Apple Shake 4 Tutorials - Manual (Page 180)

180 Chapter 5 Using Keylight
4 Connect the HoldKey node to the third input of the Keylight node.
5 In the HoldKey parameters, boost the screenRange to 0.3.
The mask becomes solid.
When the holdout matte is plugged in, it acts as a mask for the foreground. You are, of
course, not obliged to use Keylight. You can also create a holdout matte using other
nodes, including RotoShape, Primatte, ChromaKey, LumaKey, and so on. The following
illustrations show the Keylight1 alpha channel and the HoldKey alpha channel. HoldKey
contains no gray tones; Keylight1 retains the window reflections.
The holdout matte has destroyed the edge quality of Keylight1. To correct the edge
quality, “chew into the edge” of the holdout matte.
Keylight1 alpha HoldKey alpha