OpenGL Shading Language

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OpenGL Shading Language
Core in version 4.4
Core since version 2.0
ARB extension GL_ARB_shader_objects, GL_ARB_vertex_shader, GL_ARB_fragment_shader, GL_ARB_shading_language_100

The OpenGL Shading Language (GLSL) is the principle shading language for OpenGL. While there are several shading languages available for use in OpenGL, GLSL is the only one that is a part of the OpenGL core.

GLSL is a C-style language. The language has undergone a number of version changes, and it shares the deprecation model of OpenGL. The current version of GLSL is 4.40.

Compilation model

GLSL is quite unique among shading languages due to its compilation model. It's compilation model is more like the standard C paradigm. Compilation is overseen by a number of object types. Note that these do not follow the standard OpenGL Objects paradigm.

Terminology

Because of GLSL's unique compilation model, GLSL uses unique terminology.

According to GLSL's standard terminology, a shader is just a compiled set of strings for a particular programmable stage; it does not even need to have the complete code for that stage. A program is a fully linked program that covers multiple programmable stages.

For the sake of clarity, we will adjust this slightly. When the term shader is used, it will be synonymous with the GLSL concept of program. To refer to a GLSL shader, the term shader object will be used.

Language

GLSL is a lot like C/C++ in many ways. It supports most of the familiar structural components (for-loops, if-statements, etc). But it has some important language differences.

Standard library

The OpenGL Shading Language defines a number of standard functions. Some standard functions are specific to certain shader stages, while most are available in any stage. There is reference documentation for these functions available here.

Variable types

C has a number of basic types. GLSL uses some of these, but adds many more.

Type qualifiers

GLSL's uses a large number of qualifiers to specify where the values that various variables contain come from. Qualifiers also modify how those variables can be used.

Interface blocks

Certain variable definitions can be grouped into interface blocks. These can be used to make communication between different shader stages easier, or to allow storage for variables to come from a buffer object.

Predefined variables

The different shader stages have a number of predefined variables for them. These are provided by the system for various system-specific use.

Using GLSL shaders

Building shaders

Attributes and draw buffers

For the stages at the start and end of the pipeline (vertex and fragment, respectively), the initial input values and final output values do not come from or go to shader stages. The input values to a vertex shader come from vertex data specified in a vertex array object, pulled from vertex buffer objects during Vertex Rendering. The output values of a fragment shader are piped to particular buffers for the currently bound framebuffer; either the default framebuffer or a framebuffer object.

Because of this, there is a mapping layer for the program's inputs and outputs. The vertex shader's input names are mapped to attribute indices, while the fragment shader's output names are mapped to draw buffer indices. This mapping can be created before the program is linked. If it is not, or if the mapping does not cover all of the inputs and outputs, then the linker will automatically define what indices are mapped to which unmapped input or output names. This auto-generated mapping can be queried by the user after the program is linked.


Setting uniforms

Uniforms in GLSL a shader variables that are set from user code, but only are allowed to change between different glDraw*​ calls. Uniforms can be queried and set by the code external to a particular shader. Uniforms can be arranged into blocks, and the data storage for these blocks can come from buffer objects.

Setting samplers

Samplers are special types which must be defined as uniforms. They represent bound textures in the OpenGL context. They are set like integer, 1D uniform values.

Error Checking

This piece of code shows the process of loading a vertex and fragment shaders. Then it compiles them and also checks for errors. The idea here is to encourage newcomers to GLSL to always check for errors. It is in C++ but that doesn't matter.

Note that the process of loading and compiling shaders hasn't changed much over the different GL versions.

Full compile/link of a Vertex and Fragment Shader.

//Read our shaders into the appropriate buffers
std::string vertexSource = //Get source code for vertex shader.
std::string fragmentSource = //Get source code for fragment shader.
 
//Create an empty vertex shader handle
GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);
 
//Send the vertex shader source code to GL
//Note that std::string's .c_str is NULL character terminated.
const GLchar *source = (const GLchar *)vertexSource.c_str();
glShaderSource(vertexShader, 1, &source, 0);
 
//Compile the vertex shader
glCompileShader(vertexShader);
 
GLint isCompiled = 0;
glGetShaderiv(vertexShader, GL_COMPILE_STATUS, &isCompiled);
if(isCompiled == GL_FALSE)
{
        GLint maxLength = 0;
        glGetShaderiv(vertexShader, GL_INFO_LOG_LENGTH, &maxLength);
 
        //The maxLength includes the NULL character
        std::vector<GLchar> infoLog(maxLength);
        glGetShaderInfoLog(vertexShader, maxLength, &maxLength, &infoLog[0]);
 
        //We don't need the shader anymore.
        glDeleteShader(vertexShader);
 
        //Use the infoLog as you see fit.
 
        //In this simple program, we'll just leave
        return;
}
 
//Create an empty fragment shader handle
GLuint fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
 
//Send the fragment shader source code to GL
//Note that std::string's .c_str is NULL character terminated.
source = (const GLchar *)fragmentSource.c_str();
glShaderSource(fragmentShader, 1, &source, 0);
 
//Compile the fragment shader
glCompileShader(fragmentShader);
 
glGetShaderiv(fragmentShader, GL_COMPILE_STATUS, &isCompiled);
if(isCompiled == GL_FALSE)
{
        GLint maxLength = 0;
        glGetShaderiv(fragmentShader, GL_INFO_LOG_LENGTH, &maxLength);
 
        //The maxLength includes the NULL character
        std::vector<GLchar> infoLog(maxLength);
        glGetShaderInfoLog(fragmentShader, maxLength, &maxLength, &infoLog[0]);
 
        //We don't need the shader anymore.
        glDeleteShader(fragmentShader);
        //Either of them. Don't leak shaders.
        glDeleteShader(vertexShader);
 
        //Use the infoLog as you see fit.
 
        //In this simple program, we'll just leave
        return;
}
 
//Vertex and fragment shaders are successfully compiled.
//Now time to link them together into a program.
//Get a program object.
GLuint program = glCreateProgram();
 
//Attach our shaders to our program
glAttachShader(program, vertexShader);
glAttachShader(program, fragmentShader);
 
//Link our program
glLinkProgram(program);
 
//Note the different functions here: glGetProgram* instead of glGetShader*.
GLint isLinked = 0;
glGetProgramiv(shaderprogram, GL_LINK_STATUS, (int *)&isLinked);
if(isLinked == GL_FALSE)
{
        GLint maxLength = 0;
        glGetProgramiv(program, GL_INFO_LOG_LENGTH, &maxLength);
 
        //The maxLength includes the NULL character
        std::vector<GLchar> infoLog(maxLength);
        glGetProgramInfoLog(program, maxLength, &maxLength, &infoLog[0]);
 
        //We don't need the program anymore.
        glDeleteProgram(program);
        //Don't leak shaders either.
        glDeleteShader(vertexShader);
        glDeleteShader(fragmentShader);
 
        //Use the infoLog as you see fit.
 
        //In this simple program, we'll just leave
        return;
}
 
//Always detach shaders after a successful link.
glDetachShader(program, vertexShader);
glDetachShader(program, fragmentShader);

See also

External links