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Shadow volume 186
The depth fail method has the same considerations regarding the stencil buffer's precision as the depth pass method.
Also, similar to depth pass, it is sometimes referred to as the z-fail method.
William Bilodeau and Michael Songy discovered this technique in October 1998, and presented the technique at
Creativity, a Creative Labs developer's conference, in 1999. [2] Sim Dietrich presented this technique at a Creative
Labs developer's forum in 1999. [3] A few months later, William Bilodeau and Michael Songy filed a US patent
application for the technique the same year, US 6384822 [4] , entitled "Method for rendering shadows using a shadow
volume and a stencil buffer" issued in 2002. John Carmack of id Software independently discovered the algorithm in
2000 during the development of Doom 3. [5] Since he advertised the technique to the larger public, it is often known
as Carmack's Reverse.
Exclusive-or
Either of the above types may be approximated with an exclusive-or variation, which does not deal properly with
intersecting shadow volumes, but saves one rendering pass (if not fill time), and only requires a 1-bit stencil buffer.
The following steps are for the depth pass version:
1. Disable writes to the depth and color buffers.
2. Set the stencil operation to XOR on depth pass (flip on any shadow surface).
3. Render the shadow volumes.
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 [6] , 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 [7] 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. [8]
• 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.