VERTEX SHADERS, PIXEL SHADERS, AND THE FIXED FUNCTION PIPELINE

So you might be wondering, how does all this stuff fit together? Well there are two cases to consider. If you're using a shader in conjunction with the FFP, then you'll have to consider what the FFP operations are. The FFP vertex operations are going to provide your pixel shader with two color registers: one diffuse, one specular. The FFP pixel shader is going to expect two color registers as input: one diffuse, one specular. (Note that using specular is a render state.) It gets interesting when you write your own vertex and pixel shader and ignore the FFP altogether.

Your vertex shader needs to provide a valid vertex position, so you'll need to perform the transformation step and provide a valid vertex position in the oPos register. Other than that, you've got the fog, point size, texture coordinates, and of course, the two color registers. (Note that the FFP pixel operations expect only one set of texture coordinates and the two color values.) Since your pixel shader operates on these values, you are free to stick any value into them you want (within the limits of the pixel shader precision). It's simply a way of passing data directly to the pixel shader from the vertex shader. However, you should be aware that texture coordinates will always be perspective correct interpolated from the vertex positions. The fog value will always be interpolated as well. The two color values will be interpolated only if the shading mode is Gouraud. The color interpolation in this case will also be perspective correct. Setting the shading mode to flat and placing data in the color registers is the preferred method of getting values unchanged to the pixel shaders.

Specular color is added by the pixel shader. There is no rendering state for specular when using pixel shaders. Fog, however, is still part of the FFP, and the fog blend is performed after the pixel shader executes.