Global illumination in volumes can be used to simulate indirect light interactions in volumic media, such as:
Global illumination in participating media is computed much the same way as global illumination on surfaces, except that volume shaders and volume photon shaders are needed.
To change the number of photons used to compute the local intensity of global illumination in volumes, specify a photonvol accuracy (and optionally a radius) in the options (similar to volume caustics).
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