Geometric Attenuation Factor

There are many different ways that an incoming beam of light can interact with the surface locally....

The entire beam can simply reflect.

A portion of the out-going beam can be blocked.

A portion of the incoming beam can be blocked.
Cook called this self-shadowing.


In each case, the geometric configurations can be analyzed to compute the percentage of light that actually escapes from the surface. Blinn first did this analysis. The results are:

The geometric factor chooses the smallest amount of light that is lost as the local self-shadowing model.