Seeing Clint Hanson's presentation on the "Rendering Effects Used in 'Contact'" at Siggraph '98, insired me to conduct a series of experiments using the Ray Marching technique.
Here are my first attempts.
I greatly improved my shader for BMRT 2.5 (running on Linux). After some effort (space problems!) it now also works with PRman 3.9, and can still change from fluffy to
Here is the CODE
Reading about voronoi cel patters in Advanced Renderman, I just had to try that as a solid texture: Image 1, Image 2
This only works correctly on a sphere located at the origin object space. With BMRT, a ray tracer, I managed to get it to work on arbitrary geometry using rayHitTest()