Multipass 3D Compositing

Some artists, including a majority of those who work predominantly in 3D, labor under the delusion that you should finalize the look of a computer-generated element in one pass. Certainly, as it becomes more and more possible to adjust the look of a 3D model in real time (via the GPU, OpenGL) this becomes tempting.

However, it’s possible to do better by dividing the render of a single element into multiple passes. This is different from rendering in layers, which while also useful for compositing is really only about separating foreground elements from the background. Multipass rendering is the technique of isolating individual surface qualities and creating a separate render for each. By surface qualities I mean things like specularity and wear and tear, also known as grunge. In his excellent book Digital Lighting & Rendering, Second Edition (Peachpit Press, 2006), Jeremy Birn calls out multiple benefits yielded by rendering a model on multiple passes: