture, then we bind the texture to texture unit 0. To do so, first we call glActiveTexture to set the current active texture unit to GL_TEXTURE0 (the first texture unit, also called a texture channel). The subsequent texture state calls will be effective on texture unit zero. Then, we bind the new texture to that unit using glBindTexture. Finally, we set the uniform variable Tex1 in the GLSL program to zero. This is our sampler variable. Note that it is declared within the fragment shader with type sampler2D. Setting its value to zero indicates to the OpenGL system that the variable should refer to texture unit zero (the same one selected previously with glActiveTexture).

The vertex shader is very similar to the one used in the previous examples except for the addition of the texture coordinate input variable VertexTexCoord, which is bound to attribute location 2. Its value is simply passed along to the fragment shader by assigning it to the shader output variable TexCoord.

The fragment shader is also very similar to those used in the recipes of previous chapters. The primary changes are the Tex1 uniform variable and the blinnPhong function. Tex1 is a sampler2D variable that was assigned by the OpenGL program to refer to texture unit zero. In the blinnPhong function, we use that variable along with the texture coordinate (TexCoord) to access the texture. We do so by calling the built-in function texture. This is a general purpose function, which is used to access a variety of different textures. The first parameter is a sampler variable indicating which texture unit is to be accessed, and the second parameter is the texture coordinate used to access the texture. The return value is a vec4 containing the color obtained by the texture access. We select only the first three components (.rgb) and store them in texColor. Then, we use texColor as the ambient and diffuse reflectivity terms in the Blinn-Phong model.
When using a texture for both ambient and diffuse reflectivity, it is important to set the ambient light intensity to a small value, in order to avoid wash-out.

There's more...

There are several choices that could be made when deciding how to combine the texture color with other colors associated with the fragment. In this example, we used the texture color as the ambient and diffuse reflectivity, but one could have chosen to use the texture color directly or to mix it with the reflection model in some way. There are endless options—the choice is up to you!

Specifying the sampler binding within GLSL

As of OpenGL 4.2, we now have the ability to specify the default value of the sampler's binding (the value of the sampler uniform) within GLSL. In the previous example, we set the value of the uniform variable from the OpenGL side using the following code:

int loc = glGetUniformLocation(programHandle, "Tex1");
glUniform1i(loc, 0);

Instead, if we're using OpenGL 4.2, we can specify the default value within the shader using the layout qualifier, as shown in the following statement:

layout (binding=0) uniform sampler2D Tex1;

This simplifies the code on the OpenGL side and makes this one less thing we need to worry about. The example code that accompanies this book uses this technique to specify the value of Tex1, so take a look there for a more complete example. We'll also use this layout qualifier in the following recipes.

Applying multiple textures

The application of multiple textures to a surface can be used to create a wide variety of effects. The base layer texture might represent the clean surface and the second layer could provide additional detail such as shadow, blemishes, roughness, or damage. In many games, so-called light maps are applied as an additional texture layer to provide information about light exposure, effectively producing shadows, and shading without the need to explicitly calculate the reflection model. These kinds of textures are sometimes referred to as pre-baked lighting. In this recipe, we'll demonstrate this multiple texture technique by applying two layers of texture. The base layer will be a fully opaque brick image, and the second layer will be one that is partially transparent. The non-transparent parts look like moss that has grown on the bricks beneath.

The following image shows an example of multiple textures. The textures on the left are applied to the cube on the right. The base layer is the brick texture, and the moss texture is applied on top. The transparent parts of the moss texture reveal the brick texture underneath:

Getting ready

We'll start with the shaders developed in the previous recipe, Applying a 2D texture, as well as the Texture::loadTexture function described there.

How to do it...

1. In the initialization section of your OpenGL program, load the two images into texture memory in the same way as indicated in the previous recipe, Applying a 2D texture. Make sure that the brick texture is loaded into texture unit 0 and the moss texture in texture unit 1:

    GLuint brick = Texture::loadTexture("brick1.jpg");
    GLuint moss = Texture::loadTexture("moss.png"); glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, brick); glActiveTexture(GL_TEXTURE1);
    glBindTexture(GL_TEXTURE_2D, moss);

2. Starting with the fragment shader from the recipe Applying a 2D texture, replace the declaration of the sampler variable Tex1 with the following code:

    layout(binding=0) uniform sampler2D BrickTex; layout(binding=1) uniform sampler2D MossTex;

3. In the blinnPhong function, get samples from both textures and mix them together. Then, apply the mixed color to both the ambient and diffuse reflectivity:

    vec4 brickTexColor = texture( BrickTex, TexCoord ); vec4 mossTexColor = texture( MossTex, TexCoord ); vec3 col = mix(brickTexColor.rgb, mossTexColor.rgb, mossTexColor.a); vec3 ambient = Light.La * col;

How it works...

The preceding code that loads the two textures into the OpenGL program is very similar to the code from the previous recipe, Applying a 2D texture. The main difference is that we load each texture into a different texture unit. When loading the brick texture, we set the OpenGL state such that the active texture unit is unit zero:

glActiveTexture(GL_TEXTURE0);

And when loading the second texture, we set the OpenGL state to texture unit one:

glActiveTexture(GL_TEXTURE1);

In the fragment shader, we specify the texture binding for each sampler variable using the layout qualifier corresponding to the appropriate texture unit. We access the two textures using the corresponding uniform variables, and store the results in brickTexColor and mossTexColor. The two colors are blended together using the built-in function mix. The third parameter to the mix function is the percentage used when mixing the two colors. We use the alpha value of the moss texture for that parameter. This causes the result to be a linear interpolation of the two colors based on the value of the alpha in the moss texture. For those familiar with OpenGL blending functions, this is the same as the following blending function:

glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA );

In this case, the color of the moss would be the source color, and the color of the brick would be the destination color. Finally, we use the result of the mix function as the ambient and diffuse reflectivities in the Blinn-Phong reflection model.

There's more...

In this example, we mixed the two texture colors together using the alpha value of the second texture. This is just one of many options for mixing the texture colors. There are a number of different choices here, and your choice will be dependent on the kind of texture data available and the desired effect. A popular technique is to use an additional vertex attribute to augment the amount of blending between the textures. This additional vertex attribute would allow us to vary the blending factor throughout a model. For example, we could vary the amount of moss that grows on a surface by defining another vertex attribute that would control the amount of blending between the moss texture and the base texture. A value of zero might correspond to zero moss, up to a value of one, which would enable blending based on the texture's alpha value alone.

Using alpha maps to discard pixels

To create the effect of an object that has holes, we could use a texture with an appropriate alpha channel that contains information about the transparent parts of the object. However, that requires us to make the depth buffer read-only and render all of our polygons from back to front in order to avoid blending problems. We would need to sort our polygons based on the camera position and then render them in the correct order. What a pain! With GLSL shaders, we can avoid all of this by using the discard keyword to completely discard fragments when the alpha value of the texture map is below a certain value. By completely discarding the fragments, there's no need to modify the depth buffer because when discarded, they aren't evaluated against the depth buffer at all. We don't need to depth-sort our polygons because there is no blending.

The following image on the right shows a teapot with fragments discarded based upon the texture on the left. The fragment shader discards fragments that correspond to texels that have an alpha value below a certain threshold:

If we create a texture map that has an alpha channel, we can use the value of the alpha channel to determine whether or not the fragment should be discarded. If the alpha value is below a certain value, then the pixel is discarded.

As this will allow the viewer to see within the object, possibly making some back faces visible, we'll need to use two-sided lighting when rendering the object.

Getting ready

Start with the same shader pair and setup from the previous recipe, Applying a 2D texture. Load the base texture for the object into texture unit 0, and your alpha map into texture unit 1.

How to do it...

To discard fragments based on alpha data from a texture, use the following steps:

1. Use the same vertex and fragment shaders from the Applying a 2D texture recipe. However, make the following modifications to the fragment shader.

2. Replace the sampler2D uniform variable with the following:

    layout(binding=0) uniform sampler2D BaseTex; layout(binding=1) uniform sampler2D AlphaTex;

3. In the blinnPhong function, use BaseTex to look up the value of the ambient and diffuse reflectivity.

4. Replace the contents of the main function with the following code:

    void main() { vec4 alphaMap = texture( AlphaTex, TexCoord ); if(alphaMap.a < 0.15 ) discard; else { if( gl_FrontFacing ) { FragColor = vec4( blinnPhong(Position,normalize(Normal)), 1.0 ); } else { FragColor = vec4( blinnPhong(Position,normalize(-Normal)), 1.0 ); } }
    }

How it works...

Within the main function of the fragment shader, we access the alpha map texture and store the result in alphaMap. If the alpha component of alphaMap is less than a certain value (0.15, in this example), then we discard the fragment using the discard keyword.

Otherwise, we compute the Blinn-Phong lighting model using the normal vector oriented appropriately, depending on whether or not the fragment is a front facing fragment.

There's more...

This technique is fairly simple and straightforward, and is a nice alternative to traditional blending techniques. It is a great way to make holes in objects or to present the appearance of decay. If your alpha map has a gradual change in the alpha throughout the map (for example, an alpha map where the alpha values make a smoothly varying height field), then it can be used to animate the decay of an object. We could vary the alpha threshold (0.15, in the preceding example) from 0.0 to 1.0 to create an animated effect of the object gradually decaying away to nothing.

Using normal maps

Normal mapping is a technique for "faking" variations in a surface that doesn't really exist in the geometry of the surface. It is useful for producing surfaces that have bumps, dents, roughness, or wrinkles without actually providing enough position information (vertices) to fully define those deformations. The underlying surface is actually smooth, but is made to appear rough by varying the normal vectors using a texture (the normal map). The technique is closely related to bump mapping or displacement mapping. With normal maps, we modify the normal vectors based on information that is stored in a texture. This creates the appearance of a bumpy surface without actually providing the geometry of the bumps.

A normal map is a texture in which the data stored within the texture is interpreted as normal vectors instead of colors. The normal vectors are typically encoded into the RGB information of the normal map so that the red channel contains the x coordinate, the green channel contains the y coordinate, and the blue channel contains the z coordinate. The normal map can then be used as a texture in the sense that the texture values affect the normal vector used in the reflection model rather than the color of the surface. This can be used to make a surface look like it contains variations (bumps or wrinkles) that do not actually exist in the geometry of the mesh.
The following images show an ogre mesh (courtesy of Keenan Crane) with and without a normal map. The upper-left corner shows the base color texture for the ogre. In this example, we use this texture as the diffuse reflectivity in the Phong reflection model. The upper right shows the ogre with the color texture and default normal vectors. The bottom left is the normal map texture. The bottom right shows the ogre with the color texture and normal map. Note the additional detail in the wrinkles provided by the normal map:

A normal map can be produced in a number of ways. Many 3D modeling programs such as Maya, Blender, or 3D Studio Max can generate normal maps. Normal maps can also be generated directly from grayscale hightmap textures. There is a NVIDIA plugin for Adobe Photoshop that provides this functionality (see http://developer.nvidia.com/object/photoshop_dds_plugins.html).

Normal maps are interpreted as vectors in a tangent space (also called the object local coordinate system). In the tangent coordinate system, the origin is located at the surface point and the normal to the surface is aligned with the z axis (0, 0, 1). Therefore, the x and y axes are at a tangent to the surface. The following image shows an example of the tangent frames at two different positions on a surface:

The advantage of using such a coordinate system lies in the fact that the normal vectors stored within the normal map can be treated as perturbations to the true normal, and are independent of the object coordinate system. This avoids the need to transform the normals, add the perturbed normal, and renormalize. Instead, we can use the value in the normal map directly in the reflection model without any modification.

To make all of this work, we need to evaluate the reflection model in tangent space. In order to do so, we transform the vectors used in our reflection model into tangent space in the vertex shader, and then pass them along to the fragment shader where the reflection model will be evaluated. To define a transformation from the camera (eye) coordinate system to the tangent space coordinate system, we need three normalized, co-orthogonal vectors (defined in eye coordinates) that define the tangent space system. The z axis is defined by the normal vector (nnn), the x axis is defined by a vector called the tangent vector (ttt), and the y axis is often called the binormal vector (bbb). A point, PPP, defined in camera coordinates, could then be transformed into tangent space in the following way:

[SxSySz]=[txtytzbxbybznxnynz][PxPyPz]\begin{bmatrix}S_x\\ S_y \\ S_z\end{bmatrix}=\begin{bmatrix}t_x&t_y&t_z\\ b_x&b_y&b_z \\ n_x&n_y&n_z\end{bmatrix}\begin{bmatrix}P_x\\ P_y \\ P_z\end{bmatrix}​Sx​Sy​Sz​​​=​tx​bx​nx​​ty​by​ny​​tz​bz​nz​​​​Px​Py​Pz​​​

In the preceding equation, $SisthepointintangentspaceandS is the point in tangent space andSisthepointintangentspaceandPisthepointincameracoordinates.Inordertoapplythistransformationwithinthevertexshader,theOpenGLprogrammustprovideatleasttwoofthethreevectorsthatdefinetheobjectlocalsystemalongwiththevertexposition.Theusualsituationistoprovidethenormalvector(is the point in camera coordinates. In order to apply this transformation within the vertex shader, the OpenGL program must provide at least two of the three vectors that define the object local system along with the vertex position. The usual situation is to provide the normal vector (isthepointincameracoordinates.Inordertoapplythistransformationwithinthevertexshader,theOpenGLprogrammustprovideatleasttwoofthethreevectorsthatdefinetheobjectlocalsystemalongwiththevertexposition.Theusualsituationistoprovidethenormalvector(n)andthetangentvector() and the tangent vector ()andthetangentvector(t$). If the tangent vector is provided, the binormal vector can be computed as the cross product of the tangent and normal vectors.

Tangent vectors are sometimes included as additional data in mesh data structures. If the tangent data is not available, we can approximate the tangent vectors by deriving them from the variation of the texture coordinates across the surface (see Computing Tangent Space Basis Vectors for an Arbitrary Mesh, Eric Lengyel, Terathon Software 3D Graphics Library, 2001, at http://www.terathon.com/code/tangent.html).

One must take care that the tangent vectors are consistently defined across the surface. In other words, the direction of the tangent vectors should not vary greatly from one vertex to its neighboring vertex. Otherwise, it can lead to ugly shading artifacts.

In the following example, we'll read the vertex position, normal vector, tangent vector, and texture coordinate in the vertex shader. We'll transform the position, normal, and tangent to camera space, and then compute the binormal vector (in camera space). Next, we'll compute the viewing direction (vvv) and the direction toward the light source (sss) and then transform them to tangent space. We'll pass the tangent space vvv and sss vectors and the (unchanged) texture coordinate to the fragment shader, where we'll evaluate the Blinn-Phong reflection model using the tangent space vectors and the normal vector retrieved from the normal map.

Getting ready

Set up your OpenGL program to provide the position in attribute location 0, the normal in attribute location 1, the texture coordinate in location 2, and the tangent vector in location 1. For this example, the fourth coordinate of the tangent vector should contain the handedness of the tangent coordinate system (either -1 or +1). This value will be multiplied by the result of the cross product.

Load the normal map into texture unit one and the color texture into texture unit zero.

How to do it...

To render an image using normal mapping, use the following shaders:

1. In the vertex shader, find the object local coordinate system (tangent space) and transform everything into that space. Pass the tangent space light direction and view direction to the fragment shader:

    layout (location = 0) in vec3 VertexPosition; layout (location = 1) in vec3 VertexNormal; layout (location = 2) in vec2 VertexTexCoord; layout (location = 3) in vec4 VertexTangent; out vec3 LightDir; out vec2 TexCoord; out vec3 ViewDir; void main() { vec3 norm = normalize(NormalMatrix * VertexNormal); vec3 tang = normalize(NormalMatrix * VertexTangent.xyz); vec3 binormal = normalize( cross( norm, tang ) ) * VertexTangent.w; mat3 toObjectLocal = mat3( tang.x, binormal.x, norm.x, tang.y, binormal.y, norm.y, tang.z, binormal.z, norm.z ) ; vec3 pos = vec3( ModelViewMatrix * vec4(VertexPosition,1.0) ); LightDir = toObjectLocal * (Light.Position.xyz - pos); ViewDir = toObjectLocal * normalize(-pos); TexCoord = VertexTexCoord; gl_Position = MVP * vec4(VertexPosition,1.0);
    }

2. In the fragment shader, update the blinnPhong function to use the normal from the texture and to use the input variables for the light and view directions:

    in vec3 LightDir; in vec2 TexCoord; in vec3 ViewDir; layout(binding=0) uniform sampler2D ColorTex; layout(binding=1) uniform sampler2D NormalMapTex; layout( location = 0 ) out vec4 FragColor; vec3 blinnPhong( vec3 n ) { } void main() { vec3 norm = texture(NormalMapTex, TexCoord).xyz; norm = 2.0 * norm - 1.0; FragColor = vec4( blinnPhong(norm), 1.0 );
    }

How it works...

The vertex shader starts by transforming the vertex normal and the tangent vectors into eye coordinates by multiplying by the normal matrix (and renormalizing). The binormal vector is then computed as the cross product of the normal and tangent vectors. The result is multiplied by the w coordinate of the vertex tangent vector, which determines the handedness of the tangent space coordinate system. Its value will be either -1 or +1.

Next, we create the transformation matrix used to convert from eye coordinates to tangent space and store the matrix in toObjectLocal. The position is converted to eye space and stored in pos, and we compute the light direction by subtracting pos from the light position. The result is multiplied by toObjectLocal to convert it into tangent space, and the final result is stored in the output variable LightDir. This value is the direction to the light source in tangent space and will be used by the fragment shader in the reflection model.

Similarly, the view direction is computed and converted to tangent space by normalizing pos and multiplying by toObjectLocal. The result is stored in the output variable ViewDir.

The texture coordinate is passed to the fragment shader unchanged by just assigning it to the output variable TexCoord.

In the fragment shader, the tangent space values for the light direction and view direction are received in the variables LightDir and ViewDir. The blinnPhong function is slightly modified from what has been used in previous recipes. The only parameter is the normal vector. The function computes the Blinn-Phong reflection model, taking the value for the diffuse reflectivity from the texture ColorTex, and uses LightDir and ViewDir for the light and view directions rather than computing them.

In the main function, the normal vector is retrieved from the normal map texture and stored in the variable normal. Since textures store values that range from zero to one and normal vectors have components that range from -1 to +1, we need to re-scale the value to that range. We do so by multiplying by 2.0 and then subtracting 1.0.

For some normal maps, the z coordinate is never negative because in tangent space that would correspond to a normal that points into the surface. In which case, we could assume that z ranges from 0 to 1, and use the full resolution of the channel for that range. However, there is no standard convention for the z coordinate.

Finally, the blinnPhong function is called and is passed the normal. The blinnPhong function evaluates the reflection model using LightDir, ViewDir, and n, all of which are defined in tangent space. The result is applied to the output fragment by assigning it to FragColor.

Parallax mapping

Normal maps are a great way to introduce surface detail without adding additional geometry. However, they have some limitations. For example, normal maps do not provide parallax effects as the viewer's position changes and they don't support self-occlusion. Parallax mapping is a technique, originally introduced in 2001, that uses modification of texture coordinates based on a height map to simulate parallax and self-occlusion effects. It requires both a normal map and a height map. A height map (also called a bump map) is a grayscale image where each texel has a single scalar value representing the height of the surface at the texel. We can consider any height between 0 and 1 as the true surface, and then use the value in the height map as an offset from there. In this recipe, we'll use a value of 1.0 as the true surface, so a height map value of 0.0 is a distance of 1.0 below the true surface (see the following images).

To simulate parallax, we want to offset the texture coordinates by an amount that depends on the direction toward the viewer (camera). A parallax effect is stronger at steeper angles, so we want the offset amount to be stronger when the angle between the normal and the view vector (vector pointing toward the camera) is larger. In addition, we want to offset the texture coordinates in the same direction as the view vector. Similar to the normal mapping recipe, we'll work with tangent space.

As we discussed earlier, in tangent space, the normal vector is the same as the z axis. If e is the vector toward the camera in tangent space, we'll use the vector pointing in the opposite direction (v=−ev = -ev=−e). First, let's consider the case of standard normal mapping. The viewer perceives the color and normal at point PPP, but they should see the color and normal at point QQQ:

Therefore, we want to offset the texture coordinates by an amount that is proportional to Δx\Delta xΔx in the preceding diagram so that the viewer sees the shading for point QQQ, not point PPP. You could draw a similar picture for the y−zy-zy−z cross-section as well, the results would be nearly the same.

So, what we need to do is approximate Δx\Delta xΔx somehow. Consider the right triangles, as shown here:

The value of ddd is the depth of point QQQ (below the true surface), or in other words: d=1−hqd = 1 - h_qd=1−hq​, where hqh_qhq​ is the height of the bump map at point QQQ. By the rule of similar triangles, we can write the following:

Δxd=vxvz\frac{\Delta x}{d}=\frac{v_x}{v_z}dΔx​=vz​vx​​

Applying the same analysis for yyy, we get the following pair of offsets:

Δx=dvxvzΔy=dvyvz\begin{aligned} \Delta x & = \frac{dv_x}{v_z} \\ \Delta y & = \frac{dv_y}{v_z} \end{aligned}ΔxΔy​=vz​dvx​​=vz​dvy​​​

Unfortunately, we don't have a value for ddd in the preceding equations, because we don't know the value for QQQ. There's no way of quickly finding it either; we'd need to trace a ray through the height map (which is what we'll do in the next recipe). So for now, we'll just approximate ddd by using the height (depth) at P(1−hp)P (1 - h_p)P(1−hp​). It is a rough estimate, but if we assume the height map doesn't have a lot of really high frequency variation, it works fairly well in practice.

Therefore, we have the following equation for offsetting a texture coordinate (PPP) at a given surface point:

P′=P+S(1−hp)(vxvz,vyvz)P^{'}=P+S(1-h_p)(\frac{v_x}{v_z}, \frac{v_y}{v_z})P′=P+S(1−hp​)(vz​vx​​,vz​vy​​)

In the preceding equation, SSS is a scale factor that can be used to restrict the magnitude of the effect and to scale it to texture space. It is usually a very small value (between 0 and 0.05), and may need to be tuned to a particular surface.

The following images show the effect compared to basic normal mapping. On the left, a single quad rendered with simple normal mapping, and on the right is the same geometry using normal mapping, along with parallax mapping:

The effect is admittedly quite subtle, and there are some undesirable artifacts in this example, but the overall effect is clear. Note that both images use the same geometry, camera position, and texture maps. If you focus on the bricks in the distance (furthest from the viewer), you can see some simulation of occlusion, and overall the effect is more realistic on the right.

Getting ready

For parallax mapping, we need three textures: a height map texture, a normal map texture, and a color texture. We could combine the height map and normal map into a single texture, storing the height values in the alpha channel and the normal in the R, G, and B. This is a common technique and saves a significant amount of disk and memory space. In this recipe, we'll treat them as separate textures.

We also need a mesh with tangent vectors as well so that we can transform into tangent space. For more information on tangent space, see the previous recipe.

How to do it...

We can use the same vertex shader as was used in the previous recipe, Using normal maps.
The vertex shader transforms the view direction and the light direction and passes them to the fragment shader. It also passes along the texture coordinate.

The fragment shader uses the tangent space view direction and the height map value at the current texture coordinate to offset the texture coordinates. It then uses the new texture coordinate value to do shading as usual:

in vec3 LightDir; in vec2 TexCoord; in vec3 ViewDir; layout(binding=0) uniform sampler2D ColorTex; layout(binding=1) uniform sampler2D NormalMapTex; layout(binding=2) uniform sampler2D HeightMapTex; uniforms layout( location = 0 ) out vec4 FragColor; vec3 blinnPhong( ) { vec3 v = normalize(ViewDir); vec3 s = normalize(LightDir); const float bumpFactor = 0.015; float height = 1 - texture(HeightMapTex, TexCoord).r; vec2 delta = v.xy * height * bumpFactor / v.z; vec2 tc = TexCoord.xy - delta; vec3 n = texture(NormalMapTex, tc).xyz; n.xy = 2.0 * n.xy - 1.0; n = normalize(n); float sDotN = max( dot(s,n), 0.0 ); vec3 texColor = texture(ColorTex, tc).rgb; vec3 ambient = Light.La * texColor; vec3 diffuse = texColor * sDotN; vec3 spec = vec3(0.0); if ( sDotN > 0.0 ) { vec3 h = normalize( v + s ); spec = Material.Ks * pow( max( dot(h,n), 0.0 ), Material.Shininess ); } return ambient + Light.L * (diffuse + spec);
}

How it works...

In the blinnPhong method within the fragment shader, we start by computing the offset for the texture coordinate (the delta variable ). The bumpFactor constant is generally somewhere between 0 and 0.05. In this case, we use 0.015, but you'll need to tune this for your particular normal/height map. We offset the texture coordinate by the value of delta. We subtract rather than add here because ViewDir is actually pointing toward the viewer, so we need to offset in the opposite direction. Note that we also invert the height value, as discussed in the preceding analysis. Using the offset texture coordinate (tc), we compute the shading using the Blinn-Phong model with data from the normal map and color texture.

There's more...

Parallax mapping produces subtle but pleasing effects. However, it does suffer from some undesirable artifacts such as so-called texture swim and performs poorly with bump maps that have steep bumps or high frequency bumps. An improvement to parallax mapping that performs better is called steep parallax mapping, which is discussed in the next recipe.

Steep parallax mapping with self shadowing

This recipe builds on the previous one, parallax mapping, so if you haven't already done so, you may want to review that recipe prior to reading this one.

Steep parallax mapping is a technique, first published by Morgan McGuire and Max
McGuire in 2005. It improves upon parallax mapping, producing much better results at the cost of more fragment shader work. Despite the additional cost, the algorithm is still well suited to real-time rendering on modern GPUs.

The technique involves tracing the eye ray through the height map in discrete steps until a collision is found in order to more precisely determine the appropriate offset for the texture coordinate. Let's revisit the diagram from the previous recipe, but this time, we'll break up the height map into n discrete levels (indicated by the dashed lines):

As before, our goal is to offset the texture coordinates so that the surface is shaded based on the bump surface, not the true surface. The point P is the surface point on the polygon being rendered. We trace the view vector from point P to each level consecutively until we find a point that lies on or below the bump surface. In the following image, we'll find the point Q after three iterations.

As in the previous recipe, we can derive the change in x and y for a single iteration using similar triangles (see the Parallax mapping recipe):

P′=P+Sn(vxvz,vyvz)P^{'}=P+\frac{S}{n}(\frac{v_x}{v_z}, \frac{v_y}{v_z})P′=P+nS​(vz​vx​​,vz​vy​​)

As before, the scale factor (SSS) is used to vary the influence of the effect and to scale it to texture space. nnn is the number of height levels.

Using this equation, we can step through the height levels, starting at PPP and following the view vector away from the camera. We continue until we find a point that lies on or below the surface of the height map. We then use the texture coordinate at that point for shading. Essentially, we're implementing a very simple ray marcher in the fragment shader.

The results are impressive. The following image shows three versions of the same surface for comparison. On the left is the surface with normal map applied. The middle image is the same surface rendered with parallax mapping. The right-hand image is generated using steep parallax mapping. All three images use the same normal map, height map, geometry, and camera position. They are all rendered as a single quad (two triangles). Note how the steep parallax shows the varying height of each brick. The height of each brick was always included in the height map, but the parallax mapping technique didn't make it noticeable:

You may have noticed that the image on the right also includes shadows. Some bricks cast shadows onto other bricks. This is accomplished with a simple addition to the preceding technique. Once we find point Q, we march another ray in the direction of the light source. If that ray collides with the surface, the point is in shadow and we shade with ambient lighting only. Otherwise, we shade the point normally. The following diagram illustrates this idea:

In the preceding diagram, point QQQ is in shadow and point TTT is not. In each case, we march the ray along the direction toward the light source (sss). We evaluate the height map at each discrete height level. In the case of point QQQ, we find a point that lies below the bump surface, but for point TTT, all points lie above it.

The ray marchine process is nearly identical to that described before for the view vector. We start at point QQQ and move along the ray toward the light. If we find a point that is below the surface, then the point is occluded from the light source. Otherwise, the point is shaded normally. We can use the same equation we used for marching the view vector, replacing the view vector with the vector toward the light source.

Getting ready

For this algorithm, we need a height map, a normal map, and a color map. We also need tangent vectors in our mesh so that we can transform into tangent space.

How to do it...

The vertex shader is the same as the one used in the Parallax mapping recipe.

In the fragment shader, we break the process into two functions: findOffset and isOccluded. The first traces the view vector to determine the texture coordinate offset. The second traces the light vector to determine whether or not the point is in shadow:

in vec3 LightDir; in vec2 TexCoord; in vec3 ViewDir; layout(binding=0) uniform sampler2D ColorTex; layout(binding=1) uniform sampler2D NormalMapTex; layout(binding=2) uniform sampler2D HeightMapTex; layout( location = 0 ) out vec4 FragColor; const float bumpScale = 0.03; vec2 findOffset(vec3 v, out float height) { const int nSteps = int(mix(60, 10, abs(v.z))); float htStep = 1.0 / nSteps; vec2 deltaT = (v.xy * bumpScale) / (nSteps * v.z); float ht = 1.0; vec2 tc = TexCoord.xy; height = texture(HeightMapTex, tc).r; while( height < ht ) { ht -= htStep; tc -= deltaT; height = texture(HeightMapTex, tc).r; } return tc;
} bool isOccluded(float height, vec2 tc, vec3 s) { const int nShadowSteps = int(mix(60,10,s.z)); float htStep = 1.0 / nShadowSteps; vec2 deltaT = (s.xy * bumpScale) / ( nShadowSteps * s.z ); float ht = height + htStep * 0.5; while( height < ht && ht < 1.0 ) { ht += htStep; tc += deltaT; height = texture(HeightMapTex, tc).r; } return ht < 1.0;
} vec3 blinnPhong( ) { vec3 v = normalize(ViewDir); vec3 s = normalize( LightDir ); float height = 1.0; vec2 tc = findOffset(v, height); vec3 texColor = texture(ColorTex, tc).rgb; vec3 n = texture(NormalMapTex, tc).xyz; n.xy = 2.0 * n.xy - 1.0; n = normalize(n); float sDotN = max( dot(s,n), 0.0 ); vec3 diffuse = vec3(0.0), ambient = Light.La * texColor, spec = vec3(0.0); if( sDotN > 0.0 && !isOccluded(height, tc, s) ) { diffuse = texColor * sDotN; vec3 h = normalize( v + s ); spec = Material.Ks * pow( max( dot(h,n), 0.0 ), Material.Shininess ); } return ambient + Light.L * (diffuse + spec);
}

How it works...

The findOffset function determines the texture coordinate to use when shading. We pass in the vector toward the viewer (we negate the direction to move away from the eye), and the function returns the texture coordinate. It also returns the value of the height at that location via the output parameter height. The first line determines the number of discrete height levels (nSteps). We pick a number between 10 and 60 by interpolating using the value of the z coordinate of the view vector. If the z coordinate is small, then the view vector is close to vertical with respect to the height levels. When the view vector is close to vertical, we can use fewer steps because the ray travels a shorter relative horizontal distance between levels. However, when the vector is closer to horizontal, we need more steps as the ray travels a larger horizontal distance when moving from one level to the next. The deltaT variable is the amount that we move through texture space when moving from one height level to the next. This is the second term in the equation listed previously.
The ray marching proceeds with the following loop. The ht variable tracks the height level.

We start it at 1.0. The height variable will be the value of the height map at the current position. The tc variable will track our movement through texture space, initially at the texture coordinate of the fragment (TexCoord). We look up the value in the height map at tc, and then enter the loop.

The loop continues until the value in the height map (height) is less than the value of the discrete height level (ht). Within the loop, we change ht to move down one level and update the texture coordinate by deltaT. Note that we subtract deltaT because we are moving away from the viewer. Then, we look up the value of the height map (height) at the new texture coordinate and repeat.

When the loop terminates, tc should have the value of the offset texture coordinate, and height is the value in the height map at that location. We return tc, and the value of height at the end of the loop is also returned to the caller via the output parameter.

Note that this loop isn't correct when we are viewing the back side of the face. However, the loop will still terminate at some point because we always decrease ht and the height map texture is assumed to be between 0 and 1. If back faces are visible, we need to modify this to properly follow the ray or invert the normal.

