OpenGL 4.0 Shading Language Cookbook

The OpenGL Shading Language (GLSL) is now a fundamental and integral part of the OpenGL API. Going forward, every program written using OpenGL will internally utilize one or several GLSL programs. These "mini-programs" written in GLSL are often referred to as shader programs , or simply shaders. A shader program is one that runs on the GPU, and as the name implies, it (typically) implements the algorithms related to the lighting and shading effects of a 3-dimensional image. However, shader programs are capable of doing much more than just implementing a shading algorithm. They are also capable of performing animation, tessellation, and even generalized computation.

Shader programs are designed to be executed directly on the GPU and often in parallel. For example, a fragment shader might be executed once for every pixel, with each execution running simultaneously on a separate GPU thread. 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.

The computing power available in modern graphics cards is impressive. The following table shows the number of shader processors available for several models in the NVIDIA GeForce 400 series cards (source: http://en.wikipedia.org/wiki/Comparison_of_Nvidia_graphics_processing_units).

Shader programs are intended to replace parts of the OpenGL architecture referred to as the fixed-function pipeline . The default lighting/shading algorithm was a core part of this fixed-function pipeline. When we, as programmers, wanted to implement more advanced or realistic effects, we used various tricks to force the fixed-function pipeline into being more flexible than it really was. The advent of GLSL helped by providing us with the ability to replace this "hard-coded" functionality with our own programs written in GLSL, thus giving us a great deal of additional flexibility and power. For more details on the programmable pipeline, see the introduction to Chapter 2.

In fact, recent (core) versions of OpenGL 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.

OpenGL version 3.0 introduced a deprecation model, which allowed for the gradual removal of functions from the OpenGL specification. Functions or features can now 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 deprecated in version 3.0 and removed in version 3.1.

In order to maintain backwards compatibility, the concept of compatibility profiles was introduced with OpenGL 3.2. A programmer who is writing code intended for a particular version of OpenGL (with older features removed) would use the so-called core profile . Someone who also wanted to maintain compatibility with older functionality could use the compatibility profile.

The steps for selecting a core or compatibility profile are window system API dependent. For example, in recent versions of Qt (at least version 4.7), one can select a 4.0 core profile using the following code:

QGLFormat format;
format.setVersion(4,0);
format.setProfile(QGLFormat::CoreProfile);
QGLWidget *myWidget = new QGLWidget(format);

All programs in this book are designed to be compatible with an OpenGL 4.0 core profile.

Download the GLEW distribution from http://glew.sourceforge.net. There are binaries available for Windows, but it is also a relatively simple matter to compile GLEW from source (see the instructions on the website: http://glew.sourceforge.net).

Place the header files glew.h and wglew.h from the GLEW distribution into a proper location for your compiler. If you are using Windows, copy the glew32.lib to the appropriate library directory for your compiler, and place the glew32.dll into a system-wide location, or the same directory as your program's executable. Full installation instructions for all operating systems and common compilers are available on the GLEW website.

To start using GLEW in your project, use the following steps:

That's all there is to it!

Including the glew.h header file provides declarations for the OpenGL functions as function pointers, so all function entry points are available at compile time. At run time, the glewInit() function will scan the OpenGL library, and initialize all available function pointers. If a function is not available, the code will compile, but the function pointer will not be initialized.

GLEW includes a few additional features and utilities that are quite useful.


if ( ! GLEW_ARB_vertex_program )
{
   fprintf(stderr, "ARB_vertex_program is missing!\n");
   ...
}

Another option for managing OpenGL extensions is the GLee library (GL Easy Extension). It is available from http://www.elf-stone.com/glee.php and is open source under the modified BSD license. It works in a similar manner to GLEW, but does not require runtime initialization.

Download the latest GLM distribution from http://glm.g-truc.net. Unzip the archive file, and copy the glm directory contained inside to anywhere in your compiler's include path.

Using the GLM libraries is simply a matter of including the core header file (highlighted in the following code snippet) and headers for any extensions. We'll include the matrix transform extension, and the transform2 extension.

#include <glm/glm.hpp>
#include <glm/gtc/matrix_transform.hpp>
#include <glm/gtx/transform2.hpp>

The GLM classes are then 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(0.0,0.0,5.0),
                              glm::vec3(0.0,0.0,0.0),
                              glm::vec3(0.0,1.0,0.0) );

glm::mat4 model = glm::mat4(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;

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 preceding example first creates a vec4 (four coordinate vector) representing a position. Then it creates a 4x4 view matrix by using the glm::lookAt function from the transform2 extension. This works in a similar fashion to the old gluLookAt function. In this example, we set the camera's location at (0,0,5), looking towards the origin, with the "up" direction in the direction of the Y-axis. We then go on to create the modeling matrix by first storing the identity matrix in the variable model (via the constructor: glm::mat4(1.0f)), and multiplying 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 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. The net result is 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 using the combined matrix to transform the position. Note that the multiplication operator has been overloaded to behave in the expected way.

