anisotropic fog, non-uniform fog banks
visible light beams
fur and feathers
and all other effects that either have no solid substance or are otherwise difficult to model geometrically. In principle, volume shaders can be used for any interaction with rays, including geometric models. It is possible to write volume shaders that act as sub-renderers in their domain of space, performing object and volume intersection tests like mental ray does. However, in practice volume shaders are used for effects like those in the above list; mental ray can handle solid object intersections much more efficiently.
KeywordsGeometric Object Local Volume Fire Volume Global Illumination Global Volume
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