The isOccluded function returns whether or not the light source is occluded by the height map at that point. It is quite similar to the findOffset function. We pass in height previously determined by findOffset, the corresponding texture coordinate (tc), and the direction toward the light source (s). Similar to findOffset, we march the ray in the direction of s, beginning at the height and texture coordinate provided. Note that we begin the loop with a value for ht that is slightly offset from the value of the bump map there (ht= height + htStep * 0.1). This is to avoid the so-called shadow acne effect. If we don't offset it, we can sometimes get false positives when the ray collides with the surface that it starts on, producing speckled shadows.

The rest of the function contains a loop that is quite similar to the loop in findOffset. However, we move upward through the height levels, and we are careful to stop when the value of ht reaches or exceeds 1.0. At the end of the loop, we don't need the values of height or tc; we only need to know whether or not the loop stopped due to the first of the two conditions. If ht < 1.0, then we exit the loop before exceeding the range of the height map, indicating that we found a point along the ray that had a larger height. Therefore, the point must be in shadow, so we return true. Otherwise, the light source is unoccluded, so we return false.

The blinnPhong function calls findOffset to determine the appropriate texture coordinates to use. It then looks up the values in the normal map and color map at that location. Next, it evaluates the Blinn-Phong reflection model using those values. However, it uses the isOccluded function to determine whether or not we should include diffuse and specular components. We also won't evaluate those components if the value of sDotN is less than or equal to zero, meaning that the light is behind (or tangent to) the face, as determined by the shading normal.

Simulating reflection with cube maps

Textures can be used to simulate a surface that has a component that is purely reflective (a mirror-like surface such as chrome). In order to do so, we need a texture that is representative of the environment surrounding the reflective object. This texture could then be mapped onto the surface of the object in a way that represents how it would look when reflected off the surface. This general technique is known as environment mapping.

In general, environment mapping involves creating a texture that is representative of the environment and mapping it onto the surface of an object. It is typically used to simulate the effects of reflection or refraction.

A cube map is one of the more common varieties of textures used in environment mapping. A cube map is a set of six separate images that represent the environment projected onto each of the six faces of a cube. The six images represent a view of the environment from the point of view of a viewer located at the center of the cube. An example of a cube map is shown in the following image. The images are laid out as if the cube was unfolded and laid flat. The four images across the middle would make up the sides of the cube, and the top and bottom images correspond to the top and bottom of the cube:

OpenGL provides built-in support for cube map textures (using the GL_TEXTURE_CUBE_MAP target). The texture is accessed using a three-dimensional texture coordinate (s, t, r). The texture coordinate is interpreted as a direction vector from the center of the cube. The line defined by the vector and the center of the cube is extended to intersect one of the faces of the cube. The image that corresponds to that face is then accessed at the location of the intersection.

Truth be told, the conversion between the three-dimensional texture coordinate used to access the cube map and the two-dimensional texture coordinate used to access the individual face image is somewhat complicated. It can be non-intuitive and confusing. A very accessible explanation can be found in the OpenGL specification document ( https://www.khronos.org/registry/OpenGL/index_gl.php ). However, the good news is that if you are careful to orient your textures correctly within the cube map, the details of the conversion can be ignored and the texture coordinate can be visualized as a three-dimensional vector as described previously.

In this example, we'll demonstrate using a cube map to simulate a reflective surface. We'll also use the cube map to draw the environment around the reflective object (sometimes called a skybox).

Getting ready

First, prepare the six images of the cube map. In this example, the images will have the following naming convention. There is a base name (stored in the baseFileName variable) followed by an underscore, followed by one of the six possible suffixes (posx, negx, posy, negy, posz, or negz), followed by the file extension. The suffixes posx, posy, and so on, indicate the axis that goes through the center of the face (positive x, positive y, and so on).

Make sure that they are all square images (preferably with dimensions that are a power of two), and that they are all the same size.

Set up your OpenGL program to provide the vertex position in attribute location 0, and the vertex normal in attribute location 1.

This vertex shader requires the model matrix (the matrix that converts from object coordinates to world coordinates) to be separated from the model-view matrix and provided to the shader as a separate uniform. Your OpenGL program should provide the model matrix in the uniform variable ModelMatrix.

The vertex shader also requires the location of the camera in world coordinates. Make sure that your OpenGL program sets the uniform WorldCameraPosition to the appropriate value.

How to do it...

To render an image with reflection based on a cube map, and also render the cube map itself, carry out the following steps:

1. We'll start by defining a function that will load the six images of the cube map into a single texture target:

    GLuint Texture::loadCubeMap(const std::string &baseName, const std::string &extension) { GLuint texID; glGenTextures(1, &texID); glBindTexture(GL_TEXTURE_CUBE_MAP, texID); const char * suffixes[] = { "posx", "negx", "posy", "negy", "posz", "negz" }; GLint w, h; std::string texName = baseName + "_" + suffixes[0] + extension; GLubyte * data = Texture::loadPixels(texName, w, h, false); glTexStorage2D(GL_TEXTURE_CUBE_MAP, 1, GL_RGBA8, w, h); glTexSubImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_X, 0, 0, 0, w, h, GL_RGBA, GL_UNSIGNED_BYTE, data); stbi_image_free(data); for( int i = 1; i < 6; i++ ) { std::string texName = baseName + "_" + suffixes[i] + extension; data = Texture::loadPixels(texName, w, h, false); glTexSubImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_X + i, 0, 0, 0, w, h, GL_RGBA, GL_UNSIGNED_BYTE, data); stbi_image_free(data); } glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MIN_FILTER, GL_NEAREST); glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_R, GL_CLAMP_TO_EDGE); return texID;
    }

2. Use the following code for the vertex shader:

    layout (location = 0) in vec3 VertexPosition; layout (location = 1) in vec3 VertexNormal; layout (location = 2) in vec2 VertexTexCoord; out vec3 ReflectDir; uniform vec3 WorldCameraPosition; uniform mat4 ModelViewMatrix; uniform mat4 ModelMatrix; uniform mat3 NormalMatrix; uniform mat4 ProjectionMatrix; uniform mat4 MVP; void main() { vec3 worldPos = vec3(ModelMatrix * vec4(VertexPosition,1.0) ); vec3 worldNorm = vec3(ModelMatrix * vec4(VertexNormal, 0.0)); vec3 worldView = normalize( WorldCameraPosition - worldPos ); ReflectDir = reflect(-worldView, worldNorm ); gl_Position = MVP * vec4(VertexPosition,1.0);
    }

3. Use the following code for the fragment shader:

    in vec3 ReflectDir; layout(binding=0) uniform samplerCube CubeMapTex; uniform float ReflectFactor; uniform vec4 MaterialColor; layout( location = 0 ) out vec4 FragColor; void main() { vec4 cubeMapColor = texture(CubeMapTex, ReflectDir); FragColor = mix(MaterialColor, CubeMapColor, ReflectFactor);
    }

4. In the render portion of the OpenGL program, draw a cube centered at the origin and apply the cube map to the cube. You can use the normalized position as the texture coordinate. Use a separate shader for this sky box. See the example code for details.

5. Switch to the preceding shaders and draw the object(s) within the scene.

How it works...

In OpenGL, a cube map texture consists of six separate images. To fully initialize a cube map texture, we need to bind to the cube map texture and then load each image individually into the six "slots" within that texture. In the preceding code (within the Texture::loadCubeMap function), we start by binding to texture unit zero with glActiveTexture. Then, we create a new texture object by calling glGenTextures, store its handle within the variable texID, and then bind that texture object to the GL_TEXTURE_CUBE_MAP target using glBindTexture. We load the first image to determine the dimensions of the image, and then load the others in a loop. The following loop loads each texture file and copies the texture data into OpenGL memory using glTexSubImage2D. Note that the first argument to this function is the texture target, which corresponds to GL_TEXTURE_CUBE_MAP_POSITIVE_X + i. OpenGL defines consecutive constants that correspond to the six faces of the cube, so we can just add an integer to the value of the constant for the first face. After the loop is finished, the cube map texture should be fully initialized with the six images.

Following this, we set up the cube map texture environment. We use linear filtering, and we also set the texture wrap mode to GL_CLAMP_TO_EDGE for all three of the texture coordinate's components. This tends to work well, avoiding the possibility of a border color appearing between the cube edges.

Even better would be to use seamless cube map textures (available since OpenGL 3.2). It is a simple matter to enable them, just call: glEnable(GL_TEXTURE_CUBE_MAP_SEAMLESS) .

Within the vertex shader, the main goal is to compute the direction of reflection and pass that to the fragment shader to be used to access the cube map. The output variable ReflectDir will store this result. We can compute the reflected direction (in world coordinates) by reflecting the vector toward the viewer about the normal vector.

We choose to compute the reflection direction in world coordinates because, if we were to use eye coordinates, the reflection would not change as the camera moved within the scene.

In the else branch within the main function, we start by converting the position to world coordinates and storing them in worldPos. We then do the same for the normal, storing the result in worldNorm. Note that the ModelMatrix is used to transform the vertex normal. It is important when doing this to use a value of 0.0 for the fourth coordinate of the normal to avoid the translation component of the model matrix affecting the normal. Also, the model matrix must not contain any non-uniform scaling component; otherwise the normal vector will be transformed incorrectly.

The direction toward the viewer is computed in world coordinates and stored in worldView.

Finally, we reflect worldView about the normal and store the result in the output variable ReflectDir. The fragment shader will use this direction to access the cube map texture and apply the corresponding color to the fragment. One can think of this as a light ray that begins at the viewer's eye, strikes the surface, reflects off the surface, and hits the cube map. The color that the ray sees when it strikes the cube map is the color that we need for the object.

When drawing the sky box, we use the vertex position as the reflection direction. Why? Well, when the sky box is rendered, we want the location on the sky box to correspond to the equivalent location in the cube map (the sky box is really just a rendering of the cube map). Therefore, if we want to access a position on the cube map corresponding to a location on a cube centered at the origin, we need a vector that points at that location. The vector we need is the position of that point minus the origin (which is (0,0,0)). Hence, we just need the position of the vertex.

Sky boxes can be rendered with the viewer at the center of the sky box and the sky box moving along with the viewer (so the viewer is always at the center of the sky box). We have not done so in this example; however, we could do so by transforming the sky box using the rotational component of the view matrix (not the translational).

Within the fragment shader, we simply use the value of ReflectDir to access the cube map texture:

vec4 cubeMapColor = texture(CubeMapTex, ReflectDir)

We'll mix the sky box color with some material color. This allows us to provide some slight tint to the object. The amount of tint is adjusted by the variable ReflectFactor. A value of 1.0 would correspond to zero tint (all reflection), and a value of 0.0 corresponds to no reflection. The following images show a teapot rendered with different values of ReflectFactor. The teapot on the left uses a reflection factor of 0.5, and the one on the right uses a value of 0.85. The base material color is grey (the cube map used is an image of St. Peter's Basilica, Rome. ©Paul Debevec):

There's more...

There are two important points to keep in mind about this technique. First, the objects will only reflect the environment map. They will not reflect the image of any other objects within the scene. In order to do so, we would need to generate an environment map from the point of view of each object by rendering the scene six times with the view point located at the center of the object and the view direction in each of the six coordinate directions. Then, we could use the appropriate environment map for the appropriate object's reflections. Of course, if any of the objects were to move relative to one another, we'd need to regenerate the environment maps. All of this effort may be prohibitive in an interactive application.

The second point involves the reflections that appear on moving objects. In these shaders, we compute the reflection direction and treat it as a vector emanating from the center of the environment map. This means that regardless of where the object is located, the reflections will appear as if the object is in the center of the environment. In other words, the environment is treated as if it were infinitely far away. Chapter 19 of the book GPU Gems, by Randima Fernando, Addison-Wesley Professional, 2009, has an excellent discussion of this issue and provides some possible solutions for localizing the reflections.

Simulating refraction with cube maps

Objects that are transparent cause the light rays that pass through them to bend slightly at the interface between the object and the surrounding environment. This effect is called refraction. When rendering transparent objects, we simulate that effect by using an environment map and mapping the environment onto the object is such a way as to mimic the way that light would pass through the object. In other words, we can trace the rays from the viewer, through the object (bending in the process), and along to the environment. Then, we can use that ray intersection as the color for the object.

As in the previous recipe, we'll do this using a cube map for the environment. We'll trace rays from the viewer position, through the object, and finally intersect with the cube map.

The process of refraction is described by Snell's law, which defines the relationship between the angle of incidence and the angle of refraction:

Snell's law describes the angle of incidence (aia_iai​) as the angle between the incoming light ray and the normal to the surface, and the angle of refraction (ata_tat​) as the angle between the transmitted ray and the extended normal. The material through which the incident light ray travels and the material containing the transmitted light ray are each described by an index of refraction (n1n_1n1​ and n2n_2n2​ in the diagram). The ratio between the two indices of refraction defines the amount that the light ray will be bent at the interface.

Starting with Snell's law, and with a bit of mathematical effort, we can derive a formula for the transmitted vector, given the ratio of the indices of refraction, the normal vector, and the incoming vector:

sinaisinat=n2n1\frac{sina_i}{sina_t}=\frac{n_2}{n_1}sinat​sinai​​=n1​n2​​

However, there's no real need to do so because GLSL provides a built-in function for computing this transmitted vector called refract. We'll make use of that function within this example.

It is usually the case that for transparent objects, not all of the light is transmitted through the surface. Some of the light is reflected. In this example, we'll model that in a very simple way, and at the end of this recipe, we'll discuss a more accurate representation.

Getting ready

Set up your OpenGL program to provide the vertex position in attribute location 0 and the vertex normal in attribute location 1. As with the previous recipe, we'll need to provide the model matrix in the uniform variable ModelMatrix.

Load the cube map using the technique shown in the previous recipe. Place it in texture unit zero.

Set the uniform variable WorldCameraPosition to the location of your viewer in world coordinates. Set the value of the uniform variable Material.Eta to the ratio between the index of refraction of the environment n1n_1n1​ and the index of refraction of the material n2n_2n2​ (n1/n2n_1/n_2n1​/n2​). Set the value of the uniform Material.ReflectionFactor to the fraction of light that is reflected at the interface (a small value is probably what you want).
As with the preceding example, if you want to draw the environment, draw a large cube surrounding the scene and use a separate shader to apply the texture to the cube. See the example code for details.

How to do it...

To render an object with reflection and refraction, carry out the following steps:

1. Use the following code within the vertex shader:

    layout (location = 0) in vec3 VertexPosition; layout (location = 1) in vec3 VertexNormal; out vec3 ReflectDir; out vec3 RefractDir; struct MaterialInfo { float Eta; float ReflectionFactor; }; uniform MaterialInfo Material; uniform vec3 WorldCameraPosition; uniform mat4 ModelViewMatrix; uniform mat4 ModelMatrix; uniform mat3 NormalMatrix; uniform mat4 ProjectionMatrix; uniform mat4 MVP; void main() { vec3 worldPos = vec3( ModelMatrix * vec4(VertexPosition,1.0) ); vec3 worldNorm = vec3(ModelMatrix * vec4(VertexNormal, 0.0)); vec3 worldView = normalize( WorldCameraPosition - worldPos ); ReflectDir = reflect(-worldView, worldNorm ); RefractDir = refract(-worldView, worldNorm, Material.Eta ); gl_Position = MVP * vec4(VertexPosition,1.0);
    }

2. Use the following code within the fragment shader:

    in vec3 ReflectDir; in vec3 RefractDir; layout(binding=0) uniform samplerCube CubeMapTex; struct MaterialInfo { float Eta; float ReflectionFactor; }; uniform MaterialInfo Material; layout( location = 0 ) out vec4 FragColor; void main() { vec4 reflectColor = texture(CubeMapTex, ReflectDir); vec4 refractColor = texture(CubeMapTex, RefractDir); FragColor = mix(refractColor, reflectColor, Material.ReflectionFactor);
    }

How it works...

Both shaders are quite similar to the shaders in the previous recipe.

The vertex shader computes the position, normal, and view direction in world coordinates (worldPos, worldNorm, and worldView). They are then used to compute the reflected direction using the reflect function, and the result is stored in the output variable

ReflectDir. The transmitted direction is computed using the built-in function refract (which requires the ratio of the indices of refraction Material.Eta). This function makes use of Snell's law to compute the direction of the transmitted vector, which is then stored in the output variable RefractDir.

In the fragment shader, we use the two vectors ReflectDir and RefractDir to access the cube map texture. The color retrieved by the reflected ray is stored in reflectColor and the color retrieved by the transmitted ray is stored in refractColor. We then mix those two colors together based on the value of Material.ReflectionFactor. The result is a mixture between the color of the reflected ray and the color of the transmitted ray.

The following image shows the teapot rendered with 10 percent reflection and 90 percent refraction (Cubemap © Paul Debevec):

There's more...

This technique has the same drawbacks that were discussed in the There's more... section of the preceding recipe, Simulating reflection with cube maps.

Like most real-time techniques, this is a simplification of the real physics of the situation. There are a number of things about the technique that could be improved to provide more realistic-looking results.

The Fresnel equations

The amount of reflected light actually depends on the angle of incidence of the incoming light. For example, when looking at the surface of a lake from the shore, much of the light is reflected and it is easy to see reflections of the surrounding environment on the surface. However, when floating on a boat on the surface of the lake and looking straight down, there is less reflection and it is easier to see what lies below the surface. This effect is described by the Fresnel equations (after Augustin-Jean Fresnel).

The Fresnel equations describe the amount of light that is reflected as a function of the angle of incidence, the polarization of the light, and the ratio of the indices of refraction. If we ignore the polarization, it is easy to incorporate the Fresnel equations into the preceding shaders. A very good explanation of this can be found in Chapter 14 of the book The OpenGL Shading Language, 3rd Edition, Randi J Rost, Addison-Wesley Professional, 2009.

Chromatic aberration

White light is of course composed of many different individual wavelengths (or colors).
The amount that a light ray is refracted is actually wavelength dependent. This causes an effect where a spectrum of colors can be observed at the interface between materials. The most well-known example of this is the rainbow that is produced by a prism.

We can model this effect by using slightly different values of Eta for the red, green, and blue components of the light ray. We would store three different values for Eta, compute three different reflection directions (red, green, and blue), and use those three directions to look up colors in the cube map. We take the red component from the first color, the green component from the second, and the blue component for the third, and combine the three components together to create the final color for the fragment.

It is important to note that we have simplified things by only modeling the interaction of the light with one of the boundaries of the object. In reality, the light would be bent once when entering the transparent object, and again when leaving the other side. However, this simplification generally does not result in unrealistic-looking results. As is often the case in real-time graphics, we are more interested in a result that looks good than one that models the physics accurately.

Applying a projected texture

We can apply a texture to the objects in a scene as if the texture was a projection from an imaginary "slide projector" located somewhere within the scene. This technique is often called projective texture mapping and produces a very nice effect.

The following images show an example of projective texture mapping. The flower texture on the left (Stan Shebs via Wikimedia Commons) is projected onto the teapot and plane beneath:

To project a texture onto a surface, all we need do is determine the texture coordinates based on the relative position of the surface location and the source of the projection (the slide projector). An easy way to do this is to think of the projector as a camera located somewhere within the scene. In the same way that we would define an OpenGL camera, we define a coordinate system centered at the projector's location, and a view matrix (V) that converts coordinates to the projector's coordinate system. Next, we'll define a perspective projection matrix (P) that converts the view frustum (in the projector's coordinate system) into a cubic volume of size two, centered at the origin. Putting these two things together, and adding an additional matrix for re-scaling and translating the volume to a volume of size one shifted so that the volume is centered at (0.5, 0.5, 0.5), we have the following transformation matrix:

M=[0.5000.500.500.5000.50.50001]PVM=\begin{bmatrix}0.5&0&0&0.5\\ 0&0.5&0&0.5\\ 0&0&0.5&0.5\\ 0&0&0&1\end{bmatrix}PVM=​0.5000​00.500​000.50​0.50.50.51​​PV

The goal here is basically to convert the view frustum to a range between 0 and 1 in x and y. The preceding matrix can be used to do just that! It will convert world coordinates that lie within the view frustum of the projector to a range between 0 and 1 (homogeneous), which can then be used to access the texture. Note that the coordinates are homogeneous and need to be divided by the w coordinate before they can be used as a real position.

For more details on the mathematics of this technique, take a look at the following white paper, written by Cass Everitt from NVIDIA: https:// www.nvidia.com/object/Projective_Texture_Mapping.html

In this example, we'll apply a single texture to a scene using projective texture mapping.

Getting ready

Set up your OpenGL application to provide the vertex position in attribute location 0 and the normal in attribute location 1. The OpenGL application must also provide the material and lighting properties for the Phong reflection model (see the fragment shader given in the following section). Make sure to provide the model matrix (for converting to world coordinates) in the uniform variable ModelMatrix.

How to do it...

To apply a projected texture to a scene, use the following steps:

1. In the OpenGL application, load the texture into texture unit zero. While the texture object is bound to the GL_TEXTURE_2D target, use the following code to set the texture's settings:

    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_BORDER);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_BORDER);

2. Also within the OpenGL application, set up your transformation matrix for the slide projector and assign it to the uniform ProjectorMatrix. Use the following code to do this. Note that this code makes use of the GLM libraries discussed in Chapter 1, Getting Started with GLSL:

    vec3 projPos, projAt, projUp; mat4 projView = glm::lookAt(projPos, projAt, projUp);
    mat4 projProj = glm::perspective(glm::radians(30.0f), 1.0f, 0.2f, 1000.0f); mat4 bias = glm::translate(mat4(1.0f), vec3(0.5f)); bias = glm::scale(bias, vec3(0.5f)); prog.setUniform("ProjectorMatrix", bias * projProj * projView);

3. Use the following code for the vertex shader:

    layout (location = 0) in vec3 VertexPosition; layout (location = 1) in vec3 VertexNormal; out vec3 EyeNormal; out vec4 EyePosition; out vec4 ProjTexCoord; uniform mat4 ProjectorMatrix; uniform mat4 ModelViewMatrix; uniform mat4 ModelMatrix; uniform mat3 NormalMatrix; uniform mat4 MVP; void main() { vec4 pos4 = vec4(VertexPosition,1.0); EyeNormal = normalize(NormalMatrix * VertexNormal); EyePosition = ModelViewMatrix * pos4; ProjTexCoord = ProjectorMatrix * (ModelMatrix * pos4); gl_Position = MVP * pos4;
    }

4. Use the following code for the fragment shader:

    in vec3 EyeNormal; in vec4 EyePosition; in vec4 ProjTexCoord; layout(binding=0) uniform sampler2D ProjectorTex; layout( location = 0 ) out vec4 FragColor; vec3 blinnPhong( vec3 pos, vec3 norm ) { } void main() { vec3 color = blinnPhong(EyePosition.xyz, normalize(EyeNormal)); vec3 projTexColor = vec3(0.0); if ( ProjTexCoord.z > 0.0 ) projTexColor = textureProj( ProjectorTex, ProjTexCoord ).rgb; FragColor = vec4(color + projTexColor * 0.5, 1);
    }

How it works...

When loading the texture into the OpenGL application, we make sure to set the wrap mode for the s and t directions to GL_CLAMP_TO_BORDER. We do this because if the texture coordinates are outside of the range of zero to one, we do not want any contribution from the projected texture. With this mode, using the default border color, the texture will return (0,0,0,0) when the texture coordinates are outside of the range between 0 and 1 inclusive.

The transformation matrix for the slide projector is set up in the OpenGL application. We start by using the GLM function glm::lookAt to produce a view matrix for the projector. In this example, we locate the projector at (5, 5, 5), looking toward the point (-2, -4,0), with an up vector of (0, 1, 0). This function works in a similar way to the gluLookAt function. It returns a matrix for converting to the coordinate system located at (5, 5, 5), and oriented based on the second and third arguments.

Next, we create the projection matrix using glm::perspective, and the scale/translate matrix M (shown in the introduction to this recipe). These two matrices are stored in projProj and projScaleTrans, respectively. The final matrix is the product of projScaleTrans, projProj, and projView, which is stored in m and assigned to the uniform variable ProjectorTex.

In the vertex shader, we have three output variables: EyeNormal, EyePosition, and ProjTexCoord. The first two are the vertex normal and vertex position in eye coordinates. We transform the input variables appropriately, and assign the results to the output variables within the main function.

We compute ProjTexCoord by first transforming the position to world coordinates (by multiplying by ModelMatrix), and then applying the projector's transformation.

In the fragment shader, within the main function, we start by computing the reflection model and storing the result in the variable color. The next step is to look up the color from the texture. First, however, we check the z coordinate of ProjTexCoord. If this is negative, then the location is behind the projector, so we avoid doing the texture lookup. Otherwise, we use textureProj to look up the texture value and store it in projTexColor.

The function textureProj is designed for accessing textures with coordinates that have been projected. It will divide the coordinates of the second argument by its last coordinate before accessing the texture. In our case, that is exactly what we want. We mentioned earlier that after transforming by the projector's matrix, we will be left with homogeneous coordinates, so we need to divide by the w coordinate before accessing the texture. The textureProj function will do exactly that for us.

Finally, we add the projected texture's color to the base color from the Phong model. We scale the projected texture color slightly so that it is not overwhelming.

There's more...

There's one big drawback to the technique presented here. There is no support for shadows yet, so the projected texture will shine right through any objects in the scene and appear on objects that are behind them (with respect to the projector). In later recipes, we will look at some examples of techniques for handling shadows that could help solve this problem.

Rendering to a texture

Sometimes, it makes sense to generate textures on the fly during the execution of the program. The texture could be a pattern that is generated from some internal algorithm (a so-called procedural texture), or it could be that the texture is meant to represent another portion of the scene.

An example of the latter case might be a video screen where one can see another part of the world, perhaps via a security camera in another room. The video screen could be constantly updated as objects move around in the other room, by re-rendering the view from the security camera to the texture that is applied to the video screen!

In the following image, the texture appearing on the cube was generated by rendering the cow to an internal texture and then applying that texture to the faces of the cube:

In OpenGL, rendering directly to textures has been greatly simplified by the introduction of framebuffer objects (FBOs). We can create a separate rendering target buffer (the FBO), attach our texture to that FBO, and render to the FBO in exactly the same way that we would render to the default framebuffer. All that is required is to swap in the FBO, and swap it out when we are done.

Basically, the process involves the following steps when rendering:

1. Bind to the FBO
2. Render the texture
3. Unbind from the FBO (back to the default framebuffer)
4. Render the scene using the texture

There's actually not much that we need to do on the GLSL side in order to use this kind of texture. In fact, the shaders will see it as any other texture. However, there are some important points that we'll talk about regarding fragment output variables.

In this example, we'll cover the steps needed to create the FBO and its backing texture, and how to set up a shader to work with the texture.

Getting ready

For this example, we'll use the shaders from the previous recipe, Applying a 2D texture, with some minor changes. Set up your OpenGL program as described in that recipe. The only change that we'll make to the shaders is changing the name of the sampler2D variable from Tex1 to Texture.

How to do it...

To render to a texture and then apply that texture to a scene in a second pass, use the following steps:

1. Within the main OpenGL program, use the following code to set up the framebuffer object:

    GLuint fboHandle; glGenFramebuffers(1, &fboHandle);
    glBindFramebuffer(GL_FRAMEBUFFER, fboHandle); GLuint renderTex;
    glGenTextures(1, &renderTex);
    glActiveTexture(GL_TEXTURE0); glBindTexture(GL_TEXTURE_2D, renderTex);
    glTexStorage2D(GL_TEXTURE_2D, 1, GL_RGBA8, 512, 512);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glFramebufferTexture2D(GL_FRAMEBUFFER,GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, renderTex, 0); GLuint depthBuf;
    glGenRenderbuffers(1, &depthBuf);
    glBindRenderbuffer(GL_RENDERBUFFER, depthBuf);
    glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, 512, 512); glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthBuf); GLenum drawBufs[] = {GL_COLOR_ATTACHMENT0};
    glDrawBuffers(1, drawBufs); glBindFramebuffer(GL_FRAMEBUFFER, 0);

2. In your render function within the OpenGL program, bind to the framebuffer, draw the scene that is to be rendered to the texture, then unbind from that framebuffer and draw the cube:

    glBindFramebuffer(GL_FRAMEBUFFER, fboHandle); glViewport(0,0,512,512); int loc = glGetUniformLocation(programHandle, "Texture");
    glUniform1i(loc, 1); renderTextureScene(); glBindFramebuffer(GL_FRAMEBUFFER, 0);
    glViewport(0,0,width,height); int loc = glGetUniformLocation(programHandle, "Texture");
    glUniform1i(loc, 0); renderScene();

How it works...

Let's start by looking at the code for creating the framebuffer object (step 1). Our FBO will be 512 pixels square because we intend to use it as a texture. We begin by generating the FBO using glGenFramebuffers and binding the framebuffer to the GL_FRAMEBUFFER target with glBindFramebuffer. Next, we create the texture object to which we will be rendering, and use glActiveTexture to select texture unit zero. The rest is very similar to creating any other texture. We allocate space for the texture using glTexStorage2D. We don't need to copy any data into that space (using glTexSubImage2D) because we'll be writing to that memory later when rendering to the FBO.

Next, we link the texture to the FBO by calling the function glFramebufferTexture2D. This function attaches a texture object to an attachment point in the currently bound framebuffer object. The first argument (GL_FRAMEBUFFER) indicates that the texture is to be attached to the FBO currently bound to the GL_FRAMEBUFFER target. The second argument is the attachment point. Framebuffer objects have several attachment points for color buffers, one for the depth buffer, and a few others. This allows us to have several color buffers to target from our fragment shaders. We'll see more about this later. We use GL_COLOR_ATTACHMENT0 to indicate that this texture is linked to color attachment 0 of the FBO. The third argument (GL_TEXTURE_2D) is the texture target, and the fourth (renderTex) is the handle to our texture. The last argument (0) is the mip-map level of the texture that is being attached to the FBO. In this case, we only have a single level, so we use a value of zero.

As we want to render to the FBO with depth testing, we need to also attach a depth buffer. The next few lines of code create the depth buffer. The glGenRenderbuffer function creates a renderbuffer object, and glRenderbufferStorage allocates space for the renderbuffer object. The second argument to glRenderbufferStorage indicates the internal format for the buffer, and as we are using this as a depth buffer, we use the special format GL_DEPTH_COMPONENT.

Next, the depth buffer is attached to the GL_DEPTH_ATTACHMENT attachment point of the FBO using glFramebufferRenderbuffer.

The shader's output variables are assigned to the attachments of the FBO using glDrawBuffers. The second argument to glDrawBuffers is an array indicating the FBO buffers to be associated with the output variables. The i th element of the array corresponds to the fragment shader output variable at location i. In our case, we only have one shader output variable (FragColor) at location zero. This statement associates that output variable with GL_COLOR_ATTACHMENT0.

The last statement in step 1 unbinds the FBO to revert back to the default framebuffer.

In the last step (within the render function), we bind to the FBO, use the texture in unit one, and render the texture. Note that we need to be careful to set up the viewport (glViewport) and the view and projection matrices appropriately for our FBO. As our FBO is 512 x 512, we use glViewport(0,0,512,512). Similar changes should be made to the view and projection matrices to match the aspect ratio of the viewport and set up the scene to be rendered to the FBO.

Once we've rendered to the texture, we unbind from the FBO, reset the viewport, and the view and projection matrices, use the FBO's texture (texture unit 0), and draw the cube!

There's more...

As FBOs have multiple color attachment points, we can have several output targets from our fragment shaders. Note that so far, all of our fragment shaders have only had a single output variable assigned to location zero. Hence, we set up our FBO so that its texture corresponds to color attachment zero. In later chapters, we'll look at examples where we use more than one of these attachments for things like deferred shading.

Using sampler objects

Sampler objects were introduced in OpenGL 3.3 and provide a convenient way to specify the sampling parameters for a GLSL sampler variable. The traditional way to specify the parameters for a texture is to specify them using glTexParameter, typically at the time that the texture is defined. The parameters define the sampling state (sampling mode, wrapping and clamping rules, and so on) for the associated texture. This essentially combines the texture and its sampling state into a single object. If we wanted to sample from a single texture in more than one way (with and without linear filtering for example), we'd have two choices. We would either need to modify the texture's sampling state, or use two copies of the same texture.

In addition, we might want to use the same set of texture sampling parameters for multiple textures. With what we've seen up until now, there's no easy way to do that. With sampler objects, we can specify the parameters once and share them among several texture objects.

Sampler objects separate the sampling state from the texture object. We can create sampler objects that define a particular sampling state and apply that to multiple textures or bind different sampler objects to the same texture. A single sampler object can be bound to multiple textures, which allows us to define a particular sampling state once and share it among several texture objects.

