The medium which surrounds all objects in a scene is known as the atmosphere. This is normally a transparent material with a refractive index of 1.0. A procedural atmosphere can be specified by naming a volume shader that determines the attenuation of light as it travels along a ray of a given length through the atmosphere. As with all other types of shaders, a user-written shader can be used in place of the standard volume shader. This capability can be used, for example, to create procedurally defined fog.
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