"Renderman Compliant" means that everything it has to offer complies with the Renderman specification. It does not mean that it contains the full compliment of Renderman functionality. There are numerous free engines which are Renderman compliant, but that does not mean that they will render anything near the quality of the Renderman engine. I'm also leary of OpenGL - its great for fast previews and real-time activities, but the price of that speed often means living with some annoying artifacts.