Attenuation

### 6.6.6 Attenuation

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.