For my procedural shading project, I decided to depart from the typical realistic rendering and tackle cartoon shading. I used renderman to do this. Here is my first image:

The two main features of this image are the hard-edged boundary between the illuminated and shadowed portions of the sphere, and the inked sillhouette edge. In cel animation these are created with paint and ink.

For my project I tried to produce a simple but general and pleasing implementation of cel ink and paint using renderman. I also wanted to combine this "2D" looking style with raytraced effects such as reflections in order to render more interesting cartoon scenes.

One problem that I ran into remains unsolved in my implementation. As you'll see in my final images, the ink lines on objects are not of a uniform width. While this is not always undesirable, often we want uniform ink lines. The variation in width is caused by the variation in surface curvature of the surface. This is because we detect silhouettes using the cosine of the angle between the viewing direction and surface normal. I made several attempts to compensate for this, including using renderman's derivative functionality to actually compute the surface curvature along the direction of view. I was unable to accurately compute and use this to keep a constant ink width. With more time and trials, I think I could do it. I think I could also provide the ability to vary the line width with other factors, such as depth. The other problem I had was with BMRT's raytraced shadows: I couldn't get them to work.

In the images below, you will notice several features:

Cel paint incorporates diffuse, specular, and ambient illumination components. Note that unlit portions have ambient contributions, and there are specular highlights.

Raytraced reflections (I plan to add refraction, but I didn't because I didn't want it in these images). (Raytracing provided by BMRT)

Glossy reflections through distribution raytracing. (BMRT)

Here are my shaders