source: GTP/trunk/App/Demos/Illum/Standalone/HierRayEngine [DirectX]/README.txt @ 1481

GameTools standalone demo:

Ray tracing on the GPU [actual ray tracing, not approximate]
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Provided effect:
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Multiple visible refractive objects with multiple refractions, including
total internal reflection and refraction of exiting rays. The solution is
based on the hierarchical (tiled) version of the Ray Engine approach.

Algorithm benefits:
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Actual ray tracing allows accurate light simulation. The hierarchical ray engine
makes use of coherence to achive interactive performance. Therefore, it is
most suitable for rendering eye or photon paths: primarily visible refractive
objects or caustics (if augmented with photon splatting, not in this demo, but
featured in our 'approximate ray tracing' implementations. )

The algorithm is linear in the number of ray-tracing primitives. It is therefore
most useful if:
- a large number of coherent ray paths needs to be traced
- a low number of primitives are processed (low-poly models)
- non-triangle geometry (spheres, quadratics, etc.) is used

Compared to the depth approximate ray tracing solutions (a.k.a localized
environment maps, depth environment impostors) the accuracy is often
not necessary for a plausible result. Therefore, in a game, the ray
engine approach is only competitive in a few special cases matching the
conditions listed above.

Build:
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This is a standalone DirectX 9.0 application, compiles under Visual C++ 2003
with DirectX SDK April 2006 (identical to June 2006).

Usage notes:
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- Do not resize the application window. A fixed array of rays/tiles is used.
- use the 'Iterations' slider to toggle the length of ray paths traced. Ignore
the displayed number.