global illumination is a way to simulate realistic outdoor lighting, its usually very heavy on the cpu mental ray can render proper GI, GI_Joe fakes GI because maya atm can not actually do global illumination so something is needed to give the EFFECT mental ray is well known for its ray tracing ie. reflections, refractions, GI etc renderman is amazing for displacement/bump maps and is so bloody fast, its pretty much industry standard (the file format it uses, which is .rib) GI_Joe isnt actually a renderer, its just a way to simulate a light setup close to what global illumination is actually like all those compared to mayas native: mental ray, much faster, but cant render things like fur renderman, very very fast, quality of image is a lot better but requires a lot of setup and you need to know how to program shaders pretty well to get a nice result GI_Joe, nothing to compare, you cant render with it, you setup lights with it, then render using a renderer just to clarify, from the list, the only renderers are: MentalRay PRman/Renderman Maya hope that helps