ZP+: correct Z-pass stencil shadows

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       = "Hornus, Samuel and Hoberock, Jared and Lefebvre, Sylvain and Hart, John C.",
  title        = "ZP+: correct Z-pass stencil shadows",
  booktitle    = "ACM Symposium on Interactive 3D Graphics and Games",
  month        = "April",
  year         = "2005",
  publisher    = "ACM Press",
  organization = "ACM",
  url          = "http://artis.inrialpes.fr/Publications/2005/HHLH05"
}