I wonder why PDI was better than Renderman ?

“Better” is a loaded word. When you write your own renderer, you have a lot more control and a lot more support. Renderers at the time had not all converged on the same algorithms, so you could have a competitive advantage by writing a good one. PDI’s shader system was different than RenderMan’s and had some real advantages — compiled shaders, geometry & light shaders (not just materials), particles & particle shadowing, compositor integration, etc.. The renderer also needs to integrate into the pipeline deeply, so PDI’s renderer fits the PDI/DW pipeline better, whereas with RenderMan you have to try to mold the pipeline to fit the renderer in.

It wasn’t the renderer as much as the pipeline. The renderer is just one process in a stunning amount of complexity that must be managed. We may even agree that Renderman was better than the PDI renderer but there was so much much more.