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.