As stated above, the GLM library conforms as closely as possible to the GLSL specification, with additional features that go beyond what you can do in GLSL. If you are familiar with GLSL, GLM should be easy and natural to use.

#define GLM_SWIZZLE
#include <glm/glm.hpp>

It is not recommended to import all of the GLM namespace using a command like:

using namespace glm;

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

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

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

...

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

The GLM website http://glm.g-truc.net has additional documentation and examples.

The code shown below 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);  

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 (see the Introduction to this chapter).

The queries for GL_VENDOR and GL_RENDERER provide additional information about the OpenGL driver. The call glGetString(GL_VENDOR) 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 "ATI Radeon HD 5600 Series"). Note that both of these do not vary from release to release, so 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.

It is often useful to query for the supported extensions of the current OpenGL implementation. In versions prior to OpenGL 3.0, one could retrieve a full, space separated list of extension names with the following code:

GLubyte *extensions = glGetString(GL_EXTENSIONS);

The string that is returned can be extremely long and parsing it can be susceptible to error if not done carefully.

In OpenGL 3.0, a new technique was introduced, and the above functionality was deprecated (and finally removed in 3.1). Extension names are now 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: glGetString(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 ) ); 

The GLEW library has additional support for querying extension information. See Using the GLEW library to access the latest OpenGL functionality.

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.

#version 400

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 input attributes VertexPosition and VertexColor and passes them along to the fragment shader via the output variables gl_Position and Color.

Next, we'll need to build a basic shell for an OpenGL program using any standard windowing toolkit. Examples of cross-platform toolkits include GLUT, FLTK, Qt, or 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 of 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.

Finally, we need to load the shader source code into a character array named shaderCode. Don't forget to add the null character at the end! This example assumes that the variable shaderCode points to an array of GLchar that is properly terminated by a null character.

To compile a shader, use the following steps:

The first step is to create the shader object using the function glCreateShader. 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, or GL_TESS_CONTROL_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 variable vertShader. 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 function glShaderSource. This function is designed to accept an array of strings in order to support the option of compiling multiple files 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 contains the length of each source code string in the previous argument. In this case, 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 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 or not 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 or not the compilation was successful.

If the compile status is GL_FALSE, then 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 variable logLen, including 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.

The technique for compiling a shader is nearly identical for each shader type. The only significant difference is the argument to glCreateShader.

Of course, shader compilation is only the first step. To create a working shader program, we often have at least two shaders to compile, 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.

The next recipe, Linking a shader program.

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

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

#version 400

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.

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

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 parameter GL_INFO_LOG_LENGTH. 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.

With the simple fragment shader from this recipe and the vertex shader from the preceding 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 (red, green, and blue) 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.

You can compile and link 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.

We'll start with a simple, empty OpenGL program, and the following shaders.

The vertex shader (basic.vert):

#version 400

in vec3 VertexPosition;
in vec3 VertexColor;

out vec3 Color;

void main()
{
    Color = VertexColor;

    gl_Position = vec4(VertexPosition,1.0);
}

Note that there are two input attributes in the vertex shader: VertexPosition and VertexColor . Our program needs to provide 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 attribute VertexPosition is simply expanded and passed along to the built-in output variable gl_Position for further processing.

The fragment shader (basic.frag):

#version 400

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 output variable FragColor (more about fragment shader output variables in later recipes).

Write code to compile and link these shaders into a shader program (see Compiling a Shader and Linking a Shader Program). In the following code, I'll assume that the handle to the shader program is programHandle.

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

Vertex attributes are the input variables to our vertex shader. In the vertex shader above, our two attributes are VertexPosition and VertexColor. Since we can give these variables any name we like, OpenGL provides a way to refer to vertex attributes in the OpenGL program 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 refer to the vertex attributes in our OpenGL code by referring to the corresponding generic vertex attribute index.

The first step above involves making connections between the shader input variables VertexPosition and VertexColor and the generic vertex attribute indexes 0 and 1 respectively, using the function glBindAttribLocation . If this is done within the OpenGL application, we have to do this before the program is linked.

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

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. For vertex attribute data, 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 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 argument. 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 at the OpenGL documentation (http://www.opengl.org/sdk/docs/man4/). In our case, the data specified once will not be modified, and will be used many times for drawing operations, so this usage pattern best corresponds to the value GL_STATIC_DRAW.

