RENDERMAN VS. REAL TIME

RenderMan is a continually evolving product, but it's never been one that could be considered real time. A good reason for this is the flexibility needed by the artists. In 1989, there was no hardware that could render a complex ray-traced scene in real time. You can think of ray tracing as tracing each pixel through a scene—through transparent and translucent objects—back to the location where the pixel is saturated. This works nicely for scenes that have lots of complex behavior, such as scenes with reflecting surfaces, water, shadows, glass, smoke, etc. And while that is the direction 3D consumer graphics is heading, we're not quite there yet.

In the consumer 3D graphics arena, we're dealing with objects that reflect light, and we can deal with objects that are some degree of transparent. But there's no support for the effect of other objects in a scene. There's support for the concept of "lights," but a "light" in OpenGL or Direct3D is just a source of light, not an object itself. An object only "knows" about the lights in a scene, but not if it's occluded from these lights by anything else in a scene.