Apple Shake 4 - Manual (Page 684)

684 Chapter 24 Keying
You can combine keys with the holdout matte input. Typically, you pull a basic key for
soft edges and reflections. You also pull a second key which is very hard, and then
soften it with filters.
In the following example, the first image shows the initial key coming out of the
Keylight node. The reflections are good, but there is some transparency near the seat
belt and steering wheel.
Next, a key is pulled with Primatte on the same bluescreen. With just foreground and
background operators, the reflection is removed and the transparent areas filled in. This
results in a very hard matte.