PIXEL SHADERS: TECHNICAL OVERVIEW

Whereas vertex shaders replace the FFP in the traditional rendering pipeline, pixel shaders replace the pixel-blending section of the multitexture section of the pipeline. To understand how pixel shaders operate, you should be familiar with the dualistic nature of the texture pipeline. Traditionally, two paths are running simultaneously in the hardware—the color pipe (also called the vector pipe) and the alpha pipe (also called the scalar pipe). These two pathways handle the color and alpha operations of the texture processing unit. Frequently, you will have set up a mode so that the color operations are performed with one set of parameters, whereas the alpha operations are performed with a different set. At the end, the results are combined into a resulting rgba value. You can see this in Figure 4.3.

Figure 4.3: Pixel shader hardware can perform different operations simultaneously on a color vector and an alpha scalar.

Although the traditional texture pipeline allows you to specify a cascade of operations on sequential textures, pixel shaders allow you to specify these with more generality. However, the dual nature of the pipeline is still there. In fact, you'll probably be spending some time fine-tuning your pixel shaders so that you can get color and alpha operations to "pair," that is, to run simultaneously in the hardware.

Another dualistic nature of pixel shaders is they have two separate types of operations: arithmetic and texturing. Arithmetic operations are used to generate or modify color or texture coordinate values. The texture operations are operations to fetch texture coordinates or sample a texture using some texture coordinates. No matter the type of operations (coloring, texturing, or a blend), the output of the pixel shader is a single rgba value.