Customizing the OpenGL Pipeline with Shaders

OpenGL 1.x used fixed functions to deliver a useful graphics pipeline to application developers. To configure the various stages of the pipeline shown in Figure 12-1, applications called OpenGL functions to tweak the calculations that were performed for each vertex and fragment. Complex algorithms required multiple rendering passes and dozens of function calls to configure the calculations that the programmer desired. Extensions offered new configuration options, but did not change the complex nature of OpenGL programming.

Figure 12-1  OpenGL fixed-function pipeline

Starting with OpenGL 2.0, some stages of the OpenGL pipeline can be replaced with shaders. A shader is a program written in a special shading language. This program is compiled by OpenGL and uploaded directly into the graphics hardware. Figure 12-2 shows where your applications can hook into the pipeline with shaders.

Figure 12-2  OpenGL shader pipeline

Shaders offer a considerable number of advantages to your application:

If your application uses the fixed-function pipeline, a critical task is to replace those tasks with shaders.

If you are new to shaders, OpenGL Shading Language, by Randi J. Rost, is an excellent guide for those looking to learn more about writing shaders and integrating them into your application. The rest of this chapter provides some boilerplate code, briefly describe the extensions that implement shaders, and discusses tools that Apple provides to assist you in writing shaders.

Shader Basics

OpenGL 2.0 offers vertex and fragment shaders, to take over the processing of those two stages of the graphics pipeline. These same capabilities are also offered by the ARB_shader_objects, ARB_vertex_shader and ARB_fragment_shaderextensions. Vertex shading is available on all hardware running OS X v10.5 or later. Fragment shading is available on all hardware running OS X v10.6 and the majority of hardware running OS X v10.5.

Creating a shader program is an expensive operation compared to other OpenGL state changes. Listing 12-1 presents a typical strategy to load, compile, and verify a shader program.

Listing 12-1  Loading a Shader

/** Initialization-time for shader **/
            GLuint shader, prog;
            GLchar *shaderText = “... shader text ...”;

            // Create ID for shader
           shader = glCreateShader(GL_VERTEX_SHADER);

           // Define shader text

           // Compile shader

           // Associate shader with program
           glAttachShader(prog, shader);

          // Link program
           // Validate program

           // Check the status of the compile/link
           glGetProgramiv(prog, GL_INFO_LOG_LENGTH, &logLen);
           if(logLen > 0)
               // Show any errors as appropriate
               glGetProgramInfoLog(prog, logLen, &logLen, log);
               fprintf(stderr, “Prog Info Log: %s\n”, log);

     // Retrieve all uniform locations that are determined during link phase
           for(i = 0; i < uniformCt; i++)
               uniformLoc[i] = glGetUniformLocation(prog, uniformName);

           // Retrieve all attrib locations that are determined during link phase
           for(i = 0; i < attribCt; i++)
               attribLoc[i] = glGetAttribLocation(prog, attribName);

    /** Render stage for shaders **/

This code loads the text source for a vertex shader, compiles it, and adds it to the program. A more complex example might also attach fragment and geometry shaders. The program is linked and validated for correctness. Finally, the program retrieves information about the inputs to the shader and stores then in its own arrays. When the application is ready to use the shader, it calls glUseProgram to make it the current shader.

For best performance, your application should create shaders when your application is initialized, and not inside the rendering loop. Inside your rendering loop, you can quickly switch in the appropriate shaders by calling glUseProgram. For best performance, use the vertex array object extension to also switch in the vertex pointers. See Vertex Array Object for more information.

Advanced Shading Extensions

In addition to the standard shader, some Macs offer additional shading extensions to reveal advanced hardware capabilities. Not all of these extensions are available on all hardware, so you need to assess whether the features of each extension are worth implementing in your application.

Transform Feedback

The EXT_transform_feedback extension is available on all hardware running OS X v10.5 or later. With the feedback extension, you can capture the results of the vertex shader into a buffer object, which can be used as an input to future commands. This is similar to the pixel buffer object technique described in Using Pixel Buffer Objects to Keep Data on the GPU, but more directly captures the results you desire.

GPU Shader 4

The EXT_gpu_shader4 extension extends the OpenGL shading language to offer new operations, including:

  • Full integer support.

  • Built-in shader variable to reference the current vertex.

  • Built-in shader variable to reference the current primitive. This makes it easier to use a shader to use the same static vertex data to render multiple primitives, using a shader and uniform variables to customize each instance of that primitive.

  • Unfiltered texture fetches using integer coordinates.

  • Querying the size of a texture within a shader.

  • Offset texture lookups.

  • Explicit gradient and LOD texture lookups.

  • Depth Cubemaps.

Geometry Shaders

The EXT_geometry_shader4 extension allows your create geometry shaders. A geometry shader accepts transformed vertices and can add or remove vertices before passing them down to the rasterizer. This allows the application to add or remove geometry based on the calculated values in the vertex. For example, given a triangle and its neighboring vertices, your application could emit additional vertices to better create a more accurate appearance of a curved surface.

Uniform Buffers

The EXT_bindable_uniform extension allows your application to allocate buffer objects and use them as the source for uniform data in your shaders. Instead of relying on a single block of uniform memory supplied by OpenGL, your application allocates buffer objects using the same API that it uses to implement vertex buffer objects (Vertex Buffers). Instead of making a function call for each uniform variable you want to change, you can swap all of the uniform data by binding to a different uniform buffer.