Shadow Volume - Stencil Buffer Implementations

Stencil Buffer Implementations

After Crow, Tim Heidmann showed in 1991 how to use the stencil buffer to render shadows with shadow volumes quickly enough for use in real time applications. There are three common variations to this technique, depth pass, depth fail, and exclusive-or, but all of them use the same process:

1. Render the scene as if it were completely in shadow.
2. For each light source:
   1. Using the depth information from that scene, construct a mask in the stencil buffer that has holes only where the visible surface is not in shadow.
   2. Render the scene again as if it were completely lit, using the stencil buffer to mask the shadowed areas. Use additive blending to add this render to the scene.

The difference between these three methods occurs in the generation of the mask in the second step. Some involve two passes, and some only one; some require less precision in the stencil buffer.

Shadow volumes tend to cover large portions of the visible scene, and as a result consume valuable rasterization time (fill time) on 3D graphics hardware. This problem is compounded by the complexity of the shadow casting objects, as each object can cast its own shadow volume of any potential size onscreen. See optimization below for a discussion of techniques used to combat the fill time problem.