Sampler objects are defined on the OpenGL side (not in GLSL), which makes it effectively transparent to the GLSL.

In this recipe, we'll define two sampler objects and apply them to a single texture. The following image shows the result. The same texture is applied to the two planes. On the left, we use a sampler object set up for nearest-neighbor filtering, and on the right we use the same texture with a sampler object set up for linear filtering:

Getting ready

We will start with the same shaders used in the recipe Applying a 2D texture. The shader code will not change at all, but we'll use sampler objects to change the state of the sampler variable Tex1.

How to do it...

To set up the texture object and the sampler objects, perform the following steps:

1. Create and fill the texture object in the usual way, but this time, we won't set any sampling state using glTexParameter:

    GLuint texID;
    glGenTextures(1, &texID);
    glBindTexture(GL_TEXTURE_2D, texID);
    glTexStorage2D(GL_TEXTURE_2D, 1, GL_RGBA8, w, h);
    glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, w, h, GL_RGBA, GL_UNSIGNED_BYTE, data);

2. Bind the texture to texture unit 0, which is the unit that is used by the shader:

    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, texID);

3. Next, we create two sampler objects and assign their IDs to separate variables for clarity:

    GLuint samplers[2];
    glGenSamplers(2, samplers); linearSampler = samplers[0]; nearestSampler = samplers[1];

4. Set up linearSampler for linear interpolation:

    glSamplerParameteri(linearSampler, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glSamplerParameteri(linearSampler, GL_TEXTURE_MIN_FILTER, GL_LINEAR);

5. Set up nearestSampler for nearest-neighbor sampling:

    glSamplerParameteri(nearestSampler, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
    glSamplerParameteri(nearestSampler, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

6. When rendering, we bind to each sampler object when needed:

    glBindSampler(0, nearestSampler); glBindSampler(0, linearSampler);

How it works...

Sampler objects are simple to use and make it easy to switch between different sampling parameters for the same texture or use the same sampling parameters for different textures. In steps 1 and 2, we create a texture and bind it to texture unit 0. Normally, we would set the sampling parameters here using glTexParameteri, but in this case, we'll set them in the sampler objects using glSamplerParameter. In step 3, we create the sampler objects and assign their IDs to some variables. In steps 4 and 5, we set up the appropriate sampling parameters using glSamplerParameter. This function is almost exactly the same as glTexParameter except the first argument is the ID of the sampler object instead of the texture target. This defines the sampling state for each of the two sampler objects (linear for linearSampler and nearest for nearestSampler).

Finally, we use the sampler objects by binding them to the appropriate texture unit using glBindSampler just prior to rendering. In step 6 we bind nearestSampler to texture unit 0 first, render some objects, bind linearSampler to texture unit 0, and render some more objects. The result here is that the same texture uses different sampling parameters by binding different sampler objects to the texture unit during rendering.

Diffuse image-based lighting

Image-based lighting is a technique that involves using an image as a light source. The image represents an omni-directional view of the environment of the scene. With image-based lighting, the image itself is treated as a highly detailed light source that surrounds the scene completely. Objects in the scene are illuminated by the content of the image, making it possible to have a very complex lighting environment and/or to simulate a real world setting. Often, these images are produced using a special camera or special photographic techniques, and are recorded in high dynamic range. An example of one such image is shown here (image courtesy of USC Institute for Creative Technologies and Paul Debevec):

These images may be provided as a cube map (set of six images), or some other type of environment map such as a equirectangular panoramic map (the type shown previously). Conversion between the two is straightforward.

Since each texel in the equirectangular map represents a direction, the same is true for a cube map. To convert from one to the other, we just need to convert directions in one map to directions in the other.

In this recipe, we'll go through the process of using an image as a light source for diffuse reflection. Most of the effort here is involved in the creation of the diffuse convolution map. The diffuse convolution is a transformation of the environment map into a form that can be used directly when computing the diffuse reflection. In the following images, the original environment map is shown on the left, and the right side is the diffuse convolution (image courtesy of USC Institute for Creative Technologies and Paul Debevec):

To understand the diffuse convolution map, lets review the reflectance equation (presented in A Physically-based reflection model in Chapter 3, The Basics of GLSL Shaders):

L0(v)=∫Ωf(l,v)Li(l)(n⋅l)dωiL_0(v)=\int_{\Omega}f(l,v)L_i(l)(n\cdot l)d\omega_iL0​(v)=∫Ω​f(l,v)Li​(l)(n⋅l)dωi​

This equation represents the integral over all directions of incoming light (lll) on the hemisphere above the surface. The fff term in the preceding equation is the Bidirectional Reflectance Distribution Function (BRDF). It represents the fraction of light that is reflected from a surface point given the incoming (lll) and outgoing (vvv) directions. If we only consider diffuse reflection (Lambertian), we can use the following constant term for the BRDF:

f(l,v)=cdiffπf(l,v)=\frac{c_{diff}}{\pi}f(l,v)=πcdiff​​

Which gives us the following for the reflectance equation:

L0=cdiffπ∫ΩLi(l)(n⋅l)dωiL_0=\frac{c_{diff}}{\pi}\int_{\Omega}L_i(l)(n\cdot l)d\omega_iL0​=πcdiff​​∫Ω​Li​(l)(n⋅l)dωi​

Since the BRDF is just a constant, it can be factored outside of the integral. Note that there is nothing in this equation that depends upon the outgoing direction (vvv). That leads us to the following insight. The environment map that we discussed previously represents the amount of incoming radiance for a given direction, which is the Li(l)L_i(l)Li​(l) term in the preceding equation. We could estimate the value of this integral for a given value of nnn using the Monte Carlo estimator:

L0(v)≈2cdiffN∑j=1NLj(n⋅lj)L_0(v)\approx \frac{2c_{diff}}{N}\sum_{j=1}^N L_j(n\cdot l_j)L0​(v)≈N2cdiff​​∑j=1N​Lj​(n⋅lj​)

In the preceding equation, ljl_jlj​ represents a pseudo-random direction sampled uniformly from the hemisphere above the surface (around nnn), and NNN is the number of samples. The constant factor of 2π2\pi2π comes from the probability density function for uniform samples as a function of solid angle.

Sampling the directions uniformly over the hemisphere is not quite as straightforward as you might think. Common practice is to sample directions in a system where the z axis is aligned with the vector n, and then transform the sample into world coordinates. However, we have to be careful to pick directions uniformly. For example, suppose we just pick random values between -1 and 1 for x and y, and 0 and 1 for z and then normalize. That would give us directions that are biased or "clumped" around the z axis and are not uniform over the hemisphere. To get uniform directions, we can use the following formulae:

x=cos(2πξ2)1−ξ12y=sin(2πξ2)1−ξ12z=ξ1\begin{aligned} x & = cos(2\pi \xi_2)\sqrt{1-\xi_1^2} \\ y & = sin(2\pi \xi_2)\sqrt{1-\xi_1^2} \\ z & = \xi_1 \end{aligned}xyz​=cos(2πξ2​)1−ξ12​​=sin(2πξ2​)1−ξ12​​=ξ1​​

The values ξ1\xi_1ξ1​ and ξ2\xi_2ξ2​ are uniform pseudo-random values in the range [0, 1]. For a derivation of this, see Physically Based Rendering, third edition, Chapter 13, Monte Carlo Integration.

Now that we have a way to estimate the integral for a given value of n, we can convolve the original environment map in the following way. We'll create a new environment map (the diffuse convolution map), where each texel represents a direction of nnn in world coordinates. The value of the texel will be the estimated value of the preceding integral (except for the cdiffc_{diff}cdiff​ term) by taking a number of random samples (ljl_jlj​) from the original environment map. We can do this offline and pre-compute this diffuse convolution. It is a somewhat slow process, but we don't need a lot of detail. The diffuse convolution is usually fairly smoothly varying, so we can use a small resolution without sacrificing much quality.

I've admittedly glossed over some of the math here. For a very good introduction to Monte Carlo integration for graphics, please see Physically Based Rendering, third Edition by Pharr, Jakob, and Humphreys, Chapter 13, Monte Carlo Integration.

Once we have the pre-computed diffuse convolution, we can use that as a lookup table to give us the value of our diffuse integral (again, without cdiff) using the normal vector. We can multiply the value retrieved by our material's diffuse color cdiff to get the outgoing radiance. In other words, the diffuse convolution represents the outgoing radiance for a given value of n, rather than the incoming radiance.

Getting ready

Most of the preparation here is involved in convolving the environment map. The following pseudocode outlines this process:

nSamples = 1000 
foreach texel t in output map 
  n = direction towards t 
  rad = 0 
  for i = 1 to nSamples 
      li = uniform random direction in the hemisphere around n in world coords
      L = read from environment map at li 
      nDotL = dot( n, li ) 
      rad += L * nDotL 
  set texel t to (2 / nSamples) * rad

How to do it...

To render a scene with diffuse image-based lighting, the process is fairly simple. We simply need to read from our diffuse map using the normal vector.

The vertex shader just converts the position and normal to world coordinates and passes them along:

 out vec3 Position; out vec3 Normal; void main() { TexCoord = VertexTexCoord; Position = (ModelMatrix * vec4(VertexPosition,1)).xyz; Normal = normalize( ModelMatrix * vec4(VertexNormal,0) ).xyz; gl_Position = MVP * vec4(VertexPosition,1.0);
}

The fragment shader then uses the diffuse convolution map to determine the value of the integral, multiplies it by a color taken from a texture map, and applies gamma correction:

const float PI = 3.14159265358979323846; in vec3 Position;
in vec3 Normal; in vec2 TexCoord; layout(binding=0) uniform samplerCube DiffConvTex; layout(binding=1) uniform sampler2D ColorTex; layout( location = 0 ) out vec4 FragColor; const float gamma = 2.2; void main() { vec3 n = normalize(Normal); vec3 light = texture(DiffConvTex, n).rgb; vec3 color = texture(ColorTex, TexCoord).rgb; color = pow(color, vec3(gamma)); color *= light; color = pow( color, vec3(1.0/gamma)); FragColor = vec4( color, 1 );
}

Results for the environment shown in the introduction to this recipe are shown in the following screenshot:

How it works...

Once we've created the diffuse convolution, there's not much to this technique. We just simply look up the value in the convolved map DiffConvTex and multiply it with the base color of the surface. In this example, the base color for the surface is taken from a second texture map (ColorTex). We apply gamma decoding to the base color texture to move it into linear color space before multiplying with the environment map. This assumes that the texture is stored in sRGB or has already been gamma encoded. The final value is then gamma encoded before display. The values in the environment map are in linear color space, so we need to move everything to linear space before combining. For some details about gamma encoding/decoding, please see the Using gamma correction to improve image quality recipe in Chapter 6, Image Processing and Screen Space Techniques.

There's more...

It would also be nice to include a specular/glossy component to this model. We didn't cover that here. In addition, it would be nice to include proper Fresnel reflection. Including the specular component is a bit harder because it also depends on the direction of the viewer. There are techniques for creating specular convolutions and I'll refer you to the following sources. These usually involve several simplifying assumptions to make things achievable in real time.

Image Processing and Screen Space Techniques

In this chapter, we will cover the following recipes:

• Applying an edge detection filter
• Applying a Gaussian blur filter
• Implementing HDR lighting with tone mapping
• Creating a bloom effect
• Using gamma correction to improve image quality
• Using multisample anti-aliasing
• Using deferred shading
• Screen space ambient occlusion
• Configuring the depth test
• Implementing order-independent transparency

Introduction

In this chapter, we will focus on techniques that work directly with the pixels in a framebuffer. These techniques typically involve multiple passes. An initial pass produces the pixel data and subsequent passes apply effects or further processes those pixels. To implement this, we often make use of the ability provided in OpenGL for rendering directly to a texture or set of textures (refer to the Rendering to a texture recipe in Chapter 5, Using Textures).

The ability to render to a texture, combined with the power of the fragment shader, opens up a huge range of possibilities. We can implement image processing techniques such as brightness, contrast, saturation, and sharpness by applying an additional process in the fragment shader prior to output. We can apply convolution filters such as edge detection, smoothing (blur), or sharpening. We'll take a closer look at convolution filters in the recipe on edge detection.

A related set of techniques involves rendering additional information to textures beyond the traditional color information and then, in a subsequent pass, further processing that information to produce the final rendered image. These techniques fall under the general category that is often called deferred shading.

In this chapter, we'll look at some examples of each of the preceding techniques. We'll start off with examples of convolution filters for edge detection, blur, and bloom. Then, we'll move on to the important topics of gamma correction and multisample anti-aliasing. Finally, we'll finish with a full example of deferred shading.

Most of the recipes in this chapter involve multiple passes. In order to apply a filter that operates on the pixels of the final rendered image, we start by rendering the scene to an intermediate buffer (a texture). Then, in a final pass, we render the texture to the screen by drawing a single fullscreen quad, applying the filter in the process. You'll see several variations on this theme in the following recipes.

Applying an edge detection filter

Edge detection is an image processing technique that identifies regions where there is a significant change in the brightness of the image. It provides a way to detect the boundaries of objects and changes in the topology of the surface. It has applications in the field of computer vision, image processing, image analysis, and image pattern recognition. It can also be used to create some visually interesting effects. For example, it can make a 3D scene look similar to a 2D pencil sketch, as shown in the following image. To create this image, a teapot and torus were rendered normally, and then an edge detection filter was applied in a second pass:

The edge detection filter that we'll use here involves the use of a convolution filter, or convolution kernel (also called a filter kernel). A convolution filter is a matrix that defines how to transform a pixel by replacing it with the sum of the products between the values of nearby pixels and a set of pre-determined weights. As a simple example, consider the following convolution filter:

The 3x3 filter is shaded in gray, superimposed over a hypothetical grid of pixels. The numbers in bold represent the values of the filter kernel (weights), and the non-bold values are the pixel values. The values of the pixels could represent grayscale intensity or the value of one of the RGB components. Applying the filter to the center pixel in the gray area involves multiplying the corresponding cells together and summing the results. The result would be the new value for the center pixel (25). In this case, the value would be (17 + 19 + 2 * 25 + 31 + 33), or 150.

Of course, in order to apply a convolution filter, we need access to the pixels of the original image and a separate buffer to store the results of the filter. We'll achieve this here by using a two-pass algorithm. In the first pass, we'll render the image to a texture, and then in the second pass, we'll apply the filter by reading from the texture and send the filtered results to the screen.

One of the simplest convolution-based techniques for edge detection is the so-called Sobel operator. The Sobel operator is designed to approximate the gradient of the image intensity at each pixel. It does so by applying two 3x3 filters. The results of the two are the vertical and horizontal components of the gradient. We can then use the magnitude of the gradient as our edge trigger. When the magnitude of the gradient is above a certain threshold, we assume that the pixel is on an edge.

The 3x3 filter kernels used by the Sobel operator are shown in the following equation:

Sx=[−101−202−101]Sy=[−1−2−1000121]\begin{aligned} S_x & = \begin{bmatrix}-1&0&1\\ -2&0&2\\ -1&0&1\end{bmatrix} \\ S_y & = \begin{bmatrix}-1&-2&-1\\ 0&0&0\\ 1&2&1\end{bmatrix} \end{aligned}Sx​Sy​​=​−1−2−1​000​121​​=​−101​−202​−101​​​

If the result of applying Sx is sx and the result of applying Sy is sy, then an approximation of the magnitude of the gradient is given by the following equation:

g=sx2+sy2g=\sqrt{s_x^2 + s_y^2}g=sx2​+sy2​​

If the value of g is above a certain threshold, we consider the pixel to be an edge pixel and we highlight it in the resulting image.

In this example, we'll implement this filter as the second pass of a two-pass algorithm. In the first pass, we'll render the scene using an appropriate lighting model, but we'll send the result to a texture. In the second pass, we'll render the entire texture as a screen-filling quad, and apply the filter to the texture.

Getting ready

Set up a framebuffer object (refer to the Rendering to a texture recipe in Chapter 5, Using Textures) that has the same dimensions as the main window. Connect the first color attachment of the FBO to a texture object in texture unit zero. During the first pass, we'll render directly to this texture. Make sure that the mag and min filters for this texture are set to GL_NEAREST. We don't want any interpolation for this algorithm.

Provide vertex information in vertex attribute zero, normals in vertex attribute one, and texture coordinates in vertex attribute two.

The following uniform variables need to be set from the OpenGL application:

• Width: This is used to set the width of the screen window in pixels
• Height: This is used to set the height of the screen window in pixels
• EdgeThreshold: This is the minimum value of g squared required to be considered on an edge
• RenderTex: This is the texture associated with the FBO

Any other uniforms associated with the shading model should also be set from the OpenGL application.

How to do it...

To create a shader program that applies the Sobel edge detection filter, perform the following steps:

1. The vertex shader just converts the position and normal to camera coordinates and passes them along to the fragment shader.

2. The fragment shader applies the reflection model in the first pass, and applies the edge detection filter in the second pass:

    in vec3 Position; in vec3 Normal; uniform int Pass; layout( binding=0 ) uniform sampler2D RenderTex; uniform float EdgeThreshold; layout( location = 0 ) out vec4 FragColor; const vec3 lum = vec3(0.2126, 0.7152, 0.0722); vec3 blinnPhong( vec3 pos, vec3 norm ) { } float luminance( vec3 color ) { return dot(lum, color);
    } vec4 pass1() { return vec4(blinnPhong( Position, normalize(Normal) ),1.0);
    } vec4 pass2() { ivec2 pix = ivec2(gl_FragCoord.xy); float s00 = luminance( texelFetchOffset(RenderTex, pix, 0, ivec2(-1,1)).rgb); float s10 = luminance( texelFetchOffset(RenderTex, pix, 0, ivec2(-1,0)).rgb); float s20 = luminance( texelFetchOffset(RenderTex, pix, 0, ivec2(-1,-1)).rgb); float s01 = luminance( texelFetchOffset(RenderTex, pix, 0, ivec2(0,1)).rgb); float s21 = luminance( texelFetchOffset(RenderTex, pix, 0, ivec2(0,-1)).rgb); float s02 = luminance( texelFetchOffset(RenderTex, pix, 0, ivec2(1,1)).rgb); float s12 = luminance( texelFetchOffset(RenderTex, pix, 0, ivec2(1,0)).rgb); float s22 = luminance( texelFetchOffset(RenderTex, pix, 0, ivec2(1,-1)).rgb); float sx = s00 + 2 * s10 + s20 - (s02 + 2 * s12 + s22); float sy = s00 + 2 * s01 + s02 - (s20 + 2 * s21 + s22); float g = sx * sx + sy * sy; if( g > EdgeThreshold ) return vec4(1.0); else return vec4(0.0,0.0,0.0,1.0);
    } void main() { if( Pass == 1 ) FragColor = pass1(); if( Pass == 2 ) FragColor = pass2();
    }

In the render function of your OpenGL application, follow these steps for pass #1:

1.  Select FBO, and clear the color/depth buffers
2.  Set the Pass uniform to 1
3.  Set up the model, view, and projection matrices, and draw the scene

For pass #2, carry out the following steps:

1.  Deselect the FBO (revert to the default framebuffer) and clear the color/depth buffers
2.  Set the Pass uniform to 2
3.  Set the model, view, and projection matrices to the identity matrix
4.  Draw a single quad (or two triangles) that fills the screen (-1 to +1 in x and y), with texture coordinates that range from 0 to 1 in each dimension.

How it works...

The first pass renders all of the scene's geometry of sending the output to a texture. We select the function pass1, which simply computes and applies the Blinn-Phong reflection model (refer to Chapter 3, The Basics of GLSL Shaders).

In the second pass, we select the function pass2, and render only a single quad that covers the entire screen. The purpose of this is to invoke the fragment shader once for every pixel in the image. In the pass2 function, we retrieve the values of the eight neighboring pixels of the texture containing the results from the first pass, and compute their brightness by calling the luminance function. The horizontal and vertical Sobel filters are then applied and the results are stored in sx and sy.

The luminance function determines the brightness of an RGB value by computing a weighted sum of the intensities. The weights are from the ITU-R Recommendation Rec. 709. For more details on this, see the Wikipedia entry for luma.

We then compute the squared value of the magnitude of the gradient (in order to avoid the square root) and store the result in g. If the value of g is greater than EdgeThreshold, we consider the pixel to be on an edge and we output a white pixel. Otherwise, we output a solid black pixel.

There's more...

The Sobel operator is somewhat crude and tends to be sensitive to high frequency variations in the intensity. A quick look at Wikipedia will guide you to a number of other edge detection techniques that may be more accurate. It is also possible to reduce the amount of high frequency variation by adding a blur pass between the render and edge detection passes. The blur pass will smooth out the high frequency fluctuations and may improve the results of the edge detection pass.

Optimization techniques

The technique discussed here requires eight texture fetches. Texture accesses can be somewhat slow, and reducing the number of accesses can result in substantial speed improvements. Chapter 24 of GPU Gems: Programming Techniques, Tips and Tricks for Real-Time Graphics, edited by Randima Fernando (Addison-Wesley Professional 2004), has an excellent discussion of ways to reduce the number of texture fetches in a filter operation by making use of so-called helper textures.

Applying a Gaussian blur filter

A blur filter can be useful in many different situations where the goal is to reduce the amount of noise in the image. As mentioned in the previous recipe, applying a blur filter prior to the edge detection pass may improve the results by reducing the amount of high frequency fluctuation across the image. The basic idea of any blur filter is to mix the color of a pixel with that of nearby pixels using a weighted sum. The weights typically decrease with the distance from the pixel (in 2D screen space) so that pixels that are far away contribute less than those closer to the pixel being blurred.

A Gaussian blur uses the two-dimensional Gaussian function to weight the contributions of the nearby pixels:

G(x,y)=12πσ2e−x2+y22σ2G(x,y)=\frac{1}{\sqrt{2\pi \sigma^2}}e^{-\frac{x^2+y^2}{2\sigma^2}}G(x,y)=2πσ2​1​e−2σ2x2+y2​

The sigma squared term is the variance of the Gaussian, and determines the width of the
Gaussian curve. The Gaussian function is maximum at (0,0), which corresponds to the location of the pixel being blurred and its value decreases as xxx or yyy increases. The following graph shows the two-dimensional Gaussian function with a sigma squared value of 4.0:

The following images show a portion of an image before (left) and after (right) the Gaussian blur operation:

To apply a Gaussian blur, for each pixel, we need to compute the weighted sum of all pixels in the image scaled by the value of the Gaussian function at that pixel (where the xxx and yyy coordinates of each pixel are based on an origin located at the pixel being blurred). The result of that sum is the new value for the pixel. However, there are two problems with the algorithm so far:

• As this is a O(n2)O(n^2)O(n2) process (where n is the number of pixels in the image), it is likely to be too slow for real-time use

• The weights must sum to one in order to avoid changing the overall brightness of the image

As we sampled the Gaussian function at discrete locations, and didn't sum over the entire (infinite) bounds of the function, the weights almost certainly do not sum to one.

We can deal with both of the preceding problems by limiting the number of pixels that we blur with a given pixel (instead of the entire image), and by normalizing the values of the
Gaussian function. In this example, we'll use a 9x9 Gaussian blur filter. That is, we'll only compute the contributions of the 81 pixels in the neighborhood of the pixel being blurred.

Such a technique would require 81 texture fetches in the fragment shader, which is executed once for each pixel. The total number of texture fetches for an image of size 800x600 would be 800 * 600 * 81 = 38,880,000. This seems like a lot, doesn't it? The good news is that we can substantially reduce the number of texture fetches by doing the Gaussian blur in two passes.

The two-dimensional Gaussian function can be decomposed into the product of two one-dimensional Gaussians:

G(x,y)=G(x)G(y)G(x,y)=G(x)G(y)G(x,y)=G(x)G(y)

Where the one-dimensional Gaussian function is given by the following equation:

G(x)=12πσ2e−x22σ2G(x)=\frac{1}{2\pi \sigma^2}e^{-\frac{x^2}{2\sigma^2}}G(x)=2πσ21​e−2σ2x2​

So if CijC_ijCi​j is the color of the pixel at pixel location (i, j), the sum that we need to compute is given by the following equation:

Clm←∑i=−44∑j=−44G(i,j)C(l+im+j)C_{lm}\leftarrow \sum_{i=-4}^4 \sum_{j=-4}^4 G(i,j)C(_{l+i\space\space m+j})Clm​←∑i=−44​∑j=−44​G(i,j)C(l+im+j​)

This can be re-written using the fact that the two-dimensional Gaussian is a product of two one-dimensional Gaussians:

Clm←∑i=−44G(i)∑j=−44G(j)C(l+im+j)C_{lm}\leftarrow \sum_{i=-4}^4 G(i)\sum_{j=-4}^4 G(j)C(_{l+i\space\space m+j})Clm​←∑i=−44​G(i)∑j=−44​G(j)C(l+im+j​)

This implies that we can compute the Gaussian blur in two passes. In the first pass, we can compute the sum over jjj (the vertical sum) in the preceding equation and store the results in a temporary texture. In the second pass, we compute the sum over iii (the horizontal sum) using the results from the previous pass.

Now, before we look at the code, there is one important point that has to be addressed. As we mentioned previously, the Gaussian weights must sum to one in order to be a true weighted average. Therefore, we need to normalize our Gaussian weights, as in the following equation:

Clm←∑i=−44G(i)4∑j=−44G(j)4C(l+im+j)C_{lm}\leftarrow \sum_{i=-4}^4 \frac{G(i)}{4} \sum_{j=-4}^4 \frac{G(j)}{4}C(_{l+i\space\space m+j})Clm​←∑i=−44​4G(i)​∑j=−44​4G(j)​C(l+im+j​)

The value of kkk in the preceding equation is just the sum of the raw Gaussian weights:

k=∑i=−44G(i)k=\sum_{i=-4}^4 G(i)k=∑i=−44​G(i)

Phew! We've reduced the O(n2)O(n^2)O(n2) problem to one that is O(n)O(n)O(n). OK, with that, let's move on to the code.

We'll implement this technique using three passes and two textures. In the first pass, we'll render the entire scene to a texture. Then, in the second pass, we'll apply the first (vertical) sum to the texture from the first pass and store the results in another texture. Finally, in the third pass, we'll apply the horizontal sum to the texture from the second pass, and send the results to the default framebuffer.

Getting ready

Set up two framebuffer objects (refer to the Rendering to a texture recipe in Chapter 5, Using Textures), and two corresponding textures. The first FBO should have a depth buffer because it will be used for the first pass. The second FBO need not have a depth buffer because, in the second and third passes, we'll only render a single screen-filling quad in order to execute the fragment shader once for each pixel.

As with the previous recipe, we'll use a uniform variable to select the functionality of each pass. The OpenGL program should also set the following uniform variables:

• Width: This is used to set the width of the screen in pixels
• Height: This is used to set the height of the screen in pixels
• Weight[]: This is the array of normalized Gaussian weights
• Texture0: This is to set this to texture unit zero
• PixOffset[]: This is the array of offsets from the pixel being blurred

How to do it...

In the fragment shader, we apply the Blinn-Phong reflection model in the first pass. In the second pass, we compute the vertical sum. In the third, we compute the horizontal sum:

in vec3 Position; in vec3 Normal; uniform int Pass; layout(binding=0) uniform sampler2D Texture0; layout( location = 0 ) out vec4 FragColor; uniform int PixOffset[5] = int[](0,1,2,3,4); uniform float Weight[5]; vec3 blinnPhong( vec3 pos, vec3 norm ) { } vec4 pass1() { return vec4(blinnPhong( Position, normalize(Normal) ),1.0);
} vec4 pass2() { ivec2 pix = ivec2(gl_FragCoord.xy); vec4 sum = texelFetch(Texture0, pix, 0) * Weight[0]; for( int i = 1; i < 5; i++ ) { sum += texelFetchOffset( Texture0, pix, 0, ivec2(0,PixOffset[i])) * Weight[i]; sum += texelFetchOffset( Texture0, pix, 0, ivec2(0,-PixOffset[i])) * Weight[i]; } return sum;
} vec4 pass3() { ivec2 pix = ivec2(gl_FragCoord.xy); vec4 sum = texelFetch(Texture0, pix, 0) * Weight[0]; for( int i = 1; i < 5; i++ ) { sum += texelFetchOffset( Texture0, pix, 0, ivec2(PixOffset[i],0)) * Weight[i]; sum += texelFetchOffset( Texture0, pix, 0, ivec2(-PixOffset[i],0)) * Weight[i]; } return sum;
} void main() { if( Pass == 1 ) FragColor = pass1(); else if( Pass == 2 ) FragColor = pass2(); else if( Pass == 3 ) FragColor = pass3();
}

In the OpenGL application, compute the Gaussian weights for the offsets found in the uniform variable PixOffset, and store the results in the array Weight. You could use the following code to do so:

char uniName[20]; float weights[5], sum, sigma2 = 4.0f; weights[0] = gauss(0,sigma2); sum = weights[0]; for( int i = 1; i < 5; i++ ) { weights[i] = gauss(i, sigma2); sum += 2 * weights[i];
} for( int i = 0; i < 5; i++ ) { snprintf(uniName, 20, "Weight[%d]", i); prog.setUniform(uniName, weights[i] / sum);
}

In the main render function, implement the following steps for pass #1:

1. Select the render framebuffer, enable the depth test, and clear the color/depth buffers
2. Set Pass to 1
3. Draw the scene

Use the following steps for pass #2:

1. Select the intermediate framebuffer, disable the depth test, and clear the color buffer
2. Set Pass to 2
3. Set the view, projection, and model matrices to the identity matrix
4. Bind the texture from pass #1 to texture unit zero
5. Draw a fullscreen quad

Use the following steps for pass #3:

1. Deselect the framebuffer (revert to the default), and clear the color buffer
2. Set Pass to 3
3. Bind the texture from pass #2 to texture unit zero
4. Draw a fullscreen quad

How it works...

In the preceding code for computing the Gaussian weights (code segment 3), the function named gauss computes the one-dimensional Gaussian function where the first argument is the value for x and the second argument is sigma squared. Note that we only need to compute the positive offsets because the Gaussian is symmetric about zero. As we are only computing the positive offsets, we need to carefully compute the sum of the weights. We double all of the non-zero values because they will be used twice (for the positive and negative offsets).

The first pass (function pass1) renders the scene to a texture using the Blinn-Phong reflection model.

The second pass (function pass2) applies the weighted vertical sum of the Gaussian blur operation, and stores the results in yet another texture. We read pixels from the texture created in the first pass, offset in the vertical direction by the amounts in the PixOffset array. We sum using weights from the Weight array. (The dy term is the height of a texel in texture coordinates.) We sum in both directions at the same time, a distance of four pixels in each vertical direction.

The third pass (pass3) is very similar to the second pass. We accumulate the weighted, horizontal sum using the texture from the second pass. By doing so, we are incorporating the sums produced in the second pass into our overall weighted sum, as described earlier. Thereby, we are creating a sum over a 9x9 pixel area around the destination pixel. For this pass, the output color goes to the default framebuffer to make up the final result.

There's more...

Of course, we can also adapt the preceding technique to blur a larger range of texels by increasing the size of the arrays Weight and PixOffset and re-computing the weights, and/or we could use different values of sigma2 to vary the shape of the Gaussian.

Implementing HDR lighting with tone mapping

When rendering for most output devices (monitors or televisions), the device only supports a typical color precision of 8-bits per color component, or 24-bits per pixel. Therefore, for a given color component, we're limited to a range of intensities between 0 and 255. Internally, OpenGL uses floating-point values for color intensities, providing a wide range of both values and precision. These are eventually converted to 8-bit values by mapping the floating-point range [0.0, 1.0] to the range of an unsigned byte [0, 255] before rendering.

Real scenes, however, have a much wider range of luminance. For example, light sources that are visible in a scene, or direct reflections of them, can be hundreds to thousands of times brighter than the objects that are illuminated by the source. When we're working with 8-bits per channel, or the floating-point range [0.0, -1.0], we can't represent this range of intensities. If we decide to use a larger range of floating point values, we can do a better job of internally representing these intensities, but in the end, we still need to compress down to the 8-bit range.

The process of computing the lighting/shading using a larger dynamic range is often referred to as High Dynamic Range rendering (HDR rendering). Photographers are very familiar with this concept. When a photographer wants to capture a larger range of intensities than would normally be possible in a single exposure, he/she might take several images with different exposures to capture a wider range of values. This concept, called High Dynamic Range imaging (HDR imaging), is very similar in nature to the concept of HDR rendering. A post-processing pipeline that includes HDR is now considered a fundamentally essential part of any game engine.

Tone mapping is the process of taking a wide dynamic range of values and compressing them into a smaller range that is appropriate for the output device. In computer graphics, generally, tone mapping is about mapping to the 8-bit range from some arbitrary range of values. The goal is to maintain the dark and light parts of the image so that both are visible and neither is completely washed out.

For example, a scene that includes a bright light source might cause our shading model to produce intensities that are greater than 1.0. If we were to simply send that to the output device, anything greater than 1.0 would be clamped to 255 and would appear white. The result might be an image that is mostly white, similar to a photograph that is over exposed.

Or, if we were to linearly compress the intensities to the [0, 255] range, the darker parts might be too dark or completely invisible. With tone mapping, we want to maintain the brightness of the light source and also maintain detail in the darker areas.

This description just scratches the surface when it comes to tone mapping and HDR rendering/imaging. For more details, I recommend the book High Dynamic Range Imaging by Reinhard et al.

The mathematical function used to map from one dynamic range to a smaller range is called the Tone Mapping Operator (TMO). These generally come in two flavors, local operators and global operators. A local operator determines the new value for a given pixel by using its current value and perhaps the value of some nearby pixels. A global operator needs some information about the entire image in order to do its work. For example, it might need to have the overall average luminance of all pixels in the image. Other global operators use a histogram of luminance values over the entire image to help fine-tune the mapping.

In this recipe, we'll use the simple global operator described in the book Real Time
Rendering. This operator uses the log-average luminance of all pixels in the image. The log-average is determined by taking the logarithm of the luminance and averaging those values, then converting back, as shown in the following equation:

Lω′=e1N∑x,yln(0.0001+Lω(x,y))L_\omega^{'}=e^{\frac{1}{N}}\sum_{x,y}ln(0.0001+L_\omega (x,y))Lω′​=eN1​∑x,y​ln(0.0001+Lω​(x,y))

Lω(x,y)L\omega (x,y)Lω(x,y) is the luminance of the pixel at (xxx, yyy). The 0.0001 term is included in order to avoid taking the logarithm of zero for black pixels. This log-average is then used as part of the tone mapping operator shown as follows:

L(x,y)=aLω′Lω(x,y)L(x,y)=\frac{a}{L_\omega^{'}}L_\omega (x,y)L(x,y)=Lω′​a​Lω​(x,y)

The aaa term in this equation is the key. It acts in a similar way to the exposure level in a camera. The typical values for aaa range from 0.18 to 0.72. Since this tone mapping operator compresses the dark and light values a bit too much, we'll use a modification of the previous equation that doesn't compress the dark values as much, and includes a maximum luminance (LwhiteL_{white}Lwhite​), a configurable value that helps to reduce some of the extremely bright pixels:

Ld(x,y)=L(x,y)(1+L(x,y)Lwhite2)1+L(x,y)L_d(x,y)=\frac{L(x,y)(1+\frac{L(x,y)}{L_{white}^2})}{1+L(x,y)}Ld​(x,y)=1+L(x,y)L(x,y)(1+Lwhite2​L(x,y)​)​

This is the tone mapping operator that we'll use in this example. We'll render the scene to a high-resolution buffer, compute the log-average luminance, and then apply the previous tone-mapping operator in a second pass.

However, there's one more detail that we need to deal with before we can start implementing. The previous equations all deal with luminance. Starting with an RGB value, we can compute its luminance, but once we modify the luminance, how do we modify the RGB components to reflect the new luminance without changing the hue (or chromaticity)?

The chromaticity is the perceived color, independent of the brightness of that color. For example, grey and white are two brightness levels for the same color.

The solution involves switching color spaces. If we convert the scene to a color space that separates out the luminance from the chromaticity, then we can change the luminance value independently. The CIE XYZ color space has just what we need. The CIE XYZ color space was designed so that the γ\gammaγ component describes the luminance of the color and the chromaticity can be determined by two derived parameters (xxx and yyy). The derived color space is called the CIE xyγxy\gammaxyγ space, and is exactly what we're looking for. The γ\gammaγ component contains the luminance and the xxx and yyy components contain the chromaticity. By converting to the CIE xyγxy\gammaxyγ space, we've factored out the luminance from the chromaticity allowing us to change the luminance without affecting the perceived color.

So the process involves converting from RGB to CIE XYZ, then converting to CIE xyγxy\gammaxyγ, modifying the luminance, and reversing the process to get back to RGB. Converting from RGB to CIE XYZ (and vice-versa) can be described as a transformation matrix.

The conversion from XYZ to xyγxy\gammaxyγ involves the following:

x=XX+Y+Zy=YX+Y+Zx=\frac{X}{X+Y+Z}\space\space y=\frac{Y}{X+Y+Z}x=X+Y+ZX​y=X+Y+ZY​

Finally, converting from xyγxy\gammaxyγ back to XYZ is done using the following equations:

X=YyxZ=Yy(1−x−y)X=\frac{Y}{y}x\space\space Z=\frac{Y}{y}(1-x-y)X=yY​xZ=yY​(1−x−y)

The following images show an example of the results of this tone mapping operator. The left image shows the scene rendered without any tone mapping. The shading was deliberately calculated with a wide dynamic range using three strong light sources. The scene appears blown out because any values that are greater than 1.0 simply get clamped to the maximum intensity. The image on the right uses the same scene and the same shading, but with the previous tone mapping operator applied. Note the recovery of the specular highlights from the blown out areas on the sphere and teapot:

Getting ready

The steps involved are the following:

1. Render the scene to a high-resolution texture.
2. Compute the log-average luminance (on the CPU).
3. Render a screen-filling quad to execute the fragment shader for each screen pixel. In the fragment shader, read from the texture created in step 1, apply the tone mapping operator, and send the results to the screen.

To get set up, create a high-res texture (using GL_RGB32F or a similar format) attached to a framebuffer with a depth attachment. Set up your fragment shader with a uniform to select the pass. The vertex shader can simply pass through the position and normal in eye coordinates.

How to do it...

To implement HDR tone mapping, we'll perform the following steps:

1. In the first pass, we want to just render the scene to the high-resolution texture. Bind to the framebuffer that has the texture attached and render the scene normally.

2. Compute the log average luminance of the pixels in the texture. To do so, we'll pull the data from the texture and loop through the pixels on the CPU side. We do this on the CPU for simplicity; a GPU implementation, perhaps with a compute shader, would be faster:

    int size = width * height; std::vector<GLfloat> texData(size*3);
    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, hdrTex);
    glGetTexImage(GL_TEXTURE_2D, 0, GL_RGB, GL_FLOAT, texData.data()); float sum = 0.0f; for( int i = 0; i < size; i++ ) { float lum = computeLum(texData[i*3+0], texData[i*3+1], texData[i*3+2]); sum += logf( lum + 0.00001f );
    } float logAve = expf( sum / size );

3. Set the AveLum uniform variable using logAve. Switch back to the default frame buffer, and draw a screen-filling quad. In the fragment shader, apply the tone mapping operator to the values from the texture produced in step 1:

    vec4 color = texture( HdrTex, TexCoord ); vec3 xyzCol = rgb2xyz * vec3(color); float xyzSum = xyzCol.x + xyzCol.y + xyzCol.z; vec3 xyYCol = vec3(0.0); if( xyzSum > 0.0 ) xyYCol = vec3( xyzCol.x / xyzSum, xyzCol.y / xyzSum, xyzCol.y); float L = (Exposure * xyYCol.z) / AveLum;
    L = (L * ( 1 + L / (White * White) )) / ( 1 + L ); if( xyYCol.y > 0.0 ) { xyzCol.x = (L * xyYCol.x) / (xyYCol.y); xyzCol.y = L; xyzCol.z = (L * (1 - xyYCol.x - xyYCol.y))/xyYCol.y;
    } FragColor = vec4( xyz2rgb * xyzCol, 1.0);

How it works...

In the first step, we render the scene to an HDR texture. In step 2, we compute the log-average luminance by retrieving the pixels from the texture and doing the computation on the CPU (OpenGL side).

In step 3, we render a single screen-filling quad to execute the fragment shader for each screen pixel. In the fragment shader, we retrieve the HDR value from the texture and apply the tone-mapping operator. There are two tunable variables in this calculation. The Exposure variable corresponds to the aaa term in the tone mapping operator, and the variable White corresponds to LwhiteL_{white}Lwhite​. For the previous image, we used values of 0.35 and 0.928, respectively.

There's more...

Tone mapping is not an exact science. Often, it is the process of experimenting with the parameters until you find something that works well and looks good.
We could improve the efficiency of the previous technique by implementing step 2 on the GPU using compute shaders (refer to Chapter 11, Using Compute Shaders) or some other clever technique. For example, we could write the logarithms to a texture, then iteratively downsample the full frame to a 1x1 texture. The final result would be available in that single pixel. However, with the flexibility of the compute shader, we could optimize this process even more.

Creating a bloom effect

A bloom is a visual effect where the bright parts of an image seem to have fringes that extend beyond the boundaries into the darker parts of the image. This effect has its basis in the way that cameras and the human visual system perceive areas of high contrast. Sources of bright light bleed into other areas of the image due to the so-called airy disc, which is a diffraction pattern produced by light that passes through an aperture.

The following image shows a bloom effect in the animated film Elephant's Dream (© 2006, Blender Foundation / Netherlands Media Art Institute / www.elephantsdream.org). The bright white color from the light behind the door bleeds into the darker parts of the image:

Producing such an effect within an artificial CG rendering requires determining which parts of the image are bright enough, extracting those parts, blurring, and re-combining with the original image. Typically, the bloom effect is associated with HDR rendering. With HDR rendering, we can represent a larger range of intensities for each pixel (without quantizing artifacts). The bloom effect is more accurate when used in conjunction with HDR rendering due to the fact that a wider range of brightness values can be represented.

Despite the fact that HDR produces higher quality results, it is still possible to produce a bloom effect when using standard (non-HDR) color values. The result may not be as effective, but the principles involved are similar for either situation.

In the following example, we'll implement a bloom effect using five passes, consisting of four major steps:

1. In the first pass, we will render the scene to an HDR texture.

2. The second pass will extract the parts of the image that are brighter than a certain threshold value. We'll refer to this as the bright-pass filter. We'll also downsample to a lower resolution buffer when applying this filter. We do so because we will gain additional blurring of the image when we read back from this buffer using a linear sampler.

3. The third and fourth passes will apply the Gaussian blur to the bright parts (refer to the Applying a Gaussian blur filter recipe in this chapter).

4. In the fifth pass, we'll apply tone mapping and add the tone-mapped result to the blurred bright-pass filter results.

The following diagram summarizes this process. The upper-left image shows the scene rendered to an HDR buffer, with some of the colors out of gamut, causing much of the image to be blown-out. The bright-pass filter produces a smaller (about a quarter or an eighth of the original size) image with only pixels that correspond to a luminance that is above a threshold. The pixels are shown as white because they have values that are greater than one in this example. A two-pass Gaussian blur is applied to the downsampled image, and tone mapping is applied to the original image. The final image is produced by combining the tone-mapped image with the blurred bright-pass filter image. When sampling the latter, we use a linear filter to get additional blurring.The final result is shown at the bottom.

Note the bloom on the bright highlights on the sphere and the back wall:

Getting ready

For this recipe, we'll need two framebuffer objects, each associated with a texture. The first will be used for the original HDR render, and the second will be used for the two passes of the Gaussian blur operation. In the fragment shader, we'll access the original render via the variable HdrTex, and the two stages of the Gaussian blur will be accessed via BlurTex.

The uniform variable LumThresh is the minimum luminance value used in the second pass. Any pixels greater than that value will be extracted and blurred in the following passes.

Use a vertex shader that passes through the position and normal in eye coordinates.

How to do it...

To generate a bloom effect, perform the following steps:

1. In the first pass, render the scene to the framebuffer with a high-res backing texture.

2. In the second pass, switch to a framebuffer containing a high-res texture that is smaller than the size of the full render. In the example code, we use a texture that is one-eighth the size. Draw a fullscreen quad to initiate the fragement shader for each pixel, and in the fragment shader sample from the high-res texture, and write only those values that are larger than LumThresh. Otherwise, color the pixel black:

    vec4 val = texture(HdrTex, TexCoord); if( luminance(val.rgb) > LumThresh ) FragColor = val; else FragColor = vec4(0.0);

3. In the third and fourth passes, apply the Gaussian blur to the results of the second pass. This can be done with a single framebuffer and two textures. Ping-pong between them, reading from one and writing to the other. For details, refer to the Applying a Gaussian blur filter recipe in this chapter.

4. In the fifth and final pass, switch to linear filtering from the texture that was produced in the fourth pass. Switch to the default frame buffer (the screen). Apply the tone-mapping operator from the Implementing HDR lighting with tone mapping recipe to the original image texture (HdrTex), and combine the results with the blurred texture from step 3. The linear filtering and magnification should provide an additional blur:

    vec4 color = texture( HdrTex, TexCoord ); ... vec4 blurTex = texture(BlurTex1, TexCoord); FragColor = toneMapColor + blurTex;

How it works...

Due to space constraints, the entire fragment shader code isn't shown here. The full code is available from the GitHub repository. The fragment shader is implemented with five methods, one for each pass. The first pass renders the scene normally to the HDR texture.

During this pass, the active framebuffer object is the one associated with the texture corresponding to HdrTex, so the output is sent directly to that texture.

The second pass reads from HdrTex, and writes out only pixels that have a luminance above the threshold value LumThresh. The value is (0,0,0,0) for pixels that have a brightness (luma) value below LumThresh. The output goes to the second framebuffer, which contains a much smaller texture (one-eighth the size of the original).

The third and fourth passes apply the basic Gaussian blur operation (refer to the Applying a Gaussian blur filter recipe in this chapter). In these passes, we ping-pong between BlurTex1 and BlurTex2, so we must be careful to swap the appropriate texture into the framebuffer.

In the fifth pass, we switch back to the default framebuffer, and read from HdrTex and BlurTex1. BlurTex1 contains the final blurred result from step four, and HdrTex contains the original render. We apply tone mapping to the results of HdrTex and add to BlurTex1. When pulling from BlurTex1, we are applying a linear filter, gaining additional blurring.

There's more...

Note that we applied the tone-mapping operator to the original rendered image, but not to the blurred bright-pass filter image. One could choose to apply the TMO to the blurred image as well, but in practice, it is often not necessary. We should keep in mind that the bloom effect can also be visually distracting if it is overused. A little goes a long way.

Using gamma correction to improve image quality

It is common for many books about OpenGL and 3D graphics to somewhat neglect the subject of gamma correction. Lighting and shading calculations are performed, and the results are sent directly to the output buffer without modification. However, when we do this, we may produce results that don't quite end up looking the way we might expect. This may be due to the fact that computer monitors (both the old CRT and the newer LCD) have a non-linear response to pixel intensity. For example, without gamma correction, a grayscale value of 0.5 will not appear half as bright as a value of 1.0. Instead, it will appear to be darker than it should.

The lower curve in the following graph shows the response curves of a typical monitor (gamma of 2.2). The x axis is the intensity and the y axis is the perceived intensity. The dashed line represents a linear set of intensities. The upper curve represents gamma correction applied to linear values. The lower curve represents the response of a typical monitor. A grayscale value of 0.5 would appear to have a value of 0.218 on a screen that had a similar response curve:

The non-linear response of a typical monitor can usually be modeled using a simple power function. The perceived intensity (PPP) is proportional to the pixel intensity (III) raised to a power that is usually called gamma:

P=IγP=I^\gammaP=Iγ

Depending on the display device, the value of γ\gammaγ is usually somewhere between 2.0 and 2.4. Some kind of monitor calibration is often needed to determine a precise value.

In order to compensate for this non-linear response, we can apply gamma correction before sending our results to the output framebuffer. Gamma correction involves raising the pixel intensities to a power that will compensate for the monitor's non-linear response to achieve a perceived result that appears linear. Raising the linear-space values to the power of 1γ\frac{1}{\gamma}γ1​ will do the trick:

I=(I1γ)γI=(I^{\frac{1}{\gamma}})^\gammaI=(Iγ1​)γ

When rendering, we can do all of our lighting and shading computations, ignoring the fact that the monitor's response curve is non-linear. This is sometimes referred to as working in linear space. When the final result is to be written to the output framebuffer, we can apply the gamma correction by raising the pixel to the power of 1γ\frac{1}{\gamma}γ1​ just before writing. This is an important step that will help to improve the look of the rendered result.

As an example, consider the following images. The image on the left is the mesh rendered without any consideration of gamma at all. The reflection model is computed and the results are directly sent to the framebuffer. On the right is the same mesh with gamma correction applied to the color just prior to output:

The obvious difference is that the left image appears much darker than the image on the right. However, the more important distinction is the variations from light to dark across the face. While the transition at the shadow terminator seems stronger than before, the variations within the lighted areas are less extreme.

Applying gamma correction is an important technique, and can be effective in improving the results of a lighting model.

How to do it...

Adding gamma correction to an OpenGL program can be as simple as carrying out the following steps:

1. Set up a uniform variable named Gamma and set it to an appropriate value for your system.

2. Use the following code or something similar in a fragment shader:

    vec3 color = lightingModel( ... );
    FragColor = vec4( pow( color, vec3(1.0/Gamma) ), 1.0 );

If your shader involves texture data, care must be taken to make sure that the texture data is not already gamma-corrected so that you don't apply gamma correction twice (refer to the There's more... section of this recipe).

How it works...

The color determined by the lighting/shading model is computed and stored in the variable color. We think of this as computing the color in linear space. There is no consideration of the monitor's response during the calculation of the shading model (assuming that we don't access any texture data that might already be gamma-corrected).

To apply the correction, in the fragment shader, we raise the color of the pixel to the power of 1.0 / Gamma, and apply the result to the output variable FragColor. Of course, the inverse of Gamma could be computed outside the fragment shader to avoid the division operation.

We do not apply the gamma correction to the alpha component because it is typically not desired.

There's more...

The application of gamma correction is a good idea in general; however, some care must be taken to make sure that computations are done within the correct space. For example, textures could be photographs or images produced by other imaging applications that apply gamma correction before storing the data within the image file.

Therefore, if we use a texture in our application as a part of the lighting model and then apply gamma correction, we will be effectively applying gamma correction twice to the data from the texture. Instead, we need to be careful to "decode" the texture data, by raising to the power of gamma prior to using the texture data in our lighting model.

There is a very detailed discussion about these and other issues surrounding gamma correction in Chapter 24, The Importance of Being Linear in the book GPU Gems 3, edited by Hubert Nguyen (Addison-Wesley Professional 2007), and this is highly recommended supplemental reading.

Using multisample anti-aliasing

Anti-aliasing is the technique of removing or reducing the visual impact of aliasing artifacts that are present whenever high-resolution or continuous information is presented at a lower resolution. In real-time graphics, aliasing often reveals itself in the jagged appearance of polygon edges, or the visual distortion of textures that have a high degree of variation.

The following images show an example of aliasing artifacts at the edge of an object. On the left, we can see that the edge appears jagged. This occurs because each pixel is determined to lie either completely inside the polygon or completely outside it. If the pixel is determined to be inside, it is shaded, otherwise it is not. Of course, this is not entirely accurate. Some pixels lie directly on the edge of the polygon. Some of the screen area that the pixel encompasses actually lies within the polygon and some lies outside. Better results could be achieved if we were to modify the shading of a pixel based upon the amount of the pixel's area that lies within the polygon. The result could be a mixture of the shaded surface's color with the color outside the polygon, where the area that is covered by the pixel determines the proportions. You might be thinking that this sounds like it would be prohibitively expensive to do. That may be true; however, we can approximate the results by using multiple samples per pixel.

Multisample anti-aliasing involves evaluating multiple samples per pixel and combining the results of those samples to determine the final value for the pixel. The samples are located at various points within the pixel's extent. Most of these samples will fall inside the polygon, but for pixels near a polygon's edge, some will fall outside. The fragment shader will typically execute only once for each pixel as usual. For example, with 4x multisample anti-aliasing (MSAA), rasterization happens at four times the frequency. For each pixel, the fragment shader is executed once and the result is scaled based on how many of the four samples fall within the polygon.

The following image on the right shows the results when multisample anti-aliasing is used. The inset image is a zoomed portion of the inside edge of a torus. On the left, the torus is rendered without MSAA. The right-hand image shows the results with MSAA enabled:

OpenGL has supported multisampling for some time now, and it is nearly transparent to use. It is simply a matter of turning it on or off. It works by using additional buffers to store the subpixel samples as they are processed. Then, the samples are combined together to produce a final color for the fragment. Nearly all of this is automatic, and there is little that a programmer can do to fine-tune the results. However, at the end of this recipe, we'll discuss the interpolation qualifiers that can affect the results.

In this recipe, we'll see the code needed to enable multisample anti-aliasing in an OpenGL application.

Getting ready

The technique for enabling multisampling is unfortunately dependent on the window system API. In this example, we'll demonstrate how it is done using GLFW. The steps will be similar in GLUT or other APIs that support OpenGL.

How to do it...

To make sure that the multisample buffers are created and available, use the following steps:

1. When creating your OpenGL window, you need to select an OpenGL context that supports MSAA. The following is how one would do so in GLFW:

    glfwWindowHint(GLFW_SAMPLES, 8);
    ... window = glfwCreateWindow( WIN_WIDTH, WIN_HEIGHT, "Window title", NULL, NULL );

2. To determine whether multisample buffers are available and how many samples per-pixel are actually being used, you can use the following code (or something similar):

    GLint bufs, samples;
    glGetIntegerv(GL_SAMPLE_BUFFERS, &bufs);
    glGetIntegerv(GL_SAMPLES, &samples); printf("MSAA: buffers = %d samples = %dn", bufs, samples);

3. To enable multisampling, use the following:

    glEnable(GL_MULTISAMPLE);

4. To disable multisampling, use the following:

    glDisable(GL_MULTISAMPLE);

How it works...

As we just mentioned, the technique for creating an OpenGL context with multisample buffers is dependent on the API used for interacting with the window system. The preceding example demonstrates how it might be done using GLFW. Once the OpenGL context is created, it is easy to enable multisampling by simply using the glEnable call shown in the preceding example.

Stay tuned, because in the next section, we'll discuss a subtle issue surrounding interpolation of shader variables when multisample anti-aliasing is enabled.

There's more...

There are two interpolation qualifiers within the GLSL that allow the programmer to fine-tune some aspects of multisampling. They are sample and centroid.

Before we can get into how sample and centroid work, we need a bit of background.
Let's consider the way that polygon edges are handled without multisampling. A fragment is determined to be inside or outside of a polygon by determining where the center of that pixel lies. If the center is within the polygon, the pixel is shaded, otherwise it is not. The following image represents this behavior. It shows pixels near a polygon edge without MSAA. The line represents the edge of the polygon. Gray pixels are considered to be inside the polygon. White pixels are outside and are not shaded. The dots represent the pixel centers:

The values for the interpolated variables (the fragment shader's input variables) are interpolated with respect to the center of each fragment, which will always be inside the polygon.

When multisample anti-aliasing is enabled, multiple samples are computed per fragment at various locations within the fragment's extent. If any of those samples lie within the polygon, then the shader is executed at least once for that pixel (but not necessarily for each sample).

As a visual example, the following image represents pixels near a polygon's edge. The dots represent the samples. The dark samples lie within the polygon and the white samples lie outside the polygon. If any sample lies within the polygon, the fragment shader is executed (usually only once) for that pixel. Note that for some pixels, the pixel center lies outside the polygon. So, with MSAA, the fragment shader may execute slightly more often near the edges of polygons:

Now, here's the important point. The values of the fragment shader's input variables are normally interpolated to the center of the pixel rather than to the location of any particular sample. In other words, the value that is used by the fragment shader is determined by interpolating to the location of the fragment's center, which may lie outside the polygon! If we are relying on the fact that the fragment shader's input variables are interpolated strictly between their values at the vertices (and not outside that range), then this might lead to unexpected results.

As an example, consider the following portion of a fragment shader:

in vec2 TexCoord; layout( location = 0 ) out vec4 FragColor; void main() { vec3 yellow = vec3(1.0,1.0,0.0); vec3 color = vec3(0.0); if ( TexCoord.s > 1.0 ) color = yellow; FragColor = vec4( color , 1.0 );
}

This shader is designed to color the polygon black unless the s component of the texture coordinate is greater than one. In that case, the fragment gets a yellow color. If we render a square with texture coordinates that range from zero to one in each direction, we may get the results shown in the following image on the left. The following images show the enlarged edge of a polygon where the s texture coordinate is about 1.0. Both images were rendered using the preceding shader. The right-hand image was created using the centroid qualifier (more on this later in this chapter):

The left image shows that some pixels along the edge have a lighter color (yellow, if the image is in full color). This is due to the fact that the texture coordinate is interpolated to the pixel's center, rather than to any particular sample's location. Some of the fragments along the edge have a center that lies outside of the polygon and therefore end up with a texture coordinate that is greater than one!

We can ask OpenGL to instead compute the value for the input variable by interpolating to some location that is not only within the pixel but also within the polygon. We can do so by using the centroid qualifier, as shown in the following code:

centroid in vec2 TexCoord;

(The qualifier needs to also be included with the corresponding output variable in the vertex shader.) When centroid is used with the preceding shader, we get the preceding image shown on the right.

In general, we should use centroid or sample when we know that the interpolation of the input variables should not extend beyond the values of those variables at the vertices.

The sample qualifier forces OpenGL to interpolate the shader's input variables to the actual location of the sample itself: sample in vec2 TexCoord;

This, of course, requires that the fragment shader be executed once for each sample. This will produce the most accurate results, but the performance hit may not be worthwhile, especially if the visual results produced by centroid (or without the default) are good enough.

Using deferred shading

Deferred shading is a technique that involves postponing (or deferring) the lighting/shading step to a second pass. We do this (among other reasons) in order to avoid shading a pixel more than once. The basic idea is as follows:

1. In the first pass, we render the scene, but instead of evaluating the reflection model to determine a fragment color, we simply store all of the geometry information (position, normal, texture coordinate, reflectivity, and so on) in an intermediate set of buffers, collectively called the g-buffer (g for geometry).

2. In the second pass, we simply read from the g-buffer, evaluate the reflection model, and produce a final color for each pixel.

When deferred shading is used, we avoid evaluating the reflection model for a fragment that will not end up being visible. For example, consider a pixel located in an area where two polygons overlap. The fragment shader may be executed once for each polygon that covers that pixel; however, the resulting color of only one of the two executions will end up being the final color for that pixel (assuming that blending is not enabled). The cycles spent in evaluating the reflection model for one of the two fragments are effectively wasted. With deferred shading, the evaluation of the reflection model is postponed until all the geometry has been processed, and the visible geometry is known at each pixel location. Hence, the reflection model is evaluated only once for each pixel on the screen. This allows us to do lighting in a more efficient fashion. For example, we could use even hundreds of light sources because we are only evaluating the lighting once per screen pixel.

Deferred shading is fairly simple to understand and work with. It can therefore help with the implementation of complex lighting/reflection models.

In this recipe, we'll go through a simple example of deferred shading. We'll store the following information in our g-buffer: the position, normal, and diffuse color (the diffuse reflectivity). In the second pass, we'll simply evaluate the diffuse lighting model using the data stored in the g-buffer.

This recipe is meant to be a starting point for deferred shading. If we were to use deferred shading in a more substantial (real-world) application, we'd probably need more components in our g-buffer. It should be straightforward to extend this example to use more complex lighting/shading models.

Getting ready

The g-buffer will contain three textures for storing the position, normal, and diffuse color. There are three uniform variables that correspond to these three textures: PositionTex, NormalTex, and ColorTex; these textures should be assigned to texture units 0, 1, and 2, respectively. Likewise, the vertex shader assumes that position information is provided in vertex attribute 0, the normal is provided in attribute 1, and the texture coordinate in attribute 2.

The fragment shader has several uniform variables related to light and material properties that must be set from the OpenGL program. Specifically, the structures Light and Material apply to the shading model used here.

You'll need a variable named deferredFBO (type GLuint) to store the handle to the FBO.

How to do it...

To create the framebuffer object that contains our g-buffer(s) use the following code:

void createGBufTex(GLenum texUnit, GLenum format, GLuint &texid ) { glActiveTexture(texUnit); glGenTextures(1, &texid); glBindTexture(GL_TEXTURE_2D, texid); glTexStorage2D(GL_TEXTURE_2D,1,format,width,height); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
} ... GLuint depthBuf, posTex, normTex, colorTex; glGenFramebuffers(1, &deferredFBO);
glBindFramebuffer(GL_FRAMEBUFFER, deferredFBO); glGenRenderbuffers(1, &depthBuf);
glBindRenderbuffer(GL_RENDERBUFFER, depthBuf);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, width, height); createGBufTex(GL_TEXTURE0, GL_RGB32F, posTex); createGBufTex(GL_TEXTURE1, GL_RGB32F, normTex); createGBufTex(GL_TEXTURE2, GL_RGB8, colorTex); glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthBuf);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, posTex, 0);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT1, GL_TEXTURE_2D, normTex, 0);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT2, GL_TEXTURE_2D, colorTex, 0); GLenumdrawBuffers[] = {GL_NONE, GL_COLOR_ATTACHMENT0, GL_COLOR_ATTACHMENT1,GL_COLOR_ATTACHMENT2};
glDrawBuffers(4, drawBuffers);

During the first pass, the fragment shader writes to the G-buffers. In the second pass, it reads from them and applies the shading model.

in vec3 Position; in vec3 Normal; in vec2 TexCoord; layout (location = 0) out vec4 FragColor; layout (location = 1) out vec3 PositionData; layout (location = 2) out vec3 NormalData; layout (location = 3) out vec3 ColorData; layout(binding = 0) uniform sampler2D PositionTex; layout(binding = 1) uniform sampler2D NormalTex; layout(binding = 2) uniform sampler2D ColorTex; uniform int Pass; vec3 diffuseModel( vec3 pos, vec3 norm, vec3 diff ) { vec3 s = normalize( vec3(Light.Position) - pos); float sDotN = max( dot(s,norm), 0.0 ); return Light.L * diff * sDotN;
} void pass1() { PositionData = Position; NormalData = Normal; ColorData = Material.Kd;
} void pass2() { vec3 pos = vec3( texture( PositionTex, TexCoord ) ); vec3 norm = vec3( texture( NormalTex, TexCoord ) ); vec3 diffColor = vec3( texture(ColorTex, TexCoord) ); FragColor=vec4(diffuseModel(pos,norm,diffColor), 1.0);
} void main() { if ( Pass == 1 ) pass1(); else if ( Pass==2 ) pass2();
}

In the render function of the OpenGL application, use the following steps for pass #1:

1. Bind to the framebuffer object deferredFBO
2. Clear the color/depth buffers, set Pass to 1, and enable the depth test (if necessary)
3. Render the scene normally

Use the following steps for pass #2:

1. Revert to the default FBO (bind to framebuffer 0)
2. Clear the color buffer, set Pass to 2, and disable the depth test (if desired)
3. Render a screen-filling quad (or two triangles) with texture coordinates that range from zero to one in each direction

How it works...

When setting up the FBO for the g-buffer, we use textures with the internal format GL_RGB32F for the position and normal components. As we are storing geometry information, rather than simply color information, there is a need to use a higher resolution (that is more bits per pixel). The buffer for the diffuse reflectivity just uses GL_RGB8 since we don't need the extra resolution for these values.

The three textures are then attached to the framebuffer at color attachments 0, 1, and 2 using glFramebufferTexture2D. They are then connected to the fragment shader's output variables with the call to glDrawBuffers:

glDrawBuffers(4, drawBuffers);

The array drawBuffers indicates the relationship between the framebuffer's components and the fragment shader's output variable locations. The i th item in the array corresponds to the i th output variable location. This call sets color attachments 0, 1, and 2 to output variable locations 1, 2, and 3, respectively. (Note that the fragment shader's corresponding variables are PositionData, NormalData, and ColorData.)

During pass 2, it is not strictly necessary to convert and pass through the normal and position, as they will not be used in the fragment shader at all. However, to keep things simple, this optimization is not included. It would be a simple matter to add a subroutine to the vertex shader in order to switch off the conversion during pass 2. (Of course, we need to set gl_Position regardless.)

In the fragment shader, the functionality depends on the value of the variable Pass. It will either call pass1 or pass2, depending on its value. In the pass1 function, we store the values of Position, Normal, and Material.Kd in the appropriate output variables, effectively storing them in the textures that we just talked about.