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 function glGenVertexArrays . This gives us a handle to our new object, which we store in the (global) variable vaoHandle. Then we enable the generic vertex attribute indexes 0 and 1 by calling glEnableVertexAttribArray. Doing so indicates 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.

// Map index 0 to the position buffer
glBindBuffer(GL_ARRAY_BUFFER, positionBufferHandle);
glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, 
                          (GLubyte *)NULL );

First we bind the buffer object to the GL_ARRAY_BUFFER binding point, and 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 3-dimensional data, so we want 3 components per vertex. The third argument is the data type of each component in the buffer. The fourth is a Boolean which specifies whether or not 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 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 prior to the first element, so we use a value of zero (NULL).

In the render function, it is simply a matter of clearing the color buffer using glClear, binding to the vertex array object, and calling glDrawArrays to draw our triangle. The function glDrawArrays initiates rendering of primitives by stepping through the buffers for each enabled attribute array, and passing the data down the pipeline to the 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 (3 vertexes for a single triangle).

To summarize, rendering with vertex buffer objects (VBOs) involves the following steps:

You may have noticed that I've neglected saying anything about the output variable FragColor in the fragment shader. This variable receives the final output color for each fragment (pixel). Like vertex input variables, this variable also needs to be associated with a location. Of course, we typically would 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 the following command prior to program linking:

glBindFragDataLocation(programHandle, 0, "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 shading (see Chapter 5).

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

layout (location = 0) out vec4 FragColor;

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.

After linking the shader program, use the following steps to print information about the active attributes in your shader program:

We start by querying for the number of active attributes by calling glGetProgramiv with the argument GL_ACTIVE_ATTRIBUTES. The result is stored in nAttribs. Next, we query for the length of the longest attribute name (GL_ACTIVE_ATTRIBUTE_MAX_LENGTH) and store the result in maxLength. This includes the null terminating character, so we use that value to allocate space to store each variable name.

Next, we loop over each index (0 to nAttrib - 1), and retrieve information about each attribute by calling glGetActiveAttrib and glGetAttribLocation. The function glGetActiveAttrib returns a bunch of information about the attribute at the index provided as the second argument. Note that this index is not necessarily the same as the generic vertex attribute index (location) for the variable. The function provides the attribute name, size, and type, which are stored in the variables name, size, and type. Once we have the variable name, we can query for its location (the generic attribute index), by calling glGetAttribLocation , and passing in the program handle and the variable name. We then print the variable's location and name to standard out.

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 above code will not display the variable because it is not considered active and will be effectively ignored by OpenGL.

We'll use the following vertex shader.

#version 400

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 the variable RotationMatrix is declared using the uniform qualifier. We'll provide the data for this variable from the OpenGL program. The RotationMatrix is used to transform VertexPosition before assigning it to the default output position variable gl_Position.

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

#version 400

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 (see: Using the GLM library for mathematics in this chapter). Within the main OpenGL code, add the following include statements:

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

#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.

Within the render method, use the following code.

glClear(GL_COLOR_BUFFER_BIT);

mat4 rotationMatrix = glm::rotate(mat4(1.0f), angle, 
                                  vec3(0.0f,0.0f,1.0f));

GLuint location =glGetUniformLocation(programHandle,
                                     "RotationMatrix");

if( location >= 0 )
{
    glUniformMatrix4fv(location, 1, GL_FALSE, 
                      &rotationMatrix[0][0]);
}

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

The steps involved with setting the value of a uniform variable include finding the location of the variable, and then 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.

We then assign a value to the uniform variable using glUniformMatrix4fv. The first argument is the uniform variable's location. The second is the number of matrices that are being assigned (the uniform variable could be an array). The third is a Boolean value indicating whether or not 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.

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 type vec3, one would use glUniform3f or glUniform3fv.

For arrays, we can use the functions ending in "v" to initialize multiple values within the array. Note that if it is desired, we 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 we will query in the following way:

 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.

We'll start with a basic OpenGL program that compiles and links a shader program. You could use the shaders from the recipe Sending data to a shader using per-vertex attributes and vertex buffer objects. In the following example, we'll assume that the handle to the program is in a variable named programHandle.

After linking the shader program, use the following steps to print information about the active uniform variables:

In step one above, we call the function glGetProgramiv to query for the maximum length of the uniform variable names (GL_ACTIVE_UNIFORM_MAX_LENGTH), and the number of active uniforms (GL_ACTIVE_UNIFORMS). The maximum length value includes the null terminating character, so in step 2 we allocate enough space to store a name of that length.

Next, we loop from zero to the number of uniforms minus one, and call glGetActiveUniform to retrieve information about each variable. Similar to glGetActiveAttrib, this function provides several pieces of information about the variable including its size, type, and name. We then query for the location of that uniform variable by calling glGetUniformLocation. It is quite often the case that the index used in the call to glGetActiveUniform is the same as the uniform's location, but we make the call just to be sure.

