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If you happened to go to GDC this year, and then happened to attend Mark Kilgard's session on Advanced Hardware Rendering, you probably saw his shadow casting demo. That demo and a very helpful powerpoint presentation are available from nVidia's developer web pages. The demo here and Mark's demo are based on a shadow mapping technique useful for consumer hardware described by Wolfgang Heidrich in his PhD thesis, which has a lot of other interesting material.
Between Mark's presentation and Wolfgang's thesis, there's more than enough description of how this technique works, so I will refer you to that material for how to implement this technique. Instead I'll show a couple of interesting effects that are possible using this technique. This demo looks a lot like Mark's, but it is a completely independent implementation in C++ that allows for a slightly more compact representation of the algorithm.
The interesting effects are atmospheric effects and using a "slide projector"
for the light source. Here are a couple of pictures illustrating
The neat thing about these effects is that you get behavior that you expect. That is, the atmospheric effects and the projective image are both occluded by the objects in the scene. The slide projector effect is just a straightforward combination of the shadow mapping technique with a projective texture. The only wrinkle being that two texture units are used for the shadow determination, so the color buffer must be masked and the lit region is written to the stencil buffer. In a subsequent pass, the projected image is applied only in the lit region. The final pass using the illuminated region sets the stencil back to zero eliminating the need for a clear.
It is handy that the stencil can be unset in the final pass, because this is useful for the atmospheric effect. For this effect, we turn off depth buffer writes and enable blending for the entire process. Then we render a sequence of planes as follows:
One final note: this technique requires GL_EXT_texture_env_combine, but that is covered in the other documentation.
Check out the source for the definitive
description of what is happening. Below are some other rendering
modes that this demo can switch among.
|Slide projector with shadows but no atmospheric effects||range to light source from camera view||slide projector depth map projected onto scene|
|atmospheric effects, but without shadows or occlusion of atmospheric effects||view from the slide projector||depth from the slide projector|
Also, it should be pointed out that this demo makes no effort to eliminate the aliasing artifacts due to the 8-bit precision of the depth comparison. There are some simple ways to improve this significantly.