I am extending the toon shaders from the toon shader package,
but the CG code only has a vertex program. I was wondering if anybody could tell exactly what happens in the fixed function fragment processor, as I assume this is being called in the abstance of any user defined fragment programs.
I was never fully on top of the communication between the vertex and fragment stages of the GPU pipeline in general, but iirc the basic function of the vertex shader is to take the vertices, figure out their color, uv & normal, and deliver to the fragment shader appropriately-interpolated values based on the position of the pixel being rendered within the tri. Whether there are any more vert shader outputs you can go looking for than the existing fragment shader takes in... is beyond the scope of my knowledge, but you may try poking around the ShaderLab manual.