Finally, we print the name and location of the variable to standard out.

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.

Note that one could also use the function glGetActiveUniformName instead of glGetActiveUniform. The former only provides the name, while the latter also provides the size and type.

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 (see: Sending data to a shader using per-vertex attributes and vertex buffer objects).

We'll use the following vertex shader:

#version 400

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 400

in vec3 TexCoord;
layout (location = 0) out vec4 FragColor;

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 )
            );
}

The uniform block is named BlobSettings. The variables within this block define the parameters of our fuzzy circle. The variable OuterColor defines the color outside of the circle. InnerColor is the color inside of the circle. RadiusInner is the radius defining 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 it 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.

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

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 layout 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 variable names (third argument) and returns the indices of the variables in the array indices (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, 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 so-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.

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.

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

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

layout( std140 ) uniform BlobSettings {
   ...
};

layout( row_major, shared ) uniform BlobSettings {
   ...
};

There's not much to prepare for with this one, you just need to build an 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.

We'll use the following header file for our C++ class:

namespace GLSLShader {
    enum GLSLShaderType {
        VERTEX, FRAGMENT, GEOMETRY,TESS_CONTROL, 
        TESS_EVALUATION
    };
};

class GLSLProgram
{
private:
    int  handle;
    bool linked;
    string logString;
    int  getUniformLocation(const char * name );
    bool fileExists( const string & fileName );

public:
    GLSLProgram();

    bool   compileShaderFromFile( const char * fileName, 
  GLSLShader::GLSLShaderType type );
    bool   compileShaderFromString( const string & source, 
  GLSLShader::GLSLShaderType type );
    bool   link();
    void   use();

    string log();

    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 vec3 & v);
    void   setUniform(const char *name, const vec4 & v);
    void   setUniform(const char *name, const mat4 & m);
    void   setUniform(const char *name, const mat3 & m);
    void   setUniform(const char *name, float val );
    void   setUniform(const char *name, int val );
    void   setUniform(const char *name, bool val );

    void   printActiveUniforms();
    void   printActiveAttribs();
};

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 (it's available from the book's website), but we'll discuss some of the design decisions in the next section.

The state stored within a GLSLProgram object includes the handle to the OpenGL shader program object (handle), a Boolean variable indicating whether or not the program has been successfully linked (linked), and a string for storing the most recent log produced by a compile or link action (logString).

The two private functions are utilities used by other public functions. The getUniformLocation function is used by the setUniform functions to find the location of a uniform variable, and the fileExists function is used by compileShaderFromFile to check for file existence.

The constructor simply initializes linked to false, handle to zero, and logString to the empty string. The variable handle will be initialized by calling glCreateProgram when the first shader is compiled.

The compileShaderFromFile and compileShaderFromString functions attempt to compile a shader of the given type (the type is provided as the second argument). They create the shader object, load the source code, and then attempt to compile the shader. If successful, the shader object is attached to the OpenGL program object (by calling glAttachShader) and a value of true is returned. Otherwise, the log is retrieved and stored in logString, and a value of false is returned.

The link function simply attempts to link the program by calling glLinkProgram. It then checks the link status, and if successful, sets the variable linked to true and returns true. Otherwise, it gets the program log (by calling glGetProgramInfoLog), stores it in logString, and returns false.

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

The log function returns the contents of logString, which should contain the log of the most recent compile or link action.

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

The functions bindAttribLocation and bindFragDataLocation 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. Each of them calls getUniformLocation to query for the variable's location before calling the glUniform function.

Finally, the printActiveUniforms and printActiveAttribs functions are useful mainly for debugging purposes. They simply display a list of the active uniforms/attributes to standard output. The following is a simple example of the use of the GLSLProgram class:

GLSLProgram prog;

if( ! prog.compileShaderFromFile("myshader.vert",
                                 GLSLShader::VERTEX))
{
    printf("Vertex shader failed to compile!\n%s",
           prog.log().c_str());
    exit(1);
}
if( ! prog.compileShaderFromFile("myshader.frag",
                                  GLSLShader::FRAGMENT))
{
    printf("Fragment shader failed to compile!\n%s",
           prog.log().c_str());
    exit(1);
}

// Possibly call bindAttribLocation or bindFragDataLocation 
//  here...

if( ! prog.link() )
{
    printf("Shader program failed to link!\n%s",
           prog.log().c_str());
    exit(1);
}

prog.use();
prog.printActiveUniforms();
prog.printActiveAttribs();

prog.setUniform("ModelViewMatrix", matrix);
prog.setUniform("LightPosition", 1.0f, 1.0f, 1.0f);

...