In the pass2 function, the values of the position, normal, and color are retrieved from the textures, and used to evaluate the diffuse lighting model. The result is then stored in the output variable FragColor. In this pass, FragColor should be bound to the default framebuffer, so the results of this pass will appear on the screen.

There's more...

In the graphics community, the relative advantages and disadvantages of deferred shading are a source of some debate. Deferred shading is not ideal for all situations. It depends greatly on the specific requirements of your application, and one needs to carefully evaluate the benefits and drawbacks before deciding whether or not to use deferred shading.

Multi-sample anti-aliasing with deferred shading is possible in recent versions of OpenGL by making use of GL_TEXTURE_2D_MULTISAMPLE.

Another consideration is that deferred shading can't do blending/transparency very well. In fact, blending is impossible with the basic implementation we saw some time ago. Additional buffers with depth-peeling can help by storing additional layered geometry information in the g-buffer.

One notable advantage of deferred shading is that one can retain the depth information from the first pass and access it as a texture during the shading pass. Having access to the entire depth buffer as a texture can enable algorithms such as depth of field (depth blur), screen space ambient occlusion, volumetric particles, and other similar techniques.

For much more information about deferred shading, refer to Chapter 9 in GPU Gems 2 edited by Matt Pharr and Randima Fernando (Addison-Wesley Professional 2005) and Chapter 19 of GPU Gems 3 edited by Hubert Nguyen (Addison-Wesley Professional 2007).

Both combined, provide an excellent discussion of the benefits and drawbacks of deferred shading, and how to make the decision whether or not to use it in your application.

Screen space ambient occlusion

Ambient occlusion is a rendering technique that is based on the assumption that a surface receives uniform illumination from all directions. Some surface positions will receive less light than others due to objects nearby that occlude some of the light. If a surface point has a lot of local geometry nearby, some of this ambient illumination will be blocked causing the point to be darker.

An example of this is shown in the following image (generated using Blender):

This image is rendered using ambient occlusion only, without light sources. Note how the result looks like shadows in areas that have local geometry occluding the ambient illumination. The result is quite pleasing to the eye and adds a significant amount of realism to an image.

Ambient occlusion is calculated by testing the visibility of a surface point from the upper hemisphere centered at the surface point. Consider the two points A and B in the following diagram:

Point A is near a corner of the surface and point B is located on a flat area. The arrows represent directions for visibility testing. All directions in the hemisphere above point B are unoccluded, meaning that the rays do not intersect with any geometry. However, in the hemisphere above point A, roughly half of the directions are occluded (arrows with dashed lines). Therefore, A should receive less illumination and appear darker than point B.

Essentially, ambient occlusion boils down to the following process. Sample as many directions as possible in the upper hemisphere around the surface point. Test each direction for visibility (occlusion). The fraction of rays that are unoccluded gives the ambient occlusion factor at that point.

This process generally requires a large number of samples to produce acceptable results. To do this for every vertex of a mesh would be impractical in real time for complex scenes. However, the results can be precomputed and stored in a texture for static scenes. If the geometry can move, we need some approximation that is independent of the complexity of the scene.

Screen space ambient occlusion (or SSAO) is the name for a class of algorithms that attempt to approximate ambient occlusion in real time using screen space information. In other words, with SSAO, we compute the ambient occlusion in a post process after the scene has been rendered using the data stored in the depth buffer and/or geometry buffers. SSAO works naturally in conjunction with deferred shading (see the recipe Using deferred shading), but has been implemented with forward (non-deferred) renderers as well.

In this recipe, we'll implement SSAO as part of a deferred rendering process. We'll compute the ambient occlusion factor at each screen-space pixel, rather than on the surface of each object in the scene, ignoring any geometry that is occluded from the camera. After the first pass of a deferred shading renderer, we have position, normal, and color information for the visible surface locations at each screen pixel in our g-buffers (see the Using deferred shading recipe). For each pixel, we'll use the position and normal vector to define the hemisphere above the surface point. Then, we will randomly choose locations (samples) within that hemisphere and test each location for visibility:

The preceding image represents occlusion testing for the surface point P. The filled and hollow circles are random sample points chosen within the hemisphere above P, centered along the normal vector. Hollow circles fail the visibility test and the filled circles pass.

To accurately test for visibility, we would need to trace a ray from the surface point toward all sample points and check each ray for intersection with a surface. However, we can avoid that expensive process. Rather than tracing rays, we'll estimate the visibility by defining a point as visible from the surface point in the following way. If the point is visible from the camera, we'll assume that it is also visible from the surface point. This can be inaccurate for some cases, but is a good approximation for a wide variety of typical scenes.

In fact, we'll use one additional approximation. We won't trace a ray from the camera to the surface point; instead, we'll just compare the z coordinates of the point being tested and the surface point at the same (x,y) position in camera space. This introduces another small amount of error, but not enough to be objectionable in practice. The following diagram illustrates this concept:

For each sample that we test within the hemisphere, we find the corresponding location on the camera-visible surface at the same (x,y) position in camera coordinates. This position is simply the value in the position g-buffer at the (x,y) location of the sample. We then compare the z coordinates of the sample and the surface point. If the z coordinate of the surface point is larger than the sample's z coordinate (remember, we're in camera coordinates so all z coordinates will be negative), then we consider the sample to be occluded by the surface.

As the preceding diagram shows, this is an approximation of what a trace of the eye-ray might find. What precedes is a basic overview of the process and there's not much more to it than that. This algorithm boils down to testing a number of random samples in the hemisphere above each point.

The proportion of visible samples is the ambient occlusion factor. Of course, there are a bunch of details to be worked out. Let's start with an overview of the process. We'll implement this algorithm with four passes.

1. The first pass renders the data to the g-buffers: camera space position, normal, and base color.
2. The second pass computes the ambient occlusion factor for each screen pixel.
3. The third pass is a simple blur of the ambient occlusion data to remove high frequency artifacts.
4. The final pass is a lighting pass. The reflection model is evaluated, integrating the ambient occlusion.

The last three of these passes employs a screen-space technique, meaning that we invoke the fragment shader once for each pixel on the screen by rendering just a single screen filling quad. The actual geometry for the scene is only rendered during the first pass. The bulk of the interesting stuff here happens in pass 2. There we need to generate a number of random points in the hemisphere above the surface at each point. Random number generation within a shader is challenging for a number of reasons that we won't go into here. So, instead of trying to generate random numbers, we'll pre-generate a set of random points in a hemisphere centered around the z axis. We'll refer to this as our random kernel. We'll re-use this kernel at each point by transforming the points to camera space, aligning the kernel's z axis with the normal vector at the surface point. To squeeze out a bit more randomness, we'll also rotate the kernel around the normal vector by a random amount.

We'll cover the details in the steps presented in the following sections.

Getting ready

First, let's build our random kernel. We need a set of points in the positive-z hemisphere centered at the origin. We'll use a hemisphere with a radius of 1.0 so that we can scale it to any size as needed:

int kernSize = 64; std::vector<float> kern(3 * kernSize); for (int i = 0; i < kernSize; i++) { glm::vec3 randDir = rand.uniformHemisphere(); float scale = ((float)(i * i)) / (kernSize * kernSize); randDir *= glm::mix(0.1f, 1.0f, scale); kern[i * 3 + 0] = randDir.x; kern[i * 3 + 1] = randDir.y; kern[i * 3 + 2] = randDir.z;
}

The uniformHemisphere function chooses a random point on the surface of the hemisphere in a uniform fashion. The details of how to do this were covered in an earlier recipe (see Diffuse image based lighting). To get a point within the hemisphere, we scale the point by the variable scale. This value will vary from 0 to 1 and is non-linear. It will produce more points close to the origin and fewer as we move away from the origin. We do this because we want to give slightly more weight to things that are close to the surface point.
We assign the values of the kernel points to a uniform variable (array) in our shader named SampleKernel.

As mentioned earlier, we want to re-use this kernel for each surface point, but with a random rotation. To do so, we'll build a small texture containing random rotation vectors. Each vector will be a unit vector in the x-y plane:

int size = 4; std::vector<GLfloat> randDirections(3 * size * size); for (int i = 0; i < size * size; i++) { glm::vec3 v = rand.uniformCircle(); randDirections[i * 3 + 0] = v.x; randDirections[i * 3 + 1] = v.y; randDirections[i * 3 + 2] = v.z;
} glGenTextures(1, &tex);
glBindTexture(GL_TEXTURE_2D, tex);
glTexStorage2D(GL_TEXTURE_2D, 1, GL_RGB16F, size, size);
glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, size, size, GL_RGB, GL_FLOAT, randDirections.data());

The uniformCircle function gives a random point on the unit circle in the x-y plane. We're using a 4 x 4 texture here, but you could use a larger size. We'll tile this texture across the screen, and we'll make it available to the shader (uniform variable RandTex).

You might be thinking that a 4 x 4 texture is too small to give us enough randomness. Yes, it will produce high-frequency patterns, but the blur pass will help to smooth that noise out.

In this example, we'll use a single shader and a single framebuffer. You could, of course, use several if desired. We'll need framebuffer textures for the camera space position, camera space normal, base color, and ambient occlusion. The AO buffer can be a single channel texture (for example, format R_16F). We'll also need one additional AO texture for the blur pass. We will swap each one into the framebuffer as necessary.

How to do it...

1. In the first pass, render the scene to the geometry buffers (see Using deferred shading for details).

2. In the second pass, we'll use this fragment shader code to compute the AO factor. To do so, we first compute a matrix for converting the kernel points into camera space. When doing so, we use a vector from RandTex to rotate the kernel. This process is similar to computing the tangent space matrix in normal mapping. For more on this, see Using normal maps:

    vec3 randDir = normalize( texture(RandTex, TexCoord.xy * randScale).xyz ); vec3 n = normalize( texture(NormalTex, TexCoord).xyz ); vec3 biTang = cross( n, randDir ); if( length(biTang) < 0.0001 ) biTang = cross( n, vec3(0,0,1)); biTang = normalize(biTang); vec3 tang = cross(biTang, n); mat3 toCamSpace = mat3(tang, biTang, n);

3. Then, we compute the ambient occlusion factor by looping over all of the kernel points, transforming them into camera coordinates, and then finding the surface point at the same (x,y) position and comparing the z values. We write the result to the AO buffer:

    float occlusionSum = 0.0; vec3 camPos = texture(PositionTex, TexCoord).xyz; for( int i = 0; i < kernelSize; i++ ) { vec3 samplePos = camPos + Radius * (toCamSpace * SampleKernel[i]); vec4 p = ProjectionMatrix * vec4(samplePos,1); p *= 1.0 / p.w; p.xyz = p.xyz * 0.5 + 0.5; float surfaceZ = texture(PositionTex, p.xy).z; float dz = surfaceZ - camPos.z; if ( dz >= 0.0 && dz <= Radius && surfaceZ > samplePos.z ) occlusionSum += 1.0;
    } AoData = 1.0 - occlusionSum / kernelSize;

4. In the third pass, we do a simple blur, using a unweighted average of the nine nearest pixels. We read from the texture that was written in the previous pass, and write the results to our second AO buffer texture:

    ivec2 pix = ivec2( gl_FragCoord.xy ); float sum = 0.0; for( int x = -1; x <= 1; ++x ) { for( int y = -1; y <= 1; y++ ) { sum += texelFetchOffset( AoTex, pix, 0, ivec2(x,y) ).r; }
    }
    AoData = sum / 9.0;

5. The fourth pass applies the reflection model using the ambient occlusion value from the previous pass. We scale the ambient portion by the value in the AO buffer raised to the fourth power (to slightly exaggerate the effect):

    vec3 pos = texture( PositionTex, TexCoord ).xyz; vec3 norm = texture( NormalTex, TexCoord ).xyz; vec3 diffColor = texture(ColorTex, TexCoord).rgb; float aoVal = texture( AoTex, TexCoord).r; aoVal = pow(aoVal, 4); vec3 ambient = Light.La * diff * aoVal; vec3 s = normalize( vec3(Light.Position) - pos); float sDotN = max( dot(s,norm), 0.0 ); vec3 col = ambient + Light.L * diff * sDotN; col = pow(col, vec3(1.0/2.2)); FragColor = vec4(col, 1.0);

How it works...

In the second pass, we compute the ambient occlusion factor. To do so, the first step is to find the matrix that converts our kernel into camera space. We want a matrix that converts the kernel's z axis to the normal vector at the surface point, and applies a random rotation using a vector from RandTex. The columns of the matrix are the three ortho-normal vectors that define the tangent coordinate system in screen space. Since we want the kernel's z axis to be transformed to the normal vector, the third of these three vectors is the normal vector itself. The other two (tang and biTang) are determined by using cross products. To find biTang, we take the cross product of the normal vector (n) and the random rotation vector retrieved from the texture (randDir). As long as the two are not parallel, this will give us a vector that is perpendicular to both n and randDir. However, there is a small possibility that the two might be parallel. If so, the normal vector is in the x-y plane of camera space (because all of the rotation vectors in the texture are in the x-y plane). So in this case, we compute biTang by taking the cross product of n and the z axis. Next, we normalize biTang.

Note that we scale the texture coordinates when accessing the random texture to get the random rotation vector. We do this because the texture is smaller than the size of the screen and we want to tile it to fill the screen so that a texel matches the size of a screen pixel.

Now that we have two ortho-normal vectors, we can compute the third with the cross product of the two. The three vectors tang, biTang, and n make up the axes of the tangent space coordinate system. The matrix that converts from the tangent system to camera space (toCamSpace) has these three vectors as its columns.

Now that we have the toCamSpace matrix, we can loop over the 64 kernel points and test each of them. Note that we're not actually treating these as points. Instead, they are treated as vectors that define an offset from the surface point. So a sample point is determined by the following line:

vec3 samplePos = camPos + Radius * (toCamSpace * SampleKernel[i]);

Here, we take a vector from the sample kernel, convert it to camera space, scale it by Radius, and add it to the position of the surface point. The scale factor (Radius) is an important term that defines the size of the hemisphere around the point. It is a camera space value and may need to be adjusted for different scenes. In the example code, a value of 0.55 is used.

The next step is to find the visible surface at the sample point's (x,y) position. To do so, we need to look up the value of the position in the g-buffer at the sample position. We need the texture coordinates that correspond to that position. To find that, we first project the point to clip space, divide by the homogeneous w coordinate, and scale/translate to the texture space. Using that value, we then access the position g-buffer and retrieve the z coordinate of the surface position at that location (surfaceZ ). We don't need the x and y coordinates here as they are the same as the sample's.

Now that we have the z coordinate of the projected point on the surface near the sample (sampleZ), we compute the difference (dz) between it and the z coordinate of the original surface point (the point being shaded, pos). If this value is less than zero or greater than Radius, then we know that the projected point on the surface at the sample location is outside of the hemisphere. In that case, we assume the sample is unoccluded. If that is not the case, we assume the projected point is within the hemisphere and we compare the z values. If surfaceZ is greater than samplePos.z, we know that the sample point is behind the surface.

This may seem strange, but remember, we're working in camera coordinates here. All of the z coordinates will be negative. These are not depth values—they are the camera space z coordinates.

We add 1.0 to occlusionSum if we determine that the point is occluded. The final result in occlusionSum after the loop will be the total number of points that were occluded. Since we want the opposite - the fraction of points that are not occluded - we subtract one from the average before writing to the output variable AoData.

The following image (on the left) shows the results of this pass. Note that if you look closely, you can see some high frequency grid-like artifacts due to the re-use of the random rotation vectors throughout the image. This is smoothed out by the blur pass (right-hand image):

The third pass is just a simple average of the nine texels near each texel.

The fourth pass applies the reflection model. In this example, we just compute the diffuse and ambient components of the Blinn-Phong model, scaling the ambient term by the blurred ambient occlusion value (aoVal). In this example, it is raised to a power of 4.0 to make it a bit darker and increase the effect.
The following images show the scene rendered without ambient occlusion (on the left) and with ambient occlusion (on the right). The ambient term is increased substantially to demonstrate the effect:

Configuring the depth test

GLSL 4 provides the ability to configure how the depth test is performed. This gives us additional control over how and when fragments are tested against the depth buffer.

Many OpenGL implementations automatically provide an optimization known as the early depth test or early fragment test. With this optimization, the depth test is performed before the fragment shader is executed. Since fragments that fail the depth test will not appear on the screen (or the framebuffer), there is no point in executing the fragment shader at all for those fragments and we can save some time by avoiding the execution.

The OpenGL specification, however, states that the depth test must appear to be performed after the fragment shader. This means that if an implementation wishes to use the early depth test optimization, it must be careful. The implementation must make sure that if anything within the fragment shader might change the results of the depth test, then it should avoid using the early depth test.

For example, a fragment shader can change the depth of a fragment by writing to the output variable, gl_FragDepth. If it does so, then the early depth test cannot be performed because, of course, the final depth of the fragment is not known prior to the execution of the fragment shader. However, the GLSL provides ways to notify the pipeline roughly how the depth will be modified, so that the implementation may determine when it might be okay to use the early depth test.

Another possibility is that the fragment shader might conditionally discard the fragment using the discard keyword. If there is any possibility that the fragment may be discarded, some implementations may not perform the early depth test.

There are also certain situations where we want to rely on the early depth test. For example, if the fragment shader writes to memory other than the framebuffer (with image load/store, shader storage buffers, or other incoherent memory writing), we might not want the fragment shader to execute for fragments that fail the depth test. This would help us to avoid writing data for fragments that fail. The GLSL provides a technique for forcing the early depth test optimization.

How to do it...

To ask the OpenGL pipeline to always perform the early depth test optimization, use the following layout qualifier in your fragment shader:

layout(early_fragment_tests) in;

If your fragment shader will modify the fragment's depth, but you still would like to take advantage of the early depth test when possible, use the following layout qualifier in a declaration of gl_FragDepth within your fragment shader:

layout (depth_*) out float gl_FragDepth;

In this, depth_* is one of the following: depth_any, depth_greater, depth_less, or depth_unchanged.

How it works...

The following statement forces the OpenGL implementation to always perform the early depth test:

layout(early_fragment_tests) in;

We must keep in mind that if we attempt to modify the depth anywhere within the shader by writing to gl_FragDepth, the value that is written will be ignored.

If your fragment shader needs to modify the depth value, then we can't force early fragment tests. However, we can help the pipeline to determine when it can still apply the early test. We do so by using one of the layout qualifiers for gl_FragDepth as shown before. This places some limits on how the value will be modified. The OpenGL implementation can then determine if the fragment shader can be skipped. If it can be determined that the depth will not be changed in such a way that it would cause the result of the test to change, the implementation can still use the optimization.

The layout qualifier for the output variable gl_FragDepth tells the OpenGL implementation specifically how the depth might change within the fragment shader. The qualifier depth_any indicates that it could change in any way. This is the default.

The other qualifiers describe how the value may change with respect to gl_FragCoord.z:

• depth_greater: This fragment shader promises to only increase the depth.
• depth_less: This fragment shader promises to only decrease the depth.
• depth_unchanged: This fragment shader promises not to change the depth.

If it writes to gl_FragDepth, the value will be equal to gl_FragCoord.z.
If you use one of these qualifiers, but then go on to modify the depth in an incompatible way, the results are undefined. For example, if you declare gl_FragDepth with depth_greater, but decrease the depth of the fragment, the code will compile and execute, but you shouldn't expect to see accurate results.

If your fragment shader writes to gl_FragDepth, then it must be sure to write a value in all circumstances. In other words, it must write a value no matter which branches are taken within the code.
Image Processing and Screen Space Techniques Chapter 6

Implementing order-independent transparency

Transparency can be a difficult effect to do accurately in pipeline architectures like OpenGL. The general technique is to draw opaque objects first, with the depth buffer enabled, then to make the depth buffer read-only (using glDepthMask), disable the depth test, and draw the transparent geometry. However, care must be taken to ensure that the transparent geometry is drawn from back to front. That is, objects farther from the viewer should be drawn before the objects that are closer. This requires some sort of depth-sorting to take place prior to rendering.

The following images show an example of a block of small, semi-transparent spheres with some semi-transparent cubes placed evenly within them. On the right-hand side, the objects are rendered in an arbitrary order, using standard OpenGL blending. The result looks incorrect because objects are blended in an improper order. The cubes, which were drawn last, appear to be on top of the spheres, and the spheres look jumbled, especially in the middle of the block. On the left, the scene is drawn using proper ordering, so objects appear to be oriented correctly with respect to depth, and the overall look is more realistic looking:

Order Independent Transparency (OIT) means that we can draw objects in any order and still get accurate results. Depth sorting is done at some other level, perhaps within the fragment shader, so that the programmer need not sort objects before rendering. There are a variety of techniques for doing this; one of the most common technique is to keep a list of colors for each pixel, sort them by depth, and then blend them together in the fragment shader. In this recipe we'll use this technique to implement OIT, making use of some of the newest features in OpenGL 4.3.

Shader storage buffer objects (SSBO) and image load/store are some of the newest features in OpenGL, introduced in 4.3 and 4.2, respectively. They allow arbitrary read/write access to data from within a shader. Prior to this, shaders were very limited in terms of what data they could access. They could read from a variety of locations (textures, uniforms, and so on), but writing was very limited. Shaders could only write to controlled, isolated locations such as fragment shader outputs and transform feedback buffers. This was for a very good reason. Since shaders can execute in parallel and in a seemingly arbitrary order, it is very difficult to ensure that data is consistent between instantiations of a shader. Data written by one shader instance might not be visible to another shader instance whether or not that instance is executed after the other. Despite this, there are good reasons for wanting to read and write to shared locations. With the advent of SSBOs and image load/store, that capability is now available to us. We can create buffers and textures (called images) with read/write access to any shader instance. This is especially important for compute shaders, the subject of Chapter 11, Using Compute Shaders. However, this power comes at a price. The programmer must now be very careful to avoid the types of memory consistency errors that come along with writing to memory that is shared among parallel threads. Additionally, the programmer must be aware of the performance issues that come with synchronization between shader invocations.

For a more thorough discussion of the issues involved with memory consistency and shaders, refer to Chapter 11, of The OpenGL Programming Guide, 8th Edition. That chapter also includes another similar implementation of OIT.

In this recipe, we'll use SSBOs and image load/store to implement order-independent transparency. We'll use two passes. In the first pass, we'll render the scene geometry and store a linked list of fragments for each pixel. After the first pass, each pixel will have a corresponding linked list containing all fragments that were written to that pixel, including their depth and color. In the second pass, we'll draw a fullscreen quad to invoke the fragment shader for each pixel. In the fragment shader, we'll extract the linked list for the pixel, sort the fragments by depth (largest to smallest), and blend the colors in that order. The final color will then be sent to the output device.

That's the basic idea, so let's dig into the details. We'll need three memory objects that are shared among the fragment shader instances:

1. An atomic counter: This is just an unsigned integer that we'll use to keep track of the size of our linked list buffer. Think of this as the index of the first unused slot in the buffer.

2. A head-pointer texture that corresponds to the size of the screen: The texture will store a single unsigned integer in each texel. The value is the index of the head of the linked list for the corresponding pixel.

3. A buffer containing all of our linked lists: Each item in the buffer will correspond to a fragment, and contains a struct with the color and depth of the fragment as well as an integer, which is the index of the next fragment in the linked list.

In order to understand how all of this works together, let's consider a simple example. Suppose that our screen is three pixels wide and three pixels high. We'll have a head pointer texture that is the same dimensions, and we'll initialize all of the texels to a special value that indicates the end of the linked list (an empty list). In the following diagram, that value is shown as an x, but in practice, we'll use 0xffffffff. The initial value of the counter is zero, and the linked list buffer is allocated to a certain size but treated as empty initially. The initial state of our memory is shown in the following diagram:

Now suppose that a fragment is rendered at the position (0,1) with a depth of 0.75. The fragment shader will take the following steps:

1. Increment the atomic counter. The new value will be 1, but we'll use the previous value (0) as the index for our new node in the linked list.

2. Update the head pointer texture at (0,1) with the previous value of the counter (0). This is the index of the new head of the linked list at that pixel. Hold on to the previous value that was stored there (x), as we'll need that in the next step.

3. Add a new value into the linked list buffer at the location corresponding to the previous value of the counter (0). Store the color of the fragment and its depth here. Store in the next component the previous value of the head pointer texture at (0,1) that we held on to in step 2. In this case, it is the special value indicating the end of the list.

After processing this fragment, the memory layout looks as follows:

Now, suppose another fragment is rendered at (0,1), with a depth of 0.5. The fragment shader will execute the same steps as the previous ones, resulting in the following memory layout:

We now have a two-element linked list starting at index 1 and ending at index 0. Suppose, now that we have three more fragments in the following order: a fragment at (1,1) with a depth of 0.2, a fragment at (0,1) with a depth of 0.3, and a fragment at (1,1) with a depth of 0.4. Following the same steps for each fragment, we get the following result:

The linked list at (0,1) consists of fragments {3, 1, 0} and the linked list at (1,1) contains fragments {4, 2}.

Now, we must keep in mind that due to the highly parallel nature of GPUs, fragments can be rendered in virtually any order. For example, fragments from two different polygons might proceed through the pipeline in the opposite order as to when the draw instructions for polygons were issued. As a programmer, we must not expect any specific ordering of fragments. Indeed, instructions from separate instances of the fragment shader may interleave in arbitrary ways. The only thing that we can be sure of is that the statements within a particular instance of the shader will execute in order. Therefore, we need to convince ourselves that any interleaving of the previous three steps will still result in a consistent state. For example, suppose instance one executes steps 1 and 2, then another instance (another fragment, perhaps at the same fragment coordinates) executes steps 1, 2, and 3, before the first instance executes step 3. Will the result still be consistent? I think you can convince yourself that it will be, even though the linked list will be broken for a short time during the process. Try working through other interleavings and convince yourself that we're OK.

Not only can statements within separate instances of a shader interleave with each other, but the sub-instructions that make up the statements can interleave. (For example, the sub-instructions for an increment operation consist of a load, increment, and a store.) What's more, they could actually execute at exactly the same time. Consequently, if we aren't careful, nasty memory consistency issues can crop up. To help avoid this, we need to make careful use of the GLSL support for atomic operations.

Recent versions of OpenGL (4.2 and 4.3) have introduced the tools that we need to make this algorithm possible. OpenGL 4.2 introduced atomic counters and the ability to read and write to arbitrary locations within a texture (called image load/store). OpenGL 4.3 introduced shader storage buffer objects. We'll make use of all three of these features in this example, as well as the various atomic operations and memory barriers that go along with them.

Getting ready

There's a bunch of setup needed here, so we'll go into a bit of detail with some code segments. First, we'll set up a buffer for our atomic counter:

GLuint counterBuffer;
glGenBuffers(1, &counterBuffer);
glBindBufferBase(GL_ATOMIC_COUNTER_BUFFER, 0, counterBuffer);
glBufferData(GL_ATOMIC_COUNTER_BUFFER, sizeof(GLuint), NULL, GL_DYNAMIC_DRAW);

Next, we will create a buffer for our linked list storage:

GLuint llBuf;
glGenBuffers(1, &llBuf);
glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 0, llBuf);
glBufferData(GL_SHADER_STORAGE_BUFFER, maxNodes * nodeSize, NULL, GL_DYNAMIC_DRAW); 

nodeSize in the previous code is the size of struct NodeType used in the fragment shader (in the latter part of the code). This is computed based on the std430 layout. For details on the std430 layout, see the OpenGL specification document. For this example, nodeSize is 5 * sizeof(GLfloat) + sizeof(GLuint).

We also need to create a texture to hold the list head pointers. We'll use 32-bit unsigned integers, and bind it to image unit 0:

glGenTextures(1, &headPtrTex);
glBindTexture(GL_TEXTURE_2D, headPtrTex);
glTexStorage2D(GL_TEXTURE_2D, 1, GL_R32UI, width, height);
glBindImageTexture(0, headPtrTex, 0, GL_FALSE, 0, GL_READ_WRITE, GL_R32UI);

After we render each frame, we need to clear the texture by setting all texels to a value of 0xffffffff. To help with that, we'll create a buffer of the same size as the texture, with each value set to our clear value:

vector<GLuint> headPtrClear(width * height, 0xffffffff);
GLuint clearBuf;
glGenBuffers(1, &clearBuf);
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, clearBuf);
glBufferData(GL_PIXEL_UNPACK_BUFFER, headPtrClear.size()*sizeof(GLuint), &headPtrClear[0], GL_STATIC_COPY);

That's all the buffers we'll need. Note the fact that we've bound the head pointer texture to image unit 0, the atomic counter buffer to index 0 of the GL_ATOMIC_COUNTER_BUFFER binding point (glBindBufferBase), and the linked list storage buffer to index 0 of the GL_SHADER_STORAGE_BUFFER binding point. We'll refer back to that later. Use a pass-through vertex shader that sends the position and normal along in eye coordinates.

How to do it...

With all of the buffers set up, we need two render passes. Before the first pass, we want to clear our buffers to default values (that is, empty lists), and to reset our atomic counter buffer to zero:

glBindBuffer(GL_PIXEL_UNPACK_BUFFER, clearBuf);
glBindTexture(GL_TEXTURE_2D, headPtrTex);
glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, width, height, GL_RED_INTEGER, GL_UNSIGNED_INT, NULL);
GLuint zero = 0;
glBindBufferBase(GL_ATOMIC_COUNTER_BUFFER, 0, counterBuffer);
glBufferSubData(GL_ATOMIC_COUNTER_BUFFER, sizeof(GLuint), &zero);

In the first pass, we'll render the full scene geometry. Generally, we should render all the opaque geometry first and store the results in a texture. However, we'll skip that step for this example to keep things simple and focused. Instead, we'll render only transparent geometry. When rendering the transparent geometry, we need to make sure to put the depth buffer in read-only mode (use glDepthMask). In the fragment shader, we add each fragment to the appropriate linked list:

layout (early_fragment_tests) in; #define MAX_FRAGMENTS 75  in vec3 Position; in vec3 Normal; struct NodeType { vec4 color; float depth; uint next;
}; layout(binding=0, r32ui) uniform uimage2D headPointers; layout(binding=0, offset=0) uniform atomic_uint nextNodeCounter; layout(binding=0, std430) buffer linkedLists { NodeType nodes[];
}; uniform uint MaxNodes; subroutine void RenderPassType(); subroutine uniform RenderPassType RenderPass; ... subroutine(RenderPassType) void pass1() { uint nodeIdx = atomicCounterIncrement(nextNodeCounter); if ( nodeIdx < MaxNodes ) { uint prevHead = imageAtomicExchange(headPointers, ivec2(gl_FragCoord.xy), nodeIdx); nodes[nodeIdx].color = vec4(shadeFragment(), Kd.a); nodes[nodeIdx].depth = gl_FragCoord.z; nodes[nodeIdx].next = prevHead; }
}

Before rendering the second pass, we need to be sure that all of the data has been written to our buffers. In order to ensure that is indeed the case, we can use a memory barrier:

glMemoryBarrier( GL_ALL_BARRIER_BITS );

In the second pass, we don't render the scene geometry, just a single, screen-filling quad in order to invoke the fragment shader for each screen pixel. In the fragment shader, we start by copying the linked list for the fragment into a temporary array:

struct NodeType frags[MAX_FRAGMENTS]; int count = 0; uint n = imageLoad(headPointers, ivec2(gl_FragCoord.xy)).r; while( n != 0xffffffff && count < MAX_FRAGMENTS) { frags[count] = nodes[n]; n = frags[count].next; count++;
}

Then, we sort the fragments using insertion sort:

for( uint i = 1; i < count; i++ )
{ struct NodeType toInsert = frags[i]; uint j = i; while ( j > 0 && toInsert.depth > frags[j-1].depth ) { frags[j] = frags[j-1]; j--; } frags[j] = toInsert;
}

Finally, we blend the fragments manually, and send the result to the output variable:

vec4 color = vec4(0.5, 0.5, 0.5, 1.0); for( int i = 0; i < count; i++ ) { color = mix( color, frags[i].color, frags[i].color.a);
} FragColor = color;

How it works...

To clear our buffers, prior to the first pass, we bind clearBuf to the GL_PIXEL_UNPACK_BUFFER binding point, and call glTexSubImage2D to copy data from clearBuf to the the head pointer texture. Note that when a non-zero buffer is bound to GL_PIXEL_UNPACK_BUFFER, glTexSubImage2D treats the last parameter as an offset into the buffer that is bound there. Therefore, this will initiate a copy from clearBuf into headPtrTex. Clearing the atomic counter is straightforward, but the use of glBindBufferBase may be a bit confusing. If there can be several buffers bound to the binding point (at different indices), how does glBufferSubData know which buffer to target? It turns out that when we bind a buffer using glBindBufferBase, it is also bound to the generic binding point as well.

