Shadow Volume - Optimization

Optimization

• One method of speeding up the shadow volume geometry calculations is to utilize existing parts of the rendering pipeline to do some of the calculation. For instance, by using homogeneous coordinates, the w-coordinate may be set to zero to extend a point to infinity. This should be accompanied by a viewing frustum that has a far clipping plane that extends to infinity in order to accommodate those points, accomplished by using a specialized projection matrix. This technique reduces the accuracy of the depth buffer slightly, but the difference is usually negligible. Please see 2002 paper Practical and Robust Stenciled Shadow Volumes for Hardware-Accelerated Rendering, C. Everitt and M. Kilgard, for a detailed implementation.

• Rasterization time of the shadow volumes can be reduced by using an in-hardware scissor test to limit the shadows to a specific onscreen rectangle.

• NVIDIA has implemented a hardware capability called the depth bounds test that is designed to remove parts of shadow volumes that do not affect the visible scene. (This has been available since the GeForce FX 5900 model.) A discussion of this capability and its use with shadow volumes was presented at the Game Developers Conference in 2005.

• Since the depth-fail method only offers an advantage over depth-pass in the special case where the eye is within a shadow volume, it is preferable to check for this case, and use depth-pass wherever possible. This avoids both the unnecessary back-capping (and the associated rasterization) for cases where depth-fail is unnecessary, as well as the problem of appropriately front-capping for special cases of depth-pass.

• On more recent GPU pipelines, geometry shaders can be used to generate the shadow volumes.