Adobe After Effects 5.0_ Effects, part 2
This Key creates transparency from opposite starting points by dividing an image into two mattes, Matte
Partial A and Matte Partial B. Matte Partial B bases the transparency on the specified key color, and Matte
Partial A bases transparency on areas of the image that do not contain a second, different color. By
combining the two mattes into a third matte, called the alpha( ) matte, the Color Difference Key creates
well-defined transparency values.