In the fragment shader during the first pass, we start with the layout specification enabling the early fragment test optimization:

layout (early_fragment_tests) in;

This is important because if any fragments are obscured by the opaque geometry, we don't want to add them to a linked list. If the early fragment test optimization is not enabled, the fragment shader may be executed for fragments that will fail the depth test and hence will get added to the linked list. The previous statement ensures that the fragment shader will not execute for those fragments.

The definition of struct NodeType specifies the type of data that is stored in our linked list buffer. We need to store color, depth, and a pointer to the next node in the linked list.

The next three statements declare the objects related to our linked list storage.

1. The first, headPointers, is the image object that stores the locations of the heads of each linked list. The layout qualifier indicates that it is located at image unit 0 (refer to the Getting ready section of this recipe), and the data type is r32ui (red, 32-bit unsigned integer).

2. The second object is our atomic counter nextNodeCounter. The layout qualifier indicates the index within the GL_ATOMIC_COUTER_BUFFER binding point (refer to the Getting ready section of this recipe) and the offset within the buffer at that location. Since we only have a single value in the buffer, the offset is 0, but in general, you might have several atomic counters located within a single buffer.

3. Third is our linked-list storage buffer linkedLists. This is a shader storage buffer object. The organization of the data within the object is defined within the curly braces here. In this case, we just have an array of NodeType structures. The bounds of the array can be left undefined, the size being limited by the underlying buffer object that we created. The layout qualifiers define the binding and memory layout. The first, binding, indicates that the buffer is located at index 0 within the GL_SHADER_STORAGE_BUFFER binding point. The second, std430, indicates how memory is organized within the buffer. This is mainly important when we want to read the data back from the OpenGL side. As mentioned previously, this is documented in the OpenGL specification document.

The first step in the fragment shader during the first pass is to increment our atomic counter using atomicCounterIncrement. This will increment the counter in such a way that there is no possibility of memory consistency issues if another shader instance is attempting to increment the counter at the same time.

An atomic operation is one that is isolated from other threads and can be considered to be a single, uninterruptable operation. Other threads cannot interleave with an atomic operation. It is always a good idea to use atomic operations when writing to shared data within a shader.

The return value of atomicCounterIncrement is the previous value of the counter. It is the next unused location in our linked list buffer. We'll use this value as the location where we'll store this fragment, so we store it in a variable named nodeIdx. It will also become the new head of the linked list, so the next step is to update the value in the headPointers image at this pixel's location gl_FragCoord.xy. We do so using another atomic operation: imageAtomicExchange. This replaces the value within the image at the location specified by the second parameter with the value of the third parameter. The return value is the previous value of the image at that location. This is the previous head of our linked list. We hold on to this value in prevHead, because we want to link our new head to that node, thereby restoring the consistency of the linked list with our new node at the head.

Finally, we update the node at nodeIdx with the color and depth of the fragment, and set the next value to the previous head of the list (prevHead). This completes the insertion of this fragment into the linked list at the head of the list.

After the first pass is complete, we need to make sure that all changes are written to our shader storage buffer and image object before proceeding. The only way to guarantee this is to use a memory barrier. The call to glMemoryBarrier will take care of this for us. The parameter to glMemoryBarrier is the type of barrier. We can fine tune the type of barrier to specifically target the kind of data that we want to read. However, just to be safe, and for simplicity, we'll use GL_ALL_BARRIER_BITS, which ensures that all possible data has been written.

In the second pass, we start by copying the linked list for the fragment into a temporary array. We start by getting the location of the head of the list from the headPointers image using imageLoad. Then we traverse the linked list with the while loop, copying the data into the array frags.
Next, we sort the array by depth from largest to smallest, using the insertion sort algorithm.

Insertion sort works well on small arrays, so should be a fairly efficient choice here.

Finally, we combine all the fragments in order, using the mix function to blend them together based on the value of the alpha channel. The final result is stored in the output variable FragColor.

There's more...

As mentioned previously, we've skipped anything that deals with opaque geometry. In general, one would probably want to render any opaque geometry first, with the depth buffer enabled, and store the rendered fragments in a texture. Then, when rendering the transparent geometry, one would disable writing to the depth buffer, and build the linked list as shown previously. Finally, you could use the value of the opaque texture as the background color when blending the linked lists.

This is the first example in this book that makes use of reading and writing from/to arbitrary (shared) storage from a shader. This capability, has given us much more flexibility, but that comes at a price. As indicated previously, we have to be very careful to avoid memory consistency and coherence issues. The tools to do so include atomic operations and memory barriers, and this example has just scratched the surface. There's much more to come in Chapter 11, Using Compute Shaders when we look at compute shaders, and I recommend you read through the memory chapter in the OpenGL Programming Guide for much more detail than is provided here.

Using Geometry and Tessellation Shaders

In this chapter, we will cover:

• Point sprites with the geometry shader
• Drawing a wireframe on top of a shaded mesh
• Drawing silhouette lines using the geometry shader
• Tessellating a curve
• Tessellating a 2D quad
• Tessellating a 3D surface
• Tessellating based on depth

Introduction

Tessellation and geometry shaders provide programmers with additional ways to modify geometry as it progresses through the shader pipeline. Geometry shaders can be used to add, modify, or delete geometry in a very precise and user-controlled manner. Tessellation shaders can also be configured to automatically subdivide geometry to various degrees (levels of detail), potentially creating immensely dense geometry via the GPU.

In this chapter, we'll look at several examples of geometry and tessellation shaders in various contexts. However, before we get into the recipes, let's investigate how all of this fits together.

The shader pipeline extended

The following diagram shows a simplified view of the shader pipeline, when the shader program includes geometry and tessellation shaders:

The tessellation portion of the shader pipeline includes two stages: the Tessellation Control Shader (TCS) and the Tessellation Evaluation Shader (TES). The geometry shader follows the tessellation stages and precedes the fragment shader. The tessellation shader and geometry shader are optional; however, when a shader program includes a tessellation or geometry shader, a vertex shader must be included.

All shaders except the vertex shader are optional. When using a geometry shader, there is no requirement that you also include a tessellation shader and vice versa.

The geometry shader

The geometry shader (GS) is designed to execute once for each primitive. It has access to all of the vertices of the primitive, as well as the values of any input variables associated with each vertex. In other words, if a previous stage (such as the vertex shader) provides an output variable, the geometry shader has access to the value of that variable for all vertices in the primitive. As a result, the input variables within the geometry shader are always arrays.

The geometry shader can output zero, one, or more primitives. Those primitives need not be of the same kind that were received by the geometry shader.

However, the GS can only output one primitive type. For example, a GS could receive a triangle, and output several line segments as a line strip, or a GS could receive a triangle and output zero or many triangles as a triangle strip.

This enables the GS to act in many different ways. A GS could be responsible for culling (removing) geometry based on some criteria, such as visibility based on occlusions. It could generate additional geometry to augment the shape of the object being rendered. The GS could simply compute additional information about the primitive and pass the primitive along unchanged, or the GS could produce primitives that are entirely different from the input geometry.

The functionality of the GS is centered around the two built-in functions: EmitVertex and EndPrimitive. These two functions allow the GS to send multiple vertices and primitives down the pipeline. The GS defines the output variables for a particular vertex, and then calls EmitVertex. After that, the GS can proceed to redefine the output variables for the next vertex, call EmitVertex again, and so on. After emitting all of the vertices for the primitive, the GS can call EndPrimitive to let the OpenGL system know that all the vertices of the primitive have been emitted. The EndPrimitive function is implicitly called when the GS finishes execution. If GS does not call EmitVertex at all, the input primitive is effectively dropped (it is not rendered).

In the following recipes, we'll examine a few examples of the geometry shader. In the Point sprites with the geometry shader recipe, we'll see an example where the input primitive type is entirely different from the output type. In the Drawing a wireframe on top of a shaded mesh recipe, we'll pass the geometry along unchanged, but also produce some additional information about the primitive to help in drawing wireframe lines. In the Drawing silhouette lines using the geometry shader recipe, we'll see an example where the GS passes along the input primitive, but generates additional primitives as well.

The tessellation shaders

When the tessellation shaders are active, we can only render one kind of primitive: the patch (GL_PATCHES). Rendering any other kind of primitive (such as triangles, or lines) while a tessellation shader is active is an error. The patch primitive is an arbitrary chunk of geometry (or any information) that is completely defined by the programmer. It has no geometric interpretation beyond how it is interpreted within the TCS and TES. The number of vertices within the patch primitive is also configurable. The maximum number of vertices per patch is implementation-dependent, and can be queried via the following command:

glGetIntegerv(GL_MAX_PATCH_VERTICES, &maxVerts);

We can define the number of vertices per patch with the following function:

glPatchParameteri( GL_PATCH_VERTICES, numPatchVerts );

A very common application of this is when the patch primitive consists of a set of control points that define an interpolated surface or curve (such as a Bezier curve or surface). However, there is no reason why the information within the patch primitive couldn't be used for other purposes.

The patch primitive is never actually rendered; instead, it is used as additional information for the TCS and TES. The primitives that actually make their way further down the pipeline are created by the tessellation primitive generator (TPG), which lies between the TCS and TES. Think of the tessellation-primitive generator as a configurable engine that produces primitives based on a set of standard tessellation algorithms. The TCS and TES have access to the entire input patch, but have fundamentally different responsibilities. The TCS is responsible for:

• setting up the TPG
• defining how the primitives should be generated by the TPG (how many and what algorithm to use)
• producing per-vertex output attributes.

The TES has the job of determining the position (and any other information) of each vertex of the primitives that are produced by the TPG. For example, the TCS might tell the TPG to generate a line strip consisting of 100 line segments, and the TES is responsible for determining the position of each vertex of those 100 line segments. The TES would likely make use of the information within the entire patch primitive in order to do so.

The TCS is executed once for each vertex in a patch, but has access to all vertices of its associated patch. It can compute additional information about the patch and pass it along to the TES using output variables. However, the most important task of the TCS is to tell the TPG how many primitives it should produce. It does this by defining tessellation levels via the gl_TessLevelInner and gl_TessLevelOuter arrays. These arrays define the granularity of the tessellation produced by the TPG.

The TPG generates primitives based on a particular algorithm (quads, isolines, or triangles). Each algorithm produces primitives in a slightly different fashion, and we will see examples of isolines and quads in the recipes in this chapter. Each vertex of the generated primitives is associated with a position in parameter space (u, v, w). Each coordinate of this position is a number that can range from zero to one. This coordinate can be used to evaluate the location of the vertex, often by interpolation of the patch primitive's vertices.

The primitive-generation algorithms produce vertices (and the associated parametric coordinates) in a slightly different fashion. The tessellation algorithms for quads and isolines make use of only the first two parametric coordinates: u and v. The following diagram illustrates the process for an input and output patch consisting of four vertices. In the diagram, the TPG uses the quad tessellation algorithm with the inner and outer tessellation levels set at four:

The number of vertices in the input patch need not be the same as the number of vertices in the output patch, although that will be the case in all of the examples in this chapter.

The TES is executed once for each parameter-space vertex that is generated by the TPG. Somewhat strangely, the TES is actually the shader that defines the algorithm used by the TPG. It does so via its input layout qualifier. As stated earlier, its main responsibility is to determine the position of the vertex (possibly along with other information, such as normal vector and texture coordinate). Typically, the TES uses the parametric coordinate (u,v) provided by the TPG along with the positions of all of the input patch vertices to do so. For example, when drawing a curve, the patch might consists of four vertices, which are the control points for the curve. The TPG would then generate 101 vertices to create a line strip (if the tessellation level was set to 100), and each vertex might have a uuu coordinate that ranged appropriately between zero and one. The TES would then use that uuu coordinate along with the positions of the four patch vertices to determine the position of the vertex associated with the shader's execution.

If all of this seems confusing, start with the Tessellating a curve recipe, and work your way through the following recipes.

In the Tessellating a curve recipe, we'll go through a basic example where we use tessellation shaders to draw a Bezier curve with four control points. In the Tessellating a 2D quad recipe, we'll try to understand how the quad tessellation algorithm works by rendering a simple quad and visualizing the triangles produced by the TPG. In the Tessellating a 3D surface recipe, we'll use quad tessellation to render a 3D Bezier surface. Finally, in the Tessellating based on depth recipe, we'll see how the tessellation shaders make it easy to implement level-of-detail (LOD) algorithms.

Point sprites with the geometry shader

Point sprites are simple quads (usually texture mapped) that are aligned such that they are always facing the camera. They are very useful for particle systems in 3D (refer to Chapter 9, Using Noise in Shaders) or 2D games. The point sprites are specified by the OpenGL application as single-point primitives, via the GL_POINTS rendering mode. This simplifies the process, because the quad itself and the texture coordinates for the quad are determined automatically. The OpenGL side of the application can effectively treat them as point primitives, avoiding the need to compute the positions of the quad vertices.

The following image shows a group of point sprites. Each sprite is rendered as a point primitive. The quad and texture coordinates are generated automatically (within the geometry shader) and aligned to face the camera:

OpenGL already has built-in support for point sprites in the GL_POINTS rendering mode. When rendering point primitives using this mode, the points are rendered as screen-space squares that have a diameter (side length) as defined by the glPointSize function. In addition, OpenGL will automatically generate texture coordinates for the fragments of the square. These coordinates run from zero to one in each direction (horizontal and vertical), and are accessible in the fragment shader via the gl_PointCoord built-in variable.

There are various ways to fine-tune the rendering of point sprites within OpenGL. One can define the origin of the automatically-generated texture coordinates using the glPointParameter functions. The same set of functions also can be used to tweak the way that OpenGL defines the alpha value for points when multi-sampling is enabled.

The built-in support for point sprites does not allow the programmer to rotate the screen-space squares, or define them as different shapes, such as rectangles or triangles. However, one can achieve similar effects with the creative use of textures and transformations of the texture coordinates. For example, we could transform the texture coordinates using a rotation matrix to create the look of a rotating object even though the geometry itself is not actually rotating. In addition, the size of the point sprite is a screen-space size. In other words, the point size must be adjusted with the depth of the point sprite if we want to get a perspective effect (sprites get smaller with distance).

If these (and possibly other) issues make the default support for point sprites too limiting, we can use the geometry shader to generate our point sprites. In fact, this technique is a good example of using the geometry shader to generate different kinds of primitives than it receives. The basic idea here is that the geometry shader will receive point primitives (in camera coordinates) and will output a quad centered at the point and aligned so that it is facing the camera. The geometry shader will also automatically generate texture coordinates for the quad.

If desired, we could generate other shapes, such as hexagons, or we could rotate the quads before they are output from the geometry shader. The possibilities are endless.

Before jumping directly into the code, let's take a look at some of the mathematics. In the geometry shader, we'll need to generate the vertices of a quad that is centered at a point and aligned with the camera's coordinate system (eye coordinates).

Given the point location (PPP) in camera coordinates, we can generate the vertices of the corners of the quad by simply translating PPP in a plane parallel to the x-y plane of the camera's coordinate system, as shown in the following figure:

The geometry shader will receive the point location in camera coordinates, and output the quad as a triangle strip with texture coordinates. The fragment shader will then just apply the texture to the quad.

Getting ready

For this example, we'll need to render a number of point primitives. The positions can be sent via attribute location 0. There's no need to provide normal vectors or texture coordinates for this one.

The following uniform variables are defined within the shaders, and need to be set within the OpenGL program:

• Size2: This should be half the width of the sprite's square
• SpriteTex: This is the texture unit containing the point sprite texture

As usual, uniforms for the standard transformation matrices are also defined within the shaders, and need to be set within the OpenGL program.

How to do it...

To create a shader program that can be used to render point primitives as quads, use the following steps:

1. The vertex shader will convert the position to camera coordinates and assign to the gl_Position output variable. Note that we're not converting to clip coordinates just yet:

    layout (location = 0) in vec3 VertexPosition; uniform mat4 ModelViewMatrix; void main() { gl_Position = ModelViewMatrix * vec4(VertexPosition,1.0);
    }

2. The geometry shader emits two triangles as a triangle strip. We use the gl_in variable to access the position from the vertex shader (camera coordinates):

    layout( points ) in; layout( triangle_strip, max_vertices = 4 ) out; uniform float Size2; uniform mat4 ProjectionMatrix; out vec2 TexCoord; void main() { mat4 m = ProjectionMatrix; gl_Position = m * (vec4(-Size2,-Size2,0.0,0.0) + gl_in[0].gl_Position); TexCoord = vec2(0.0,0.0); EmitVertex(); gl_Position = m * (vec4(Size2,-Size2,0.0,0.0) + gl_in[0].gl_Position); TexCoord = vec2(1.0,0.0); EmitVertex(); gl_Position = m * (vec4(-Size2,Size2,0.0,0.0) + gl_in[0].gl_Position); TexCoord = vec2(0.0,1.0); EmitVertex(); gl_Position = m * (vec4(Size2,Size2,0.0,0.0) + gl_in[0].gl_Position); TexCoord = vec2(1.0,1.0); EmitVertex(); EndPrimitive();
    }

3. The fragment shader applies the texture:

    in vec2 TexCoord; uniform sampler2D SpriteTex; layout( location = 0 ) out vec4 FragColor; void main() { FragColor = texture(SpriteTex, TexCoord);
    }

4. Within the OpenGL render function, render a set of point primitives.

How it works...

The vertex shader is almost as simple as it can get. It converts the point's position to camera coordinates by multiplying by the model-view matrix, and assigns the result to the built-in output variable, gl_Position.

In the geometry shader, we start by defining the kind of primitive that this geometry shader expects to receive. The first layout statement indicates that this geometry shader will receive point primitives:

layout( points ) in;

The next layout statement indicates the kind of primitives produced by this geometry shader, and the maximum number of vertices that will be output:

layout( triangle_strip, max_vertices = 4 ) out;

In this case, we want to produce a single quad for each point received, so we indicate that the output will be a triangle strip with a maximum of four vertices.

The input primitive is available to the geometry shader via the built-in input variable, gl_in. Note that it is an array of structures. You might be wondering why this is an array since a point primitive is only defined by a single position.

Well, in general, the geometry shader can receive triangles, lines, or points (and possibly adjacency information). So, the number of values available may be more than one. If the input were triangles, the geometry shader would have access to three input values (associated with each vertex). In fact, it could have access to as many as six values when triangles_adjacency is used (more on that in a later recipe).

The gl_in variable is an array of structs. Each struct contains the following fields: gl_Position, gl_PointSize, and gl_ClipDistance[]. In this example, we are only interested in gl_Position. However, the others can be set in the vertex shader to provide additional information to the geometry shader.

Within the main function of the geometry shader, we produce the quad (as a triangle strip) in the following way. For each vertex of the triangle strip, we execute the following steps:

1. Compute the attributes for the vertex (in this case the position and texture coordinate), and assign their values to the appropriate output variables (gl_Position and TexCoord). Note that the position is also transformed by the projection matrix. We do this because the gl_Position variable must be provided in clip coordinates to later stages of the pipeline.

2. Emit the vertex (send it down the pipeline) by calling the built-in EmitVertex() function.

Once we have emitted all vertices for the output primitive, we call EndPrimitive() to finalize the primitive and send it along.

It is not strictly necessary to call EndPrimitive() in this case because it is implicitly called when the geometry shader finishes. However, like closing files, it is a good practice.

The fragment shader is also very simple. It just applies the texture to the fragment using the (interpolated) texture coordinate provided by the geometry shader.

There's more...

This example is fairly straightforward and is intended as a gentle introduction to geometry shaders. We could expand on this by allowing the quad to rotate or to be oriented in different directions. We could also use the texture to discard fragments (in the fragment shader) in order to create point sprites of arbitrary shapes. The power of the geometry shader opens up plenty of possibilities!

Drawing a wireframe on top of a shaded mesh

The preceding recipe demonstrated the use of a geometry shader to produce a different variety of primitives than it received. Geometry shaders can also be used to provide additional information to later stages. They are quite well-suited to doing so because they have access to all of the vertices of the primitive at once, and can do computations based on the entire primitive rather than a single vertex.

This example involves a geometry shader that does not modify the triangle at all. It essentially passes the primitive along unchanged. However, it computes additional information about the triangle that will be used by the fragment shader to highlight the edges of the polygon. The basic idea here is to draw the edges of each polygon directly on top of the shaded mesh.

The following image shows an example of this technique. The mesh edges are drawn on top of the shaded surface by using information computed within the geometry shader:

There are many techniques for producing wireframe structures on top of shaded surfaces. This technique comes from an NVIDIA whitepaper published in 2007. We make use of the geometry shader to produce the wireframe and shaded surface in a single pass. We also provide some simple anti-aliasing of the mesh lines that are produced, and the results are quite nice (refer to the preceding image).

To render the wireframe on top of the shaded mesh, we'll compute the distance from each fragment to the nearest triangle edge. When the fragment is within a certain distance from the edge, it will be shaded and mixed with the edge color. Otherwise, the fragment will be shaded normally.

To compute the distance from a fragment to the edge, we use the following technique. In the geometry shader, we compute the minimum distance from each vertex to the opposite edge (also called the triangle altitude). In the following diagram, the desired distances are hahaha, hbhbhb, and hchchc:

We can compute these altitudes using the interior angles of the triangle, which can be determined using the law of cosines. For example, to find ha, we use the interior angle at vertex CCC (β\betaβ):

The other altitudes can be computed in a similar way. (Note that β\betaβ could be greater than 90 degrees, in which case, we would want the sine of 180−β180-\beta180−β. However, the sine of 180−β180-\beta180−β is the same as the sine of β\betaβ.) Once we have computed these triangle altitudes, we can create an output vector (an edge-distance vector) within the geometry shader for interpolation across the triangle. The components of this vector represent the distances from the fragment to each edge of the triangle.

The x component represents the distance from edge aaa, the y component is the distance from edge bbb, and the z component is the distance from edge ccc. If we assign the correct values to these components at the vertices, the hardware will automatically interpolate them for us to provide the appropriate distances at each fragment. At vertex AAA, the value of this vector should be (hahaha, 0, 0) because vertex AAA is at a distance of ha from edge aaa and directly on edges bbb and ccc. Similarly, the value for vertex BBB is (0, hbhbhb, 0) and for vertex CCC is (0, 0, hchchc). When these three values are interpolated across the triangle, we should have the distance from the fragment to each of the three edges. We will calculate all of this in screen space. That is, we'll transform the vertices to screen space within the geometry shader before computing the altitudes.

Since we are working in screen space, there's no need (and it would be incorrect) to interpolate the values in a perspective-correct manner. So we need to be careful to tell the hardware to interpolate linearly. Within the fragment shader, all we need to do is find the minimum of the three distances, and if that distance is less than the line width, we mix the fragment color with the line color. However, we'd also like to apply a bit of anti-aliasing while we're at it. To do so, we'll fade the edge of the line using the GLSL smoothstep function. We'll scale the intensity of the line in a two-pixel range around the edge of the line. Pixels that are at a distance of one or less from the true edge of the line get 100% of the line color, and pixels that are at a distance of one or more from the edge of the line get 0% of the line color. In between, we'll use the smoothstep function to create a smooth transition. Of course, the edge of the line itself is a configurable distance (we'll call it Line.Width) from the edge of the polygon.

Getting ready

The typical setup is needed for this example. The vertex position and normal should be provided in attributes zero and one, respectively, and you need to provide the appropriate parameters for your shading model. As usual, the standard matrices are defined as uniform variables and should be set within the OpenGL application. However, note that this time we also need the viewport matrix (the ViewportMatrix uniform variable) in order to transform into screen space. There are a few uniforms related to the mesh lines that need to be set:

• Line.Width: This should be half the width of the mesh lines
• Line.Color: This is the color of the mesh lines

How to do it...

To create a shader program that utilizes the geometry shader to produce a wireframe on top of a shaded surface, use the following steps:

1. Use the following code for the vertex shader:

    layout (location = 0 ) in vec3 VertexPosition; layout (location = 1 ) in vec3 VertexNormal; out vec3 VNormal; out vec3 VPosition; uniform mat4 ModelViewMatrix; uniform mat3 NormalMatrix; uniform mat4 ProjectionMatrix; uniform mat4 MVP; void main() { VNormal = normalize( NormalMatrix * VertexNormal); VPosition = vec3(ModelViewMatrix * vec4(VertexPosition,1.0)); gl_Position = MVP * vec4(VertexPosition,1.0);
    }

2. Use the following code for the geometry shader:

    layout( triangles ) in; layout( triangle_strip, max_vertices = 3 ) out; out vec3 GNormal; out vec3 GPosition; noperspective out vec3 GEdgeDistance; in vec3 VNormal[]; in vec3 VPosition[]; uniform mat4 ViewportMatrix; void main() { vec3 p0 = vec3(ViewportMatrix * (gl_in[0].gl_Position / gl_in[0].gl_Position.w)); vec3 p1 = vec3(ViewportMatrix * (gl_in[1].gl_Position / gl_in[1].gl_Position.w)); vec3 p2 = vec3(ViewportMatrix * (gl_in[2].gl_Position / gl_in[2].gl_Position.w)); float a = length(p1 - p2); float b = length(p2 - p0); float c = length(p1 - p0); float alpha = acos( (b*b + c*c - a*a) / (2.0*b*c) ); float beta = acos( (a*a + c*c - b*b) / (2.0*a*c) ); float ha = abs( c * sin( beta ) ); float hb = abs( c * sin( alpha ) ); float hc = abs( b * sin( alpha ) ); GEdgeDistance = vec3( ha, 0, 0 ); GNormal = VNormal[0]; GPosition = VPosition[0]; gl_Position = gl_in[0].gl_Position; EmitVertex(); GEdgeDistance = vec3( 0, hb, 0 ); GNormal = VNormal[1]; GPosition = VPosition[1]; gl_Position = gl_in[1].gl_Position; EmitVertex(); GEdgeDistance = vec3( 0, 0, hc ); GNormal = VNormal[2]; GPosition = VPosition[2]; gl_Position = gl_in[2].gl_Position; EmitVertex(); EndPrimitive();
    }

3. Use the following code for the fragment shader:

     uniform struct LineInfo { float Width; vec4 Color;
    } Line; in vec3 GPosition; in vec3 GNormal; noperspective in vec3 GEdgeDistance; layout( location = 0 ) out vec4 FragColor; vec3 blinnPhong( vec3 pos, vec3 norm ) { } void main() { vec4 color=vec4(blinnPhong(GPosition, GNormal), 1.0); float d = min( GEdgeDistance.x, GEdgeDistance.y ); d = min( d, GEdgeDistance.z ); float mixVal = smoothstep( Line.Width - 1, Line.Width + 1, d ); FragColor = mix( Line.Color, color, mixVal );
    }

How it works...

The vertex shader is pretty simple. It passes the normal and position along to the geometry shader after converting them into camera coordinates. The built-in gl_Position variable gets the position in clip coordinates. We'll use this value in the geometry shader to determine the screen space coordinates. In the geometry shader, we begin by defining the input and output primitive types for this shader:

layout( triangles ) in; layout( triangle_strip, max_vertices = 3 ) out;

We don't actually change anything about the geometry of the triangle, so the input and output types are essentially the same. We will output exactly the same triangle that was received as input. The output variables for the geometry shader are GNormal, GPosition, and GEdgeDistance. The first two are simply the values of the normal and position in camera coordinates, passed through unchanged. The third is the vector that will store the distance to each edge of the triangle (described previously). Note that it is defined with the noperspective qualifier:

noperspective out vec3 GEdgeDistance;

The noperspective qualifier indicates that the values are to be interpolated linearly, instead of the default perspective correct interpolation. As mentioned previously, these distances are in screen space, so it would be incorrect to interpolate them in a non-linear fashion. Within the main function, we start by transforming the position of each of the three vertices of the triangle from clip coordinates to screen space coordinates by multiplying with the viewport matrix. (Note that it is also necessary to divide by the w coordinate as the clip coordinates are homogeneous and may need to be converted back to true Cartesian coordinates.)

Next, we compute the three altitudes—ha, hb, and hc—using the law of cosines. Once we have the three altitudes, we set GEdgeDistance appropriately for the first vertex, pass along GNormal, GPosition, and gl_Position unchanged, and emit the first vertex by calling EmitVertex(). This finishes the vertex and emits the vertex position and all of the per-vertex output variables. We then proceed similarly for the other two vertices of the triangle, finishing the polygon by calling EndPrimitive(). In the fragment shader, we start by evaluating the basic shading model and storing the resulting color in color. At this stage in the pipeline, the three components of the GEdgeDistance variable should contain the distance from this fragment to each of the three edges of the triangle. We are interested in the minimum distance, so we find the minimum of the three components and store that in the d variable. The smoothstep function is then used to determine how much to mix the line color with the shaded color (mixVal):

float mixVal = smoothstep( Line.Width - 1, Line.Width + 1, d );

If the distance is less than Line.Width - 1, then smoothstep will return a value of 0, and if it is greater than Line.Width + 1, it will return 1. For values of d that are in between the two, we'll get a smooth transition. This gives us a value of 0 when inside the line, a value of 1 when outside the line, and in a two-pixel area around the edge, we'll get a smooth variation between 0 and 1. Therefore, we can use the result to mix the color directly with the line color. Finally, the fragment color is determined by mixing the shaded color with the line color using mixVal as the interpolation parameter.

There's more...

This technique produces very nice-looking results and has relatively few drawbacks. However, it does have some issues with triangles that are large in screen space (extend outside the view volume). If the w coordinate is small or zero, the position in viewport space can approach infinity, producing some ugly artifacts. This happens when the vertex is at or near the x-y plane in camera space.

However, it is a good example of how geometry shaders can be useful for tasks other than the modification of the actual geometry. In this case, we used the geometry shader simply to compute additional information about the primitive as it was being sent down the pipeline. This shader can be dropped in and applied to any mesh without any modification to the OpenGL side of the application. It can be useful when debugging mesh issues or when implementing a mesh modeling program. Other common techniques for accomplishing this effect typically involve rendering the shaded object and wireframe in two passes with a polygon offset (via the glPolygonOffset function) applied to avoid z-fighting, which takes place between the wireframe and the shaded surface beneath. This technique is not always effective because the modified depth values might not always be correct, or as desired, and it can be difficult to find the sweet spot for the polygon offset value. For a good survey of techniques, refer to Section 11.4.2 in Real Time Rendering, third edition, by T Akenine-Moller, E Haines, and N Hoffman, AK Peters, 2008.

Drawing silhouette lines using the geometry shader

When a cartoon or hand-drawn effect is desired, we often want to draw black outlines around the edges of a model and along ridges or creases (silhouette lines). In this recipe, we'll discuss one technique for doing this using the geometry shader, to produce the additional geometry for the silhouette lines. The geometry shader will approximate these lines by generating small, skinny quads aligned with the edges that make up the silhouette of the object. The following image shows the ogre mesh with black silhouette lines generated by the geometry shader.

The lines are made up of small quads that are aligned with certain mesh edges:

The technique shown in this recipe is based on a technique published in a blog post by Philip Rideout: prideout.net/blog/?p=54. His implementation uses two passes (base geometry and silhouette), and includes many optimizations, such as anti-aliasing and custom depth testing (with g-buffers). To keep things simple, as our main goal is to demonstrate the features of the geometry shader, we'll implement the technique using a single pass without anti-aliasing or custom depth testing. If you are interested in adding these additional features, refer to Philip's excellent blog post. One of the most important features of the geometry shader is that it allows us to provide additional vertex information beyond just the primitive being rendered. When geometry shaders were introduced in OpenGL, several additional primitive rendering modes were also introduced. These adjacency modes allow additional vertex data to be associated with each primitive. Typically, this additional information is related to the nearby primitives within a mesh, but there is no requirement that this be the case (we could actually use the additional information for other purposes if desired). The following list includes the adjacency modes along with a short description:

• GL_LINES_ADJACENCY: This mode defines lines with adjacent vertices (four vertices per line segment)

• GL_LINE_STRIP_ADJACENCY: This mode defines a line strip with adjacent vertices (for n lines, there are n+3 vertices)

• GL_TRIANGLES_ADJACENCY: This mode defines triangles along with vertices of adjacent triangles (six vertices per primitive)

• GL_TRIANGLE_STRIP_ADJACENCY: This mode defines a triangle strip along with vertices of adjacent triangles (for n triangles, there are 2(n+2) vertices provided)

For full details on each of these modes, check out the official OpenGL documentation. In this recipe, we'll use the GL_TRIANGLES_ADJACENCY mode to provide information about adjacent triangles in our mesh. With this mode, we provide six vertices per primitive. The following diagram illustrates the locations of these vertices:

In the preceding diagram, the solid line represents the triangle itself, and the dotted lines represent adjacent triangles. The first, third, and fifth vertices (0, 2, and 4) make up the triangle itself. The second, fourth, and sixth are vertices that make up the adjacent triangles.

