Given some geometry plus its surface ("material") description and light specifications, how to compute shading on the surface? This is a KEY step in achieving photorealistic or alternative looks.

Slides - Part I

Slides - Part II

Here are links related to toon-shading, FYI (you could think of adding toon-shading support to your renderer):

And the foll. have to do with uv-mapping, ie. assigning texture coordinates to meshes: