We present a novel algorithm for the rendering of hard shadows cast by a point light source. The well-known Z-pass method for rasterizing shadow volumes is not always correct. Our algorithm, which we call ZP+, elegantly corrects Z-pass defects. ZP+ takes advantage of triangle strips and fast culling capabilities of graphics hardware not available to conventional robust methods like Z-fail. While Z-fail can be up to 80% slower than Z-pass, our new method ZP+ is typically less than 10% slower than Z-pass. Finally, we give a comparison of the three methods. When a scene is geometry-bound, ZP+ is always faster than Z-fail. We also explain why, in some situations, Z-pass (hence ZP+) is surprisingly slower than Z-fail on more recent graphics hardware.

@inproceedings{HHLH05,
author = {Samuel Hornus and Jared Hoberock and Sylvain Lefebvre and John C. Hart},
title = {{ZP+}: correct {Z}-pass stencil shadows},
booktitle = {ACM Symposium on Interactive 3D Graphics and Games, I3D, April, 2005},
organization = {ACM},
address = {Washington, DC, Etats-Unis},
publisher = {ACM Press},
pages = {195--202},
year = 2005
}