Atmospheric Effects . .
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 thinness for each color channel. If the distance is equal to thinness , a ray's new color will be half of the fog color plus half its original color. Add global low-altitude mist of the specified color. The color of a ray is modulated by a fog with density that varies linearly with the difference in z coordinate between the ray origin and the point of intersection. The thinness values specify the transmissivity of the fog for each color channel.