Andrew the Orchid <voi### [at] devnull> wrote:
> How does POV-Ray's radiosity system actually work? Does it actually sit
> there and perform forward ray calculations like photon mapping does? Or
> does it just modify the normal reverse ray calculations to shoot some
> extra says to try to simulate diffuse reflections?
>
> Andrew.


Hi Andrew,

    POV-Ray uses hemisphere sampling combined with irradiance caching for
its "radiosity" system. The total illuminance at a point (which is what you
see) equals the direct illuminance + the indirect illuminance at that
point.

The direct illumination is computed using backward (AKA Whitted) raytracing.
The indirect illumination is gathered by shooting many rays through the
hemisphere over that point, and averaging the total illuminances of where
those rays intersect with objects in the scene.

     As you may guess, this is a recursive process because the illuminance
gathered by each hemisphere ray consists of the total (direct+indirect)
illuminance for the point where it intersected. This happens recursively
until the recursion_limit is reached. When the recursion bottoms out, a
constant ambient term is used for the indirect illuminance.

     Alone this method would be unworkable due to the combinatorial
explosion of rays that would need to be traced. However, the irradiance
cache provides a way out of this dilemma. After an illuminance sample is
gathered, its location is recorded into a data structure called an octree.
This is the irradiance cache. The next time POV-Ray needs to gather
indirect illuminance data it first checks this cache. If there are enough
samples nearby that have already been gathered the hard way, it averages
those and avoids gathering altogether. As tracing the scene progresses, the
cache fills up with values and makes gathering the hard way less and less
necessary. This makes the algorithm workable and avoids that combinatorial
explosion of rays!

Also see "A Ray Tracing Solution to Diffuse Interreflection" by Ward,
Rubenstein, and Clear.

Hope this helps,
George

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