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<H1><A ID="SECTION00120000000000000000"> Atmospheric Effects</A> </H1>
<P> Any number of atmospheric effects may be associated with the default medium (``air'').
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Fog is simulated by blending the color of the fog with the color of
each ray. The amount of fog color blended into a ray color is an exponential
function of the distance from the ray origin to the point of intersection
divided by the specified <EM>thinness</EM> for each color channel.
If the distance is equal to <EM>thinness</EM>,
a ray's new color will be half of the fog color plus half its
original color.
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