The "soft_fog" box can be used to define different densities and colors of fog. Both the densities and colors can be modified prodecually. This is the SoftImage implimentation of a Mental Ray 'volume shader':
The rendered image looks like:
Volumetric Shaders are used to generate an "atmosphere" inside a
prescribed volume, such as fire or steam or jet exhaust. They are
are assigned on a per-object basis. In ShadeTree you must
have the Shader Type set to volume in the
Render Options Editor.
Volumetric shaders are still in development at this time.
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