Mesh data is not usually provided in this form, so we need to preprocess our mesh to include the additional vertex information. Typically, this only means expanding the element index array by a factor of two. The position, normal, and texture coordinate arrays can remain unchanged.

When a mesh is rendered with adjacency information, the geometry shader has access to all six vertices associated with a particular triangle. We can then use the adjacent triangles to determine whether a triangle edge is part of the silhouette of the object. The basic assumption is that an edge is a silhouette edge if the triangle is front-facing and the corresponding adjacent triangle is not front-facing.

We can determine whether a triangle is front-facing within the geometry shader by computing the triangle's normal vector (using a cross product). If we are working within eye coordinates (or clip coordinates), the z coordinate of the normal vector will be positive for front-facing triangles. Therefore, we only need to compute the z coordinate of the normal vector, which should save a few cycles. For a triangle with vertices AAA, BBB, and CCC, the z coordinate of the normal vector is given by the following equation:

nz=(AxBy−BxA−y)+(BxCy−CxBy)+(CxAy−AxCy)n_z=(A_xB_y-B_xA-y)+(B_xC_y-C_xB_y)+(C_xA_y-A_xC_y)nz​=(Ax​By​−Bx​A−y)+(Bx​Cy​−Cx​By​)+(Cx​Ay​−Ax​Cy​)

Once we determine which edges are silhouette edges, the geometry shader will produce additional skinny quads aligned with the silhouette edge. These quads, taken together, will make up the desired dark lines (refer to the previous figure). After generating all the silhouette quads, the geometry shader will output the original triangle.

In order to render the mesh in a single pass with appropriate shading for the base mesh, and no shading for the silhouette lines, we'll use an additional output variable. This variable will let the fragment shader know when we are rendering the base mesh and when we are rendering the silhouette edge.

Getting ready

Set up your mesh data so that adjacency information is included. As just mentioned, this probably requires expanding the element index array to include the additional information. This can be done by passing through your mesh and looking for shared edges. Due to space limitations, we won't go through the details here, but the blog post mentioned some time back has some information about how this might be done. Also, the source code for this example contains a simple (albeit not very efficient) technique. The important uniform variables for this example are as follows:

• EdgeWidth: This is the width of the silhouette edge in clip (normalized device) coordinates
• PctExtend: This is a percentage to extend the quads beyond the edge
• LineColor: This is the color of the silhouette edge lines

As usual, there are also the appropriate uniforms for the shading model, and the standard matrices.

How to do it...

To create a shader program that utilizes the geometry shader to render silhouette edges, use the following steps:

1. Use the following code for the vertex shader:

    layout (location = 0 ) in vec3 VertexPosition; layout (location = 1 ) in vec3 VertexNormal; out vec3 VNormal; out vec3 VPosition; uniform mat4 ModelViewMatrix; uniform mat3 NormalMatrix; uniform mat4 ProjectionMatrix; uniform mat4 MVP; void main() { VNormal = normalize( NormalMatrix * VertexNormal); VPosition = vec3(ModelViewMatrix * vec4(VertexPosition,1.0)); gl_Position = MVP * vec4(VertexPosition,1.0);
    }

2. Use the following code for the geometry shader:

    layout( triangles_adjacency ) in; layout( triangle_strip, max_vertices = 15 ) out; out vec3 GNormal; out vec3 GPosition; flat out bool GIsEdge; in vec3 VNormal[]; in vec3 VPosition[]; uniform float EdgeWidth; uniform float PctExtend; bool isFrontFacing( vec3 a, vec3 b, vec3 c ) { return ((a.x * b.y - b.x * a.y) + (b.x * c.y - c.x * b.y) + (c.x * a.y - a.x * c.y)) > 0;
    } void emitEdgeQuad( vec3 e0, vec3 e1 ) { vec2 ext = PctExtend * (e1.xy - e0.xy); vec2 v = normalize(e1.xy - e0.xy); vec2 n = vec2(-v.y, v.x) * EdgeWidth; GIsEdge = true; gl_Position = vec4( e0.xy - ext, e0.z, 1.0 ); EmitVertex(); gl_Position = vec4( e0.xy - n - ext, e0.z, 1.0 ); EmitVertex(); gl_Position = vec4( e1.xy + ext, e1.z, 1.0 ); EmitVertex(); gl_Position = vec4( e1.xy - n + ext, e1.z, 1.0 ); EmitVertex(); EndPrimitive();
    } void main() { vec3 p0 = gl_in[0].gl_Position.xyz / gl_in[0].gl_Position.w; vec3 p1 = gl_in[1].gl_Position.xyz / gl_in[1].gl_Position.w; vec3 p2 = gl_in[2].gl_Position.xyz / gl_in[2].gl_Position.w; vec3 p3 = gl_in[3].gl_Position.xyz / gl_in[3].gl_Position.w; vec3 p4 = gl_in[4].gl_Position.xyz / gl_in[4].gl_Position.w; vec3 p5 = gl_in[5].gl_Position.xyz / gl_in[5].gl_Position.w; if ( isFrontFacing(p0, p2, p4) ) { if ( ! isFrontFacing(p0,p1,p2) ) emitEdgeQuad(p0,p2); if ( ! isFrontFacing(p2,p3,p4) ) emitEdgeQuad(p2,p4); if ( ! isFrontFacing(p4,p5,p0) ) emitEdgeQuad(p4,p0); } GIsEdge = false; GNormal = VNormal[0]; GPosition = VPosition[0]; gl_Position = gl_in[0].gl_Position; EmitVertex(); GNormal = VNormal[2]; GPosition = VPosition[2]; gl_Position = gl_in[2].gl_Position; EmitVertex(); GNormal = VNormal[4]; GPosition = VPosition[4]; gl_Position = gl_in[4].gl_Position; EmitVertex(); EndPrimitive();
    }

3. Use the following code for the fragment shader:

    uniform vec4 LineColor; in vec3 GPosition; in vec3 GNormal; flat in bool GIsEdge; layout( location = 0 ) out vec4 FragColor; vec3 toonShade( ) { } void main() { if ( GIsEdge ) { FragColor = LineColor; } else { FragColor = vec4( toonShade(), 1.0 ); }
    }

How it works...

The vertex shader is a simple passthrough shader. It converts the vertex position and normal to camera coordinates and sends them along, via VPosition and VNormal. These will be used for shading within the fragment shader and will be passed along (or ignored) by the geometry shader. The position is also converted to clip coordinates (or normalized device coordinates) by transforming with the model-view projection matrix, and it is then assigned to the built-in gl_Position.

The geometry shader begins by defining the input and output primitive types using the layout directive:

layout( triangles_adjacency ) in; layout( triangle_strip, max_vertices = 15 ) out;

This indicates that the input primitive type is triangles with adjacency information, and the output type is triangle strips. This geometry shader will produce a single triangle (the original triangle) and at most one quad for each edge. This corresponds to a maximum of 15 vertices that could be produced, and we indicate that maximum within the output layout directive.

The GIsEdge output variable is used to indicate to the fragment shader whether or not the polygon is an edge quad. The fragment shader will use this value to determine whether to shade the polygon. There is no need to interpolate the value and since it is a Boolean, interpolation doesn't quite make sense, so we use the flat qualifier.

The first few lines within the main function take the position for each of the six vertices (in clip coordinates) and divide it by the fourth coordinate in order to convert it from its homogeneous representation to the true Cartesian value. This is necessary if we are using a perspective projection, but is not necessary for orthographic projections.

Next, we determine whether the main triangle (defined by points 0, 2, and 4) is front-facing. The isFrontFacing function returns whether the triangle defined by its three parameters is front-facing using the equation described previously. If the main triangle is front-facing, we will emit a silhouette edge quad only if the adjacent triangle is not front-facing.

The emitEdgeQuad function produces a quad that is aligned with an edge defined by the e0 and e1 points. It begins by computing ext, which is the vector from e0 to e1, scaled by PctExtend (in order to slightly lengthen the edge quad). We lengthen the edge quad in order to cover gaps that may appear between quads (we'll discuss this further in There's more...).

Note also that we drop the z coordinate here. As the points are defined in clip coordinates, and we are going to produce a quad that is aligned with the x-y plane (facing the camera), we want to compute the positions of the vertices by translating within the x-y plane. Therefore we can ignore the z coordinate for now. We'll use its value unchanged in the final position of each vertex.

Next, the v variable is assigned to the normalized vector from e0 to e1. The n variable gets a vector that is perpendicular to v (in 2D, this can be achieved by swapping the x and y coordinates and negating the new x coordinate). This is just a counter-clockwise 90-degree rotation in 2D. We scale the n vector by EdgeWidth because we want the length of the vector to be the same as the width of the quad. The ext and n vectors will be used to determine the vertices of the quad, as shown in the following diagram:

The four corners of the quad are given by e0−exte0 - exte0−ext, e0−n−exte0 - n - exte0−n−ext, e1+exte1 + exte1+ext, and e1−n+exte1 - n + exte1−n+ext. The z coordinate for the lower two vertices is the same as the z coordinate for e0, and the z coordinate for the upper two vertices is the z coordinate for e1e1e1.

We then finish up the emitEdgeQuad function by setting GIsEdge to true in order to let the fragment shader know that we are rendering a silhouette edge, and then emitting the four vertices of the quad. The function ends with a call to EndPrimitive to terminate the processing of the triangle strip for the quad.

Back within the main function, after producing the silhouette edges, we proceed by emitting the original triangle unchanged. VNormal, VPosition, and gl_Position for vertices 0, 2, and 4 are passed along without any modification to the fragment shader. Each vertex is emitted with a call to EmitVertex, and the primitive is completed with EndPrimitive.

Within the fragment shader, we either shade the fragment (using the toon shading algorithm), or simply give the fragment a constant color. The GIsEdge input variable will indicate which option to choose. If GIsEdge is true, then we are rendering a silhouette edge so the fragment is given the line color. Otherwise, we are rendering a mesh polygon, so we shade the fragment using the toon shading technique from Chapter 4, Lighting and Shading.

There's more...

One of the problems with the preceding technique is that feathering can occur due to the gaps between consecutive edge quads:

The preceding diagram shows the feathering of a silhouette edge. The gaps between the polygons can be filled with triangles, but in our example, we simply extend the length of each quad to fill in the gap. This can, of course, cause artifacts if the quads are extended too far, but in practice they haven't been very distracting in my experience.

A second issue is related to depth testing. If an edge polygon extends into another area of the mesh, it can be clipped due to the depth test. The following is an example:

The edge polygon should extend vertically throughout the middle of the preceding image, but is clipped because it falls behind the part of the mesh that is nearby. This issue can be solved by using custom depth testing when rendering the silhouette edges. Refer to Philip Rideout's blog post mentioned earlier for details on this technique. It may also be possible to turn depth testing off when rendering the edges, being careful not to render any edges from the opposite side of the model.

Tessellating a curve

In this recipe, we'll take a look at the basics of tessellation shaders by drawing a cubic Bezier curve. A Bezier curve is a parametric curve defined by four control points. The control points define the overall shape of the curve. The first and last of the four points define the start and end of the curve, and the middle points guide the shape of the curve, but do not necessarily lie directly on the curve itself. The curve is defined by interpolating the four control points using a set of blending functions. The blending functions define how much each control point contributes to the curve for a given position along the curve. For Bezier curves, the blending functions are known as the Bernstein polynomials:

Bin=(ni)(1−t)n−itiB_i^n=\begin{pmatrix}n\\ i\end{pmatrix}(1-t)^{n-i}t^iBin​=(ni​)(1−t)n−iti

In the preceding equation, the first term is the binomial coefficient function (shown in the following equation), nnn is the degree of the polynomial, iii is the polynomial number, and ttt is the parametric parameter:

(ni)=n!i!(n−i)!\begin{pmatrix}n\\ i\end{pmatrix}=\frac{n!}{i!(n-i)!}(ni​)=i!(n−i)!n!​

The general parametric form for the Bezier curve is then given as a sum of the products of the Bernstein polynomials with the control points (PiP_iPi​):

P(t)=∑i=0nBin(t)PiP(t)=\sum_{i=0}^nB_i^n(t)P_iP(t)=∑i=0n​Bin​(t)Pi​

In this example, we will draw a cubic Bezier curve, which involves four control points (n=3n = 3n=3):

P(t)=B03(t)P0+B13(t)P1+B23(t)P2+B33(t)P3P(t)=B_0^3(t)P_0+B_1^3(t)P_1+B_2^3(t)P_2+B_3^3(t)P_3P(t)=B03​(t)P0​+B13​(t)P1​+B23​(t)P2​+B33​(t)P3​

And the cubic Bernstein polynomials are:

B03(t)=(1−t)3B13(t)=3(1−t)2tB23(t)=3(1−t)t2B33(t)=t3\begin{aligned} B_0^3(t) & = (1-t)^3 \\ B_1^3(t) & = 3(1-t)^2t \\ B_2^3(t) & = 3(1-t)t^2 \\ B_3^3(t) & = t^3 \end{aligned}B03​(t)B13​(t)B23​(t)B33​(t)​=(1−t)3=3(1−t)2t=3(1−t)t2=t3​

As stated in the introduction to this chapter, the tessellation functionality within OpenGL involves two shader stages. They are the tessellation control shader (TCS) and the tessellation evaluation shader (TES). In this example, we'll define the number of line segments for our Bezier curve within the TCS (by defining the outer tessellation levels), and evaluate the Bezier curve at each particular vertex location within the TES. The following image shows the output of this example for three different tessellation levels. The left figure uses three line segments (level 3), the middle uses level 5, and the right-hand figure is created with tessellation level 30. The small squares are the control points:

The control points for the Bezier curve are sent down the pipeline as a patch primitive consisting of four vertices. A patch primitive is a programmer-defined primitive type. Basically, it is a set of vertices that can be used for anything that the programmer chooses. The TCS is executed once for each vertex within the patch, and the TES is executed, a variable number of times, depending on the number of vertices produced by the TPG. The final output of the tessellation stages is a set of primitives. In our case, it will be a line strip.

Part of the job of the TCS is to define the tessellation level. In very rough terms, the tessellation level is related to the number of vertices that will be generated. In our case, the TCS will be generating a line strip, so the tessellation level is the number of line segments in the line strip. Each vertex that is generated for this line strip will be associated with a tessellation coordinate that will vary between zero and one. We'll refer to this as the u coordinate, and it will correspond to the parametric t parameter in the preceding Bezier curve equation.

What we've looked at so far is not, in fact, the whole story. Actually, the TCS will trigger the generation of a set of line strips called isolines. Each vertex in this set of isolines will have a u and a v coordinate. The u coordinate will vary from zero to one along a given isoline, and v will be constant for each isoline. The number of distinct values of u and v is associated with two separate tessellation levels, the so-called outer levels. For this example, however, we'll only generate a single line strip, so the second tessellation level (for v) will always be one.

Within the TES, the main task is to determine the position of the vertex associated with this execution of the shader. We have access to the u and v coordinates associated with the vertex, and we also have (read-only) access to all of the vertices of the patch. We can then determine the appropriate position for the vertex by using the parametric equation, with u as the parametric coordinate (t in the preceding equation).

Getting ready

The following are the important uniform variables for this example:

• NumSegments: This is the number of line segments to be produced.
• NumStrips: This is the number of isolines to be produced. For this example, this should be set to 1.
• LineColor: This is the color for the resulting line strip.

Set the uniform variables within the main OpenGL application. There are a total of four shaders to be compiled and linked. They are the vertex, fragment, tessellation control, and tessellation evaluation shaders.

How to do it...

To create a shader program that will generate a Bezier curve from a patch of four control points, use the following steps:

1. Use the following code for the vertex shader. Note that we send the vertex position along to the TCS unmodified:

    layout (location = 0 ) in vec2 VertexPosition; void main() { gl_Position = vec4(VertexPosition, 0.0, 1.0);
    }

2. Use the following code as the tessellation control shader:

    layout( vertices=4 ) out; uniform int NumSegments; uniform int NumStrips; void main() { gl_out[gl_InvocationID].gl_Position = gl_in[gl_InvocationID].gl_Position; gl_TessLevelOuter[0] = float(NumStrips); gl_TessLevelOuter[1] = float(NumSegments);
    }

3. Use the following code as the tessellation evaluation shader:

    layout( isolines ) in; uniform mat4 MVP; void main() { float u = gl_TessCoord.x; vec3 p0 = gl_in[0].gl_Position.xyz; vec3 p1 = gl_in[1].gl_Position.xyz; vec3 p2 = gl_in[2].gl_Position.xyz; vec3 p3 = gl_in[3].gl_Position.xyz; float u1 = (1.0 - u); float u2 = u * u; float b3 = u2 * u; float b2 = 3.0 * u2 * u1; float b1 = 3.0 * u * u1 * u1; float b0 = u1 * u1 * u1; vec3 p = p0 * b0 + p1 * b1 + p2 * b2 + p3 * b3; gl_Position = MVP * vec4(p, 1.0);
    }

4. Use the following code for the fragment shader:

    uniform vec4 LineColor; layout ( location = 0 ) out vec4 FragColor; void main() { FragColor = LineColor;
    }

5. It is important to define the number of vertices per patch within the OpenGL application. You can do so using the glPatchParameter function:

    glPatchParameteri( GL_PATCH_VERTICES, 4);

6. Render the four control points as a patch primitive within the OpenGL application's render function:

    glDrawArrays(GL_PATCHES, 0, 4);

How it works...

The vertex shader is just a passthrough shader. It sends the vertex position along to the next stage without any modification. The tessellation control shader begins by defining the number of vertices in the output patch:

layout (vertices = 4) out;

Note that this is not the same as the number of vertices that will be produced by the tessellation process. In this case, the patch is our four control points, so we use a value of four.

The main method within the TCS passes the input position (of the patch vertex) to the output position without modification. The gl_out and gl_in arrays contain the input and output information associated with each vertex in the patch. Note that we assign and read from gl_InvocationID in these arrays. The gl_InvocationID variable defines the output patch vertex for which this invocation of the TCS is responsible. The TCS can access all of the gl_in array, but should only write to the location in gl_out corresponding to gl_InvocationID. The other indices will be written by other invocations of the TCS.

Next, the TCS sets the tessellation levels by assigning to the gl_TessLevelOuter array. Note that the values for gl_TessLevelOuter are floating point numbers rather than integers. They will be rounded up to the nearest integer and clamped automatically by the OpenGL system.

The first element in the array defines the number of isolines that will be generated. Each isoline will have a constant value for v. In this example, the value of gl_TessLevelOuter[0] should be one since we only want to create a single curve. The second one defines the number of line segments that will be produced in the line strip. Each vertex in the strip will have a value for the parametric u coordinate that will vary from zero to one.

In the TES, we start by defining the input primitive type using a layout declaration:

layout (isolines) in;

This indicates the type of subdivision that is performed by the tessellation primitive generator. Other possibilities here include quads and triangles.

Within the main function of the TES, the gl_TessCoord variable contains the tessellation's u and v coordinates for this invocation. As we are only tessellating in one dimension, we only need the u coordinate, which corresponds to the x coordinate of gl_TessCoord.

The next step accesses the positions of the four control points (all the points in our patch primitive). These are available in the gl_in array.

The cubic Bernstein polynomials are then evaluated at u and stored in b0, b1, b2, and b3. Next, we compute the interpolated position using the Bezier curve equation. The final position is converted to clip coordinates and assigned to the gl_Position output variable.

The fragment shader simply applies LineColor to the fragment.

There's more...

There's a lot more to be said about tessellation shaders, but this example is intended to be a simple introduction so we'll leave that for the following recipes. Next, we'll look at tessellation across surfaces in two dimensions.

Tessellating a 2D quad

One of the best ways to understand OpenGL's hardware tessellation is to visualize the tessellation of a 2D quad. When linear interpolation is used, the triangles that are produced are directly related to the tessellation coordinates (u,v) that are produced by the tessellation primitive generator. It can be extremely helpful to draw a few quads with different inner and outer tessellation levels, and study the triangles produced. We will do exactly that in this recipe.

When using quad tessellation, the tessellation primitive generator subdivides (u,v) parameter space into a number of subdivisions based on six parameters. These are the inner tessellation levels for u and v (inner level 0 and inner level 1), and the outer tessellation levels for u and v along both edges (outer levels 0 to 3). These determine the number of subdivisions along the edges of the parameter space and internally. Let's look at each of these individually:

• Outer level 0 (OL0): This is the number of subdivisions along the v direction where u = 0
• Outer level 1 (OL1): This is the number of subdivisions along the u direction where v = 0
• Outer level 2 (OL2): This is the number of subdivisions along the v direction where u = 1
• Outer level 3 (OL3): This is the number of subdivisions along the u direction where v = 1
• Inner level 0 (IL0): This is the number of subdivisions along the u direction for all internal values of v
• Inner level 1 (IL1): This is the number of subdivisions along the v direction for all internal values of u

The following diagram represents the relationship between the tessellation levels and the areas of parameter space that are affected by each. The outer levels define the number of subdivisions along the edges, and the inner levels define the number of subdivisions internally:

The six tessellation levels described some time back can be configured via the gl_TessLevelOuter and gl_TessLevelInner arrays. For example, gl_TessLevelInner[0] corresponds to IL0, gl_TessLevelOuter[2] corresponds to OL2, and so on.

If we draw a patch primitive that consists of a single quad (four vertices), and use linear interpolation, the triangles that result can help us to understand how OpenGL does quad tessellation. The following diagram shows the results for various tessellation levels:

When we use linear interpolation, the triangles that are produced represent a visual representation of parameter (u, v) space. The x axis corresponds to the u coordinate and the y axis corresponds to the v coordinate. The vertices of the triangles are the (u,v) coordinates generated by the tessellation primitive generator. The number of subdivisions can be clearly seen in the mesh of triangles. For example, when the outer levels are set to 2 and the inner levels are set to 8, you can see that the outer edges have two subdivisions, but within the quad, u and v are subdivided into eight intervals.

Before jumping into the code, let's discuss linear interpolation. If the four corners of the quad are as shown in the following figure, then any point within the quad can be determined by linearly interpolating the four corners with respect to the u and v parameters:

We'll let the tessellation-primitive generator create a set of vertices with appropriate parametric coordinates, and we'll determine the corresponding positions by interpolating the corners of the quad using the preceding equation.

Getting ready

The outer and inner tessellation levels will be determined by the Inner and Outer uniform variables. In order to display the triangles, we will use the geometry shader.

Set up your OpenGL application to render a patch primitive consisting of four vertices in counterclockwise order, as shown in the previous figure.

How to do it...

To create a shader program that will generate a set of triangles using quad tessellation from a patch of four vertices, use the following steps:

1. Use the following code for the vertex shader:

    layout (location = 0 ) in vec2 VertexPosition; void main() { gl_Position = vec4(VertexPosition, 0.0, 1.0);
    }

2. Use the following code as the tessellation control shader:

    layout( vertices=4 ) out; uniform int Outer; uniform int Inner; void main() { gl_out[gl_InvocationID].gl_Position = gl_in[gl_InvocationID].gl_Position; gl_TessLevelOuter[0] = float(Outer); gl_TessLevelOuter[1] = float(Outer); gl_TessLevelOuter[2] = float(Outer); gl_TessLevelOuter[3] = float(Outer); gl_TessLevelInner[0] = float(Inner); gl_TessLevelInner[1] = float(Inner);
    }

3. Use the following code as the tessellation evaluation shader:

    layout( quads, equal_spacing, ccw ) in; uniform mat4 MVP; void main() { float u = gl_TessCoord.x; float v = gl_TessCoord.y; vec4 p0 = gl_in[0].gl_Position; vec4 p1 = gl_in[1].gl_Position; vec4 p2 = gl_in[2].gl_Position; vec4 p3 = gl_in[3].gl_Position; gl_Position = p0 * (1-u) * (1-v) + p1 * u * (1-v) + p3 * v * (1-u) + p2 * u * v; gl_Position = MVP * gl_Position;
    }

4. Use the geometry shader from the Drawing a wireframe on top of a shaded mesh recipe

5. Use the following code as the fragment shader:

    uniform float LineWidth; uniform vec4 LineColor; uniform vec4 QuadColor; noperspective in vec3 GEdgeDistance; layout ( location = 0 ) out vec4 FragColor; float edgeMix() { } void main() { float mixVal = edgeMix(); FragColor = mix( QuadColor, LineColor, mixVal );
    }

6. Within the render function of your main OpenGL program, define the number of vertices within a patch:

    glPatchParameteri(GL_PATCH_VERTICES, 4);

7. Render the patch as four 2D vertices in counterclockwise order

How it works...

The vertex shader passes the position along to the TCS unchanged.

The TCS defines the number of vertices in the patch using the layout directive:

layout (vertices=4) out;

In the main function, it passes along the position of the vertex without modification, and sets the inner and outer tessellation levels. All four of the outer tessellation levels are set to the value of Outer, and both of the inner tessellation levels are set to Inner.

In the tessellation evaluation shader, we define the tessellation mode and other tessellation parameters with the input layout directive:

layout ( quads, equal_spacing, ccw ) in;

The quads parameter indicates that the tessellation-primitive generator should tessellate the parameter space using quad tessellation. The equal_spacing parameter says that the tessellation should be performed such that all subdivisions have equal length. The last parameter, ccw, indicates that the primitives should be generated with counterclockwise winding.

The main function in the TES starts by retrieving the parametric coordinates for this vertex by accessing the gl_TessCoord variable. Then we move on to read the positions of the four vertices in the patch from the gl_in array. We store them in temporary variables to be used in the interpolation calculation.

The built-in gl_Position output variable then gets the value of the interpolated point using the preceding equation. Finally, we convert the position into clip coordinates by multiplying by the model-view projection matrix.

Within the fragment shader, we give all fragments a color that is possibly mixed with a line color in order to highlight the edges.

Tessellating a 3D surface

As an example of tessellating a 3D surface, let's render (yet again) the teapotahedron. It turns out that the teapot's dataset is actually defined as a set of 4 x 4 patches of control points, suitable for cubic Bezier interpolation. Therefore, drawing the teapot really boils down to drawing a set of cubic Bezier surfaces.

Of course, this sounds like a perfect job for tessellation shaders! We'll render each patch of
16 vertices as a patch primitive, use quad tessellation to subdivide the parameter space, and implement the Bezier interpolation within the tessellation evaluation shader.

The following image shows an example of the desired output. The left teapot is rendered with inner and outer tessellation level 2, the middle uses level 4, and the teapot on the right uses tessellation level 16. The tessellation evaluation shader computes the Bezier surface interpolation:

First, let's take a look at how cubic Bezier surface-interpolation works. If our surface is defined by a set of 16 control points (laid out in a 4x4 grid) PijP_{ij}Pij​, with iii and jjj ranging from 0 to 3, the parametric Bezier surface is given by the following equation:

P(u,v)=∑i=03∑j=03Bi3(u)Bj3(v)PijP(u,v)=\sum_{i=0}^3\sum_{j=0}^3B_i^3(u)B_j^3(v)P_{ij}P(u,v)=∑i=03​∑j=03​Bi3​(u)Bj3​(v)Pij​

The instances of B in the preceding equation are the cubic Bernstein polynomials (refer to the previous recipe, Tessellating a 2D quad).

We also need to compute the normal vector at each interpolated location. To do so, we have to compute the cross product of the partial derivatives of the preceding equation:

n(u,v)=ðPðu×ðPðvn(u,v)=\frac{\eth P}{\eth u}\times \frac{\eth P}{\eth v}n(u,v)=ðuðP​×ðvðP​

The partial derivatives of the Bezier surface boil down to the partial derivatives of the Bernstein polynomials:

ðPðu=∑i=03∑j=03ðBi3(u)ðuBj3(v)PijðPðv=∑i=03∑j=03Bi3(u)ðBj3(v)ðvPij\begin{aligned} \frac{\eth P}{\eth u} & = \sum_{i=0}^3\sum_{j=0}^3\frac{\eth B_i^3(u)}{\eth u}B_j^3(v)P_{ij} \\ \frac{\eth P}{\eth v} & = \sum_{i=0}^3\sum_{j=0}^3B_i^3(u)\frac{\eth B_j^3(v)}{\eth v}P_{ij} \end{aligned}ðuðP​ðvðP​​=i=0∑3​j=0∑3​ðuðBi3​(u)​Bj3​(v)Pij​=i=0∑3​j=0∑3​Bi3​(u)ðvðBj3​(v)​Pij​​

We'll compute the partials within the TES and compute the cross product to determine the normal to the surface at each tessellated vertex.

Getting ready

Set up your shaders with a vertex shader that simply passes the vertex position along without any modification (you can use the same vertex shader as was used in the Tessellating a 2D quad recipe). Create a fragment shader that implements whatever shading model you choose. The fragment shader should receive the TENormal and TEPosition input variables, which will be the normal and position in camera coordinates.

The TessLevel uniform variable should be given the value of the desired tessellation level. All of the inner and outer levels will be set to this value.

How to do it...

To create a shader program that creates Bezier patches from input patches of 16 control points, use the following steps:

1. Use the vertex shader from the Tessellating a 2D quad recipe.

2. Use the following code for the tessellation control shader:

    layout( vertices=16 ) out; uniform int TessLevel; void main() { gl_out[gl_InvocationID].gl_Position = gl_in[gl_InvocationID].gl_Position; gl_TessLevelOuter[0] = float(TessLevel); gl_TessLevelOuter[1] = float(TessLevel); gl_TessLevelOuter[2] = float(TessLevel); gl_TessLevelOuter[3] = float(TessLevel); gl_TessLevelInner[0] = float(TessLevel); gl_TessLevelInner[1] = float(TessLevel);
    }

3. Use the following code for the tessellation evaluation shader:

    layout( quads ) in; out vec3 TENormal; out vec4 TEPosition; uniform mat4 MVP; uniform mat4 ModelViewMatrix; uniform mat3 NormalMatrix; void basisFunctions(out float[4] b, out float[4] db, float t) { float t1 = (1.0 - t); float t12 = t1 * t1; b[0] = t12 * t1; b[1] = 3.0 * t12 * t; b[2] = 3.0 * t1 * t * t; b[3] = t * t * t; db[0] = -3.0 * t1 * t1; db[1] = -6.0 * t * t1 + 3.0 * t12; db[2] = -3.0 * t * t + 6.0 * t * t1; db[3] = 3.0 * t * t;
    } void main() { float u = gl_TessCoord.x; float v = gl_TessCoord.y; vec4 p00 = gl_in[0].gl_Position; vec4 p01 = gl_in[1].gl_Position; vec4 p02 = gl_in[2].gl_Position; vec4 p03 = gl_in[3].gl_Position; vec4 p10 = gl_in[4].gl_Position; vec4 p11 = gl_in[5].gl_Position; vec4 p12 = gl_in[6].gl_Position; vec4 p13 = gl_in[7].gl_Position; vec4 p20 = gl_in[8].gl_Position; vec4 p21 = gl_in[9].gl_Position; vec4 p22 = gl_in[10].gl_Position; vec4 p23 = gl_in[11].gl_Position; vec4 p30 = gl_in[12].gl_Position; vec4 p31 = gl_in[13].gl_Position; vec4 p32 = gl_in[14].gl_Position; vec4 p33 = gl_in[15].gl_Position; float bu[4], bv[4]; float dbu[4], dbv[4]; basisFunctions(bu, dbu, u); basisFunctions(bv, dbv, v); TEPosition = p00*bu[0]*bv[0] + p01*bu[0]*bv[1] + p02*bu[0]*bv[2] + p03*bu[0]*bv[3] + p10*bu[1]*bv[0] + p11*bu[1]*bv[1] + p12*bu[1]*bv[2] + p13*bu[1]*bv[3] + p20*bu[2]*bv[0] + p21*bu[2]*bv[1] + p22*bu[2]*bv[2] + p23*bu[2]*bv[3] + p30*bu[3]*bv[0] + p31*bu[3]*bv[1] + p32*bu[3]*bv[2] + p33*bu[3]*bv[3]; vec4 du = p00*dbu[0]*bv[0]+p01*dbu[0]*bv[1]+p02*dbu[0]*bv[2]+ p03*dbu[0]*bv[3]+ p10*dbu[1]*bv[0]+p11*dbu[1]*bv[1]+p12*dbu[1]*bv[2]+ p13*dbu[1]*bv[3]+ p20*dbu[2]*bv[0]+p21*dbu[2]*bv[1]+p22*dbu[2]*bv[2]+ p23*dbu[2]*bv[3]+ p30*dbu[3]*bv[0]+p31*dbu[3]*bv[1]+p32*dbu[3]*bv[2]+ p33*dbu[3]*bv[3]; vec4 dv = p00*bu[0]*dbv[0]+p01*bu[0]*dbv[1]+p02*bu[0]*dbv[2]+ p03*bu[0]*dbv[3]+ p10*bu[1]*dbv[0]+p11*bu[1]*dbv[1]+p12*bu[1]*dbv[2]+ p13*bu[1]*dbv[3]+ p20*bu[2]*dbv[0]+p21*bu[2]*dbv[1]+p22*bu[2]*dbv[2]+ p23*bu[2]*dbv[3]+ p30*bu[3]*dbv[0]+p31*bu[3]*dbv[1]+p32*bu[3]*dbv[2]+ p33*bu[3]*dbv[3]; vec3 n = normalize( cross(du.xyz, dv.xyz) ); gl_Position = MVP * TEPosition; TEPosition = ModelViewMatrix * TEPosition; TENormal = normalize(NormalMatrix * n);
    }

4. Implement your favorite shading model within the fragment shader utilizing the output variables from the TES.

5. Render the Bezier control points as a 16-vertex patch primitive. Don't forget to set the number of vertices per patch within the OpenGL application:

    glPatchParameteri(GL_PATCH_VERTICES, 16);

How it works...

