Reference: "A Ray Tracing Algorithm for Progressive Radiosity", John R.
Wallace, Kells A. Elmquist, Eric A. Haines, **Computer Graphics 23**:3, pp.
315-324, July 1989 (SIGGRAPH 89).

This is a method to compute form factors using ray casting in the context of progressive radiosity using a sort and shoot algorithm. Instead of using a hemi-cube for patch i to compute all the form factors Fij, the algorithm shoots one or more rays from patch i to each patch j.

The original equation for computing a form factor was:

dF dA-1-

The variation of accuracy with source geometry. The configuration is shown at the upper left. The length of side b was varied with the other parameters kept contstant at a= 1.0 and r = 1.0. The distribution of samples on the source for 1, 4, and 16 points is shown at the top right. | |

Variation of accuracy with source-receiver orientation. The source and receiver lie in parallel palnes. Angle Q was varie, while a and r were kept constant at 1.0. The distribution of samples on the source for 1, 4, and 16 points is shown at the top right. |

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