How do I use a stencil buffer to render shadow volumes?
The key to this and other volumetric stencil buffer effects, is the interaction between the stencil buffer and the z-buffer. A scene with a shadow volume is rendered in three stages. First, the scene without the shadow is rendered as usual, using the z-buffer. Next, the shadow is marked out in the stencil buffer as follows. The front faces of the shadow volume are drawn using invisible polygons, with z-testing enabled but z-writes disabled and the stencil buffer incremented at every pixel passing the z-test. The back faces of the shadow volume are rendered similarly, but decrementing the stencil value instead. Now, consider a single pixel. Assuming the camera is not in the shadow volume there are four possibilities for the corresponding point in the scene. If the ray from the camera to the point does not intersect the shadow volume, then no shadow polygons will have been drawn there and the stencil buffer is still zero. Otherwise, if the point lies in front of the shadow volume the sha