The tessellation control shader starts by defining the number of vertices in the patch using the layout directive:

layout( vertices=16 ) out;

It then simply sets the tessellation levels to the value of TessLevel. It passes the vertex position along, without any modification.

The tessellation evaluation shader starts by using a layout directive to indicate the type of tessellation to be used. As we are tessellating a 4 x 4 Bezier surface patch, quad tessellation makes the most sense.

The basisFunctions function evaluates the Bernstein polynomials and their derivatives for a given value of the t parameter. The results are returned in the b and db output parameters.

Within the main function, we start by assigning the tessellation coordinates to the u and v variables, and reassigning all 16 of the patch vertices to variables with shorter names (to shorten the code that appears later).

We then call basisFunctions to compute the Bernstein polynomials and their derivatives at u and v, storing the results in bu, dbu, bv, and dbv.

The next step is the evaluation of the sums from the preceding equations for the position (TEPosition), the partial derivative with respect to u (du), and the partial derivative with respect to v (dv). We compute the normal vector as the cross product of du and dv.

Finally, we convert the position (TEPosition) to clip coordinates and assign the result to
gl_Position. We also convert it to camera coordinates before it is passed along to the fragment shader.

The normal vector is converted to camera coordinates by multiplying it with NormalMatrix, and the result is normalized and passed along to the fragment shader via TENormal.

Tessellating based on depth

One of the greatest things about tessellation shaders is how easy it is to implement level-of-detail (LOD) algorithms. LOD is a general term in computer graphics that refers to the process of increasing/decreasing the complexity of an object's geometry with respect to the distance from the viewer (or other factors). As an object moves farther away from the camera, less geometric detail is needed to represent the shape because the overall size of the object becomes smaller. However, as the object moves closer to the camera, the object fills more and more of the screen, and more geometric detail is needed to maintain the desired appearance (smoothness or lack of other geometric artifacts).
The following image shows a few teapots rendered with tessellation levels that depend on distance from the camera. Each teapot is rendered using exactly the same code on the OpenGL side. The TCS automatically varies the tessellation levels based on depth:

When tessellation shaders are used, the tessellation level is what determines the geometric complexity of the object. As the tessellation levels can be set within the tessellation control shader, it is a simple matter to vary the tessellation levels with respect to the distance from the camera.

In this example, we'll vary the tessellation levels linearly (with respect to distance) between a minimum level and a maximum level. We'll compute the distance from the camera as the absolute value of the z coordinate in camera coordinates, (of course, this is not the true distance, but should work fine for the purposes of this example). The tessellation level will then be computed based on that value. We'll also define two additional values (as uniform variables): MinDepth and MaxDepth. Objects that are closer to the camera than MinDepth get the maximum tessellation level, and any objects that are further from the camera than MaxDepth will get the minimum tessellation level. The tessellation level for objects in between will be linearly interpolated.

Getting ready

This program is nearly identical to the one in the Tessellating a 3D surface recipe. The only difference lies within the TCS. We'll remove the TessLevel uniform variable, and add a few new ones that are described as follows:

• MinTessLevel: This is the lowest desired tessellation level
• MaxTessLevel: This is the highest desired tessellation level
• MinDepth: This is the minimum distance from the camera, where the tessellation level is maximal
• MaxDepth: This is the maximum distance from the camera, where the tessellation level is at a minimum

Render your objects as 16-vertex patch primitives as indicated in the Tessellating a 3D surface recipe.

How to do it...

To create a shader program that varies the tessellation level based on the depth, use the following steps:

1. Use the vertex shader and tessellation evaluation shader from the Tessellating a 3D surface recipe.

2. Use the following code for the tessellation control shader:

    layout( vertices=16 ) out; uniform int MinTessLevel; uniform int MaxTessLevel; uniform float MaxDepth;
    uniform float MinDepth; uniform mat4 ModelViewMatrix; void main() { vec4 p = ModelViewMatrix * gl_in[gl_InvocationID].gl_Position; float depth = clamp( (abs(p.z) - MinDepth) / (MaxDepth - MinDepth), 0.0, 1.0 ); float tessLevel = mix(MaxTessLevel, MinTessLevel, depth); gl_TessLevelOuter[0] = float(tessLevel); gl_TessLevelOuter[1] = float(tessLevel); gl_TessLevelOuter[2] = float(tessLevel); gl_TessLevelOuter[3] = float(tessLevel); gl_TessLevelInner[0] = float(tessLevel); gl_TessLevelInner[1] = float(tessLevel); gl_out[gl_InvocationID].gl_Position = gl_in[gl_InvocationID].gl_Position;
    }

3. As with the previous recipe, implement your favorite shading model within the fragment shader.

How it works...

The TCS takes the position and converts it to camera coordinates and stores the result in the p variable. The absolute value of the z coordinate is then scaled and clamped so that the result is between zero and one. If the z coordinate is equal to MaxDepth, the value of the depth will be 1.0, if it is equal to MinDepth, the depth will be 0.0. If z is between MinDepth and MaxDepth, the depth will get a value between zero and one. If z is outside that range, it will be clamped to 0.0 or 1.0 by the clamp function.

The value of depth is then used to linearly interpolate between MaxTessLevel and
MinTessLevel using the mix function. The result (tessLevel) is used to set the inner and outer tessellation levels.

There's more...

There is a somewhat subtle aspect to this example. Recall that the TCS is executed once for each output vertex in the patch. Therefore, assuming that we are rendering cubic Bezier surfaces, this TCS will be executed 16 times for each patch. Each time it is executed, the value of depth will be slightly different because it is evaluated based on the z coordinate of the vertex. You might be wondering, which of the 16 possible different tessellation levels will be the one that is used? It doesn't make sense for the tessellation level to be interpolated across the parameter space. What's going on?

The gl_TessLevelInner and gl_TessLevelOuter output arrays are per-patch output variables. This means that only a single value will be used per patch, similar to the way that the flat qualifier works for fragment shader input variables. The OpenGL specification seems to indicate that any of the values from each of the invocations of the TCS could be the value that ends up being used.

We should also note that if the tessellation level is different for patches that share an edge, then there is the potential for cracks to appear or other visual artifacts. Therefore we should take care to make sure that neighboring patches use the same tessellation level.

Shadows

In this chapter, we will cover the following recipes:

• Rendering shadows with shadow maps
• Anti-aliasing shadow edges with PCF
• Creating soft shadow edges with random sampling
• Creating shadows using shadow volumes and the geometry shader

Introduction

Shadows add a great deal of realism to a scene. Without shadows, it can be easy to misjudge the relative location of objects, and the lighting can appear unrealistic, as light rays seem to pass right through objects.

Shadows are important visual cues for realistic scenes, but can be challenging to produce in an efficient manner in interactive applications. One of the most popular techniques for creating shadows in real-time graphics is the shadow mapping algorithm (also called depth shadows). In this chapter, we'll look at several recipes surrounding the shadow mapping algorithm. We'll start with the basic algorithm, and discuss it in detail in the first recipe. Then, we'll look at a couple of techniques for improving the look of the shadows produced by the basic algorithm.

We'll also look at an alternative technique for shadows called shadow volumes. Shadow volumes produce near perfect hard-edged shadows, but are not well-suited for creating shadows with soft edges.

Rendering shadows with shadow maps

One of the most common and popular techniques for producing shadows is called shadow mapping. In its basic form, the algorithm involves two passes. In the first pass, the scene is rendered from the point of view of the light source. The depth information from this pass is saved into a texture called the shadow map. This map will help provide information about the visibility of objects from the light's perspective. In other words, the shadow map stores the distance (actually the pseudo-depth) from the light to whatever the light can see. Anything that is closer to the light than the corresponding depth stored in the map is lit; anything else must be in shadow.

In the second pass, the scene is rendered normally, but each fragment's depth (from the light's perspective) is first tested against the shadow map to determine whether or not the fragment is in shadow. The fragment is then shaded differently depending on the result of this test. If the fragment is in shadow, it is shaded with ambient lighting only; otherwise, it is shaded normally.

The following image shows an example of shadows produced by the basic shadow mapping technique:

Let's look at each step of the algorithm in detail.

The first step is the creation of the shadow map. We set up our view matrix so that we are rendering the scene as if the camera is located at the position of the light source, and is oriented toward the shadow-casting objects. We set up a projection matrix so that the view frustum encloses all objects that may cast shadows as well as the area where the shadows will appear. We then render the scene normally and store the information from the depth buffer in a texture. This texture is called the shadow map (or simply depth map). We can think of it (roughly) as a set of distances from the light source to various surface locations.

Technically, these are depth values, not distances. A depth value is not a true distance (from the origin), but can be roughly treated as such for the purposes of depth testing.

The following diagrams represent an example of the basic shadow mapping setup. The left diagram shows the light's position and its associated perspective frustum. The right-hand diagram shows the corresponding shadow map. The greyscale intensities in the shadow map correspond to the depth values (darker is closer):

Once we have created the shadow map and stored the map in a texture, we render the scene again from the point of view of the camera. This time, we use a fragment shader that shades each fragment based on the result of a depth test with the shadow map. The position of the fragment is first converted into the coordinate system of the light source and projected using the light source's projection matrix. The result is then biased (in order to get valid texture coordinates) and tested against the shadow map. If the depth of the fragment is greater than the depth stored in the shadow map, then there must be some surface that is between the fragment and the light source. Therefore, the fragment is in shadow and is shaded using ambient lighting only. Otherwise, the fragment must have a clear view to the light source, and so it is shaded normally.

The key aspect here is the conversion of the fragment's 3D coordinates to the coordinates appropriate for a lookup into the shadow map. As the shadow map is just a 2D texture, we need coordinates that range from zero to one for points that lie within the light's frustum. The light's view matrix will transform points in world coordinates to points within the light's coordinate system. The light's projection matrix will transform points that are within the light's frustum to homogeneous clip coordinates.

These are called clip coordinates because the built-in clipping functionality takes place when the position is defined in these coordinates. Points within the perspective (or orthographic) frustum are transformed by the projection matrix to the (homogeneous) space that is contained within a cube centered at the origin, with each side of length two. This space is called the canonical viewing volume. The term homogeneous means that these coordinates should not necessarily be considered to be true Cartesian positions until they are divided by their fourth coordinate. For full details about homogeneous coordinates, refer to your favorite textbook on computer graphics.

The x and y components of the position in clip coordinates are roughly what we need to access the shadow map. The z coordinate contains the depth information that we can use to compare with the shadow map. However, before we can use these values we need to do two things. First, we need to bias them so that they range from zero to one (instead of -1 to 1), and second, we need to apply perspective division (more on this later).

To convert the value from clip coordinates to a range appropriate for use with a shadow map, we need the x, y, and z coordinates to range from zero to one (for points within the light's view frustum). The depth that is stored in an OpenGL depth buffer (and also our shadow map) is simply a fixed or floating-point value between zero and one (typically). A value of zero corresponds to the near plane of the perspective frustum, and a value of one corresponds to points on the far plane. Therefore, if we are to use our z coordinate to accurately compare with this depth buffer, we need to scale and translate it appropriately.

In clip coordinates (after perspective division) the z coordinate ranges from -1 to 1. It is the viewport transformation that (among other things) converts the depth to a range between zero and one. Incidentally, if so desired, we can configure the viewport transformation to use some other range for the depth values (say between 0 and 100) via the glDepthRange function.

Of course, the x and y components also need to be biased between zero and one because that is the appropriate range for texture access.

We can use the following bias matrix to alter our clip coordinates:

B=[0.5000.500.500000.5050001]B=\begin{bmatrix}0.5&0&0&0.5\\ 0&0.5&0&0\\ 0&0&0.5&05\\ 0&0&0&1\end{bmatrix}B=​0.5000​00.500​000.50​0.50051​​

This matrix will scale and translate our coordinates such that the x, y, and z components range from 0 to 1 (after perspective division) for points within the light's frustum. Now, combining the bias matrix with the light's view (VlV_lVl​) and projection (PlP_lPl​) matrices, we have the following equation for converting positions in world coordinates (WWW) to homogeneous positions that can be used for shadow map access (QQQ):

Q=BPlVlWQ=BP_lV_lWQ=BPl​Vl​W

Finally, before we can use the value of QQQ directly, we need to divide by the fourth (www) component. This step is sometimes called perspective division. This converts the position from a homogeneous value to a true Cartesian position, and is always required when using a perspective projection matrix.

Perspective division is automatically done by the OpenGL pipeline prior to rasterization. However, since we're working with a value that is not transformed by the pipeline, we need to perform the division manually.

In the following equation, we'll define a shadow matrix (SSS) that also includes the model matrix (MMM), so that we can convert directly from the object coordinates (CCC) (note that W=MCW = MCW=MC, because the model matrix takes object coordinates as world coordinates):

Q=SCQ=SCQ=SC

Here, SSS is the shadow matrix, the product of the model matrix with all of the preceding matrices:

S=BPlVlMS=BP_lV_lMS=BPl​Vl​M

In this recipe, in order to keep things simple and clear, we'll cover only the basic shadow mapping algorithm, without any of the usual improvements. We'll build upon this basic algorithm in the following recipes. Before we get into the code, we should note that the results will likely be less than satisfying. This is because the basic shadow mapping algorithm suffers from significant aliasing artifacts. Nevertheless, it is still an effective technique when combined with one of many techniques for anti-aliasing. We'll look at some of those techniques in the recipes that follow.

Getting ready

The position should be supplied in vertex attribute zero and the normal in vertex attribute one. Uniform variables for the ADS shading model should be declared and assigned, as well as uniforms for the standard transformation matrices. The ShadowMatrix variable should be set to the matrix for converting from object coordinates to shadow map coordinates (S in the preceding equation).
The uniform variable ShadowMap is a handle to the shadow map texture, and should be assigned to texture unit zero.

How to do it...

To create an OpenGL application that creates shadows using the shadow mapping technique, perform the following steps. We'll start by setting up a framebuffer object (FBO) to contain the shadow map texture, and then move on to the required shader code:

1. In the main OpenGL program, set up an FBO with a depth buffer only. Declare a GLuint variable named shadowFBO to store the handle to this framebuffer. The depth buffer storage should be a texture object. You can use something similar to the following code to accomplish this:

    GLfloat border[]={1.0f,0.0f,0.0f,0.0f}; GLuint depthTex;
    glGenTextures(1,&depthTex);
    glBindTexture(GL_TEXTURE_2D,depthTex);
    glTexStorage2D(GL_TEXTURE_2D, 1, GL_DEPTH_COMPONENT24, shadowMapWidth, shadowMapHeight);
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S, GL_CLAMP_TO_BORDER);
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T, GL_CLAMP_TO_BORDER);
    glTexParameterfv(GL_TEXTURE_2D,GL_TEXTURE_BORDER_COLOR, border);
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_COMPARE_MODE, GL_COMPARE_REF_TO_TEXTURE);
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_COMPARE_FUNC, GL_LESS); glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D,depthTex); glGenFramebuffers(1,&shadowFBO);
    glBindFramebuffer(GL_FRAMEBUFFER,shadowFBO);
    glFramebufferTexture2D(GL_FRAMEBUFFER,GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D,depthTex,0);
    GLenum drawBuffers[]={GL_NONE};
    glDrawBuffers(1,drawBuffers); glBindFramebuffer(GL_FRAMEBUFFER,0);

2. Use the following code for the vertex shader:

    layout (location=0) in vec3 VertexPosition; layout (location=1) in vec3 VertexNormal; out vec3 Normal; out vec3 Position; vec4 ShadowCoord; uniform mat4 ModelViewMatrix; uniform mat3 NormalMatrix; uniform mat4 MVP; uniform mat4 ShadowMatrix; void main() { Position = (ModelViewMatrix * vec4(VertexPosition,1.0)).xyz; Normal = normalize( NormalMatrix * VertexNormal ); ShadowCoord =ShadowMatrix * vec4(VertexPosition,1.0); gl_Position = MVP * vec4(VertexPosition,1.0);
    }

3. Use the following code for the fragment shader:

    uniform sampler2DShadow ShadowMap; in vec3 Position; in vec3 Normal; in vec4 ShadowCoord; layout (location = 0) out vec4 FragColor; vec3 diffAndSpec() { } subroutine void RenderPassType(); subroutine uniform RenderPassType RenderPass; subroutine (RenderPassType) void shadeWithShadow() { vec3 ambient = ...; vec3 diffSpec = diffAndSpec(); float shadow = textureProj(ShadowMap, ShadowCoord); FragColor = vec4(diffSpec * shadow + ambient, 1.0);
    } subroutine (RenderPassType) void recordDepth() { } void main() { RenderPass();
    }

Within the main OpenGL program, perform the following steps when rendering. For pass one:

1. Set the viewport, view, and projection matrices to those that are appropriate for the light source.
2. Bind to the framebuffer containing the shadow map (shadowFBO).
3. Clear the depth buffer.
4. Select the subroutine recordDepth function.
5. Enable front-face culling.
6. Draw the scene.

For pass two:

1. Select the viewport, view, and projection matrices appropriate for the scene.
2. Bind to the default framebuffer.
3. Disable culling (or switch to back-face culling).
4. Select the subroutine function shadeWithShadow function.
5. Draw the scene.

How it works...

The first block of the preceding code demonstrates how to create an FBO for our shadow map texture. The FBO contains only a single texture connected to its depth buffer attachment. The first few lines of code create the shadow map texture. The texture is allocated using the glTexStorage2D function with an internal format of GL_DEPTH_COMPONENT24.

We use GL_NEAREST for GL_TEXTURE_MAG_FILTER and GL_TEXTURE_MIN_FILTER here, although GL_LINEAR could also be used, and might provide slightly better-looking results. We use GL_NEAREST here so that we can see the aliasing artifacts clearly, and the performance will be slightly better.

Next, the GL_TEXTURE_WRAP_* modes are set to GL_CLAMP_TO_BORDER. When a fragment is found to lie completely outside of the shadow map (outside of the light's frustum), then the texture coordinates for that fragment will be greater than one or less than zero. When that happens, we need to make sure that those points are not treated as being in shadow.

When GL_CLAMP_TO_BORDER is used, the value that is returned from a texture lookup (for coordinates outside the 0–1 range) will be the border value. The default border value is
(0,0,0,0). When the texture contains depth components, the first component is treated as the depth value. A value of zero will not work for us here because a depth of zero corresponds to points on the near plane. Therefore, all points outside of the light's frustum will be treated as being in shadow! Instead, we set the border color to (1,0,0,0) using the glTexParameterfv function, which corresponds to the maximum possible depth.

The next two calls to glTexParameteri affect settings that are specific to depth textures. The first call sets GL_TEXTURE_COMPARE_MODE to GL_COMPARE_REF_TO_TEXTURE. When this setting is enabled, the result of a texture access is the result of a comparison, rather than a color value retrieved from the texture. The third component of the texture coordinate (the p component) is compared against the value in the texture at location (s,t). The result of the comparison is returned as a single floating-point value. The comparison function that is used is determined by the value of GL_TEXTURE_COMPARE_FUNC, which is set on the next line. In this case, we set it to GL_LESS, which means that the result will be 1.0 if the p value of the texture coordinate is less than the value stored at (s,t). (Other options include GL_LEQUAL, GL_ALWAYS, GL_GEQUAL, and so on.)

The next few lines create and set up the FBO. The shadow map texture is attached to the FBO as the depth attachment with the glFramebufferTexture2D function. For more details about FBOs, check out the Rendering to a texture recipe in Chapter 5, Using Textures.

The vertex shader is fairly simple. It converts the vertex position and normal-to-camera coordinates and passes them along to the fragment shader via the output variables Position and Normal. The vertex position is also converted into shadow map coordinates using ShadowMatrix. This is the matrix S that we referred to in the previous section. It converts a position from object coordinates to shadow coordinates. The result is sent to the fragment shader via the ShadowCoord output variable.

As usual, the position is also converted to clip coordinates and assigned to the built-in gl_Position output variable.

In the fragment shader, we provide different functionality for each pass. In the main function, we call RenderPass, which is a subroutine uniform that will call either recordDepth or shadeWithShadow. For the first pass (shadow map generation), the recordDepth subroutine function is executed. This function does nothing at all! This is because we only need to write the depth to the depth buffer. OpenGL will do this automatically (assuming that gl_Position was set correctly by the vertex shader), so there is nothing for the fragment shader to do.

During the second pass, the shadeWithShadow function is executed. We compute the ambient component of the shading model and store the result in the ambient variable. We then compute the diffuse and specular components and store those in the diffuseAndSpec variable.

The next step is the key to the shadow mapping algorithm. We use the built-in textureProj texture access function to access the ShadowMap shadow map texture. Before using the texture coordinate to access the texture, the textureProj function will divide the first three components of the texture coordinate by the fourth component. Remember that this is exactly what is needed to convert the homogeneous position (ShadowCoord) to a true Cartesian position.

After this perspective division, the textureProj function will use the result to access the texture. As this texture's sampler type is sampler2DShadow, it is treated as a texture containing depth values, and rather than returning a value from the texture, it returns the result of a comparison. The first two components of ShadowCoord are used to access a depth value within the texture. That value is then compared against the value of the third component of ShadowCoord.

We need to use vec3 as the lookup coordinate when using a sampler of type sampler2DShadow, because we need a 2D position and a depth.

When GL_NEAREST is the interpolation mode (as it is in our case) the result will be 1.0, or 0.0. As we set the comparison function to GL_LESS, this will return 1.0, but only if the value of the third component of ShadowCoord is less than the value within the depth texture at the sampled location. This result is then stored in the shadow variable. Finally, we assign a value to the output variable FragColor. The result of the shadow map comparison (shadow) is multiplied by the diffuse and specular components, and the result is added to the ambient component. If shadow is 0.0, that means that the comparison failed, meaning that there is something between the fragment and the light source. Therefore, the fragment is only shaded with ambient light. Otherwise, shadow is 1.0, and the fragment is shaded with all three shading components.

When rendering the shadow map, note that we culled the front faces. This is to avoid the z-fighting that can occur when front faces are included in the shadow map. Note that this only works if our mesh is completely closed. If back faces are exposed, you may need to use another technique (that uses glPolygonOffset) to avoid this. I'll talk a bit more about this in the next section.

There's more...

There's a number of challenging issues with the shadow mapping technique. Let's look at just a few of the most immediate ones.

Aliasing

As mentioned earlier, this algorithm often suffers from severe aliasing artifacts at the shadow's edges. This is due to the fact that the shadow map is essentially projected onto the scene when the depth comparison is made. If the projection causes the map to be magnified, aliasing artifacts appear.

The following image shows the aliasing of the shadow's edges:

The easiest solution is to simply increase the size of the shadow map. However, that may not be possible due to memory, CPU speed, or other constraints. There is a large number of techniques for improving the quality of the shadows produced by the shadow mapping algorithm such as resolution-matched shadow maps, cascaded shadow maps, variance shadow maps, perspective shadow maps, and many others. In the following recipes, we'll look at some ways to help you soften and anti-alias the edges of the shadows.

Rendering back faces only for the shadow map

When creating the shadow map, we only rendered back faces. This is because if we were to render front faces, points on certain faces would have nearly the same depth as the shadow map's depth, which can cause fluctuations between light and shadow across faces that should be completely lit. The following image shows an example of this effect:

Since the majority of faces that cause this issue are those that are facing the light source, we avoid much of the problem by only rendering back faces during the shadow map pass. This, of course, will only work correctly if your meshes are completely closed. If that is not the case, glPolygonOffset can be used to help the situation by offsetting the depth of the geometry from that in the shadow map. In fact, even when back faces are only rendered when generating the shadow map, similar artifacts can appear on faces that are facing away from the light (back faces in the shadow map, but the front from the camera's perspective). Therefore, it is quite often the case that a combination of front-face culling and glPolygonOffset is used when generating the shadow map.

Anti-aliasing shadow edges with PCF

One of the simplest and most common techniques for dealing with the aliasing of shadow edges is called percentage-closer filtering (PCF). The name comes from the concept of sampling the area around the fragment and determining the percentage of the area that is closer to the light source (in shadow). The percentage is then used to scale the amount of shading (diffuse and specular) that the fragment receives. The overall effect is a blurring of the shadow's edges.

The basic technique was first published by Reeves et al. in a 1987 paper (SIGGRAPH Proceedings, Volume 21, Number 4, July 1987). The concept involves transforming the fragment's extents into shadow space, sampling several locations within that region, and computing the percent that is closer than the depth of the fragment. The result is then used to attenuate the shading. If the size of this filter region is increased, it can have the effect of blurring the shadow's edges.

A common variant of the PCF algorithm involves just sampling a constant number of nearby texels within the shadow map. The percent of those texels that are closer to the light is used to attenuate the shading. This has the effect of blurring the shadow's edges. While the result may not be physically accurate, the result is not objectionable to the eye. The following images show shadows rendered with PCF (right) and without PCF (left). Note that the shadows in the right-hand image have fuzzier edges and the aliasing is less visible:

In this recipe, we'll use the latter technique, and sample a constant number of texels around the fragment's position in the shadow map. We'll calculate an average of the resulting comparisons and use that result to scale the diffuse and specular components.

We'll make use of OpenGL's built-in support for PCF by using linear filtering on the depth texture. When linear filtering is used with this kind of texture, the hardware can automatically sample four nearby texels (execute four depth comparisons) and average the results (the details of this are implementation dependent). Therefore, when linear filtering is enabled, the result of the textureProj function can be somewhere between 0.0 and 1.0.

We'll also make use of the built-in functions for texture accesses with offsets. OpenGL provides the textureProjOffset texture access function, which has a third parameter (the offset) that is added to the texel coordinates before the lookup/comparison.

Getting ready

Start with the shaders and FBO presented in the previous Rendering shadows with shadow maps recipe. We'll just make a few minor changes to the code presented there.

How to do it...

To add the PCF technique to the shadow mapping algorithm, use the following steps:

1. When setting up the FBO for the shadow map, make sure to use linear filtering on the depth texture. Replace the corresponding lines with the following code:

    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);

2. Use the following code for the shadeWithShadow function within the fragment shader:

    subroutine (RenderPassType) void shadeWithShadow() { vec3 ambient = vec3(0.2); vec3 diffSpec = diffAndSpec(); float sum = 0; sum += textureProjOffset(ShadowMap, ShadowCoord, ivec2(-1,-1)); sum += textureProjOffset(ShadowMap, ShadowCoord, ivec2(-1,1)); sum += textureProjOffset(ShadowMap, ShadowCoord, ivec2(1,1)); sum += textureProjOffset(ShadowMap, ShadowCoord, ivec2(1,-1)); float shadow = sum * 0.25; FragColor = vec4(ambient + diffSpec * shadow,1.0);
    }

How it works...

The first step enables linear filtering on the shadow map texture. When this is enabled, the
OpenGL driver can repeat the depth comparison on the four nearby texels within the texture. The results of the four comparisons will be averaged and returned.

Within the fragment shader, we use the textureProjOffset function to sample the four texels (diagonally) surrounding the texel nearest to ShadowCoord. The third argument is the offset. It is added to the texel's coordinates (not the texture coordinates) before the lookup takes place.

As linear filtering is enabled, each lookup will sample an additional four texels, for a total of 16 texels. The results are then averaged together and stored within the variable shadow.

As before, the value of shadow is used to attenuate the diffuse and specular components of the lighting model.

There's more...

An excellent survey of the PCF technique was written by Fabio Pellacini of Pixar, and can be found in Chapter 11, Shadow Map Anti-aliasing, of GPU Gems, edited by Randima Fernando, Addison-Wesley Professional, 2004. If more details are desired, I highly recommend reading this short, but informative, chapter.

Because of its simplicity and efficiency, the PCF technique is an extremely common method for anti-aliasing the edges of shadows produced by shadow mapping. Since it has the effect of blurring the edges, it can also be used to simulate soft shadows. However, the number of samples must be increased with the size of the blurred edge (the penumbra) to avoid certain artifacts. This can, of course, be a computational roadblock. In the next recipe, we'll look at a technique for producing soft shadows by randomly sampling a larger region.

A penumbra is the region of a shadow where only a portion of the light source is obscured.

Creating soft shadow edges with random sampling

The basic shadow mapping algorithm combined with PCF can produce shadows with soft edges. However, if we desire blurred edges that are substantially wide (to approximate true soft shadows), then a large number of samples is required. Additionally, there is a good deal of wasted effort when shading fragments that are in the center of large shadows, or completely outside of the shadow. For those fragments, all of the nearby shadow map texels will evaluate to the same value. Therefore, the work of accessing and averaging those texels is essentially a wasted effort.

The technique presented in this recipe is based on a chapter published in GPU Gems 2, edited by Matt Pharr and Randima Fernando, Addison-Wesley Professional, 2005. (Chapter 17 by Yury Uralsky.) It provides an approach that can address both of the preceding issues to create shadows with soft edges of various widths, while avoiding unnecessary texture accesses in areas inside and outside of the shadow.

The basic idea is as follows:

• Instead of sampling texels around the fragment's position (in shadow map space) using a constant set of offsets, we use a random, circular pattern of offsets

• In addition, we sample only the outer edges of the circle first in order to determine whether or not the fragment is in an area that is completely inside or outside of the shadow

The following diagram is a visualization of a possible set of shadow map samples. The center of the crosshairs is the fragment's location in the shadow map, and each x is a sample. The samples are distributed randomly within a circular grid around the fragment's location (one sample per grid cell):

Additionally, we vary the sample locations through a set of precomputed sample patterns. We compute random sample offsets and store them in a texture prior to rendering. Then, in the fragment shader, the samples are determined by first accessing the offset texture to grab a set of offsets and using them to vary the fragment's position in the shadow map. The results are then averaged together in a similar manner to the basic PCF algorithm. The following diagram shows the difference between shadows using the PCF algorithm (left), and the random sampling technique described in this recipe (right):

We'll store the offsets in a three-dimensional texture (n x n x d). The first two dimensions are of arbitrary size, and the third dimension contains the offsets. Each (s,t) location contains a list (size d) of random offsets packed into an RGBA color. Each RGBA color in the texture contains two 2D offsets. The R and G channels contain the first offset, and the B and A channels contain the second.
Therefore, each (s,t) location contains a total of 2*d offsets.

For example, location (1, 1, 3) contains the sixth and seventh offset at location (1,1). The entire set of values at a given (s,t) comprise a full set of offsets. We'll rotate through the texture based on the fragment's screen coordinates. The location within the offset texture will be determined by taking the remainder of the screen coordinates divided by the texture's size. For example, if the fragment's coordinates are (10.0,10.0) and the texture's size is (4,4), then we use the set of offsets located in the offset texture at location (2,2).

Getting ready

Start with the code presented in the Rendering shadows with shadow maps recipe. There are three additional uniforms that need to be set. They are as follows:

• OffsetTexSize: This gives the width, height, and depth of the offset texture.
• Note that the depth is same as the number of samples per fragment divided by two.
• OffsetTex: This is a handle to the texture unit containing the offset texture.
• Radius: This is the blur radius in pixels divided by the size of the shadow map texture (assuming a square shadow map). This could be considered as the softness of the shadow.