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Light attenuation is used to model the decrease in light intensity as the light travels through a transparent
object. The keywords fade_power, fade_distance and fade_color are specified in
the interior statement.
The fade_distance value determines the distance the light has to travel to reach half intensity while
the fade_power value determines how fast the light will fall off. fade_color colorizes the
attenuation. For realistic effects a fade power of 1 to 2 should be used. Default values for fade_power
and fade_distance is 0.0 which turns this feature off. Default for fade_color is <0,0,0>,
if fade_color is <1,1,1> there is no attenuation. The actual colors give colored
attenuation. <1,0,0> looks red, not cyan as in media.
The attenuation is calculated by a formula similar to that used for light source attenuation.
If you set fade_power in the interior of an object at 1000 or above, a realistic exponential attenuation function
will be used:
Attenuation = exp(-depth/fade_dist)
The fade_power and fade_distance keywords were originally specified in finish
but are now properly specified in interior. They are accepted in finish for backward
compatibility and generate a warning message.
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