Apple Shake 4 Tutorials - Manual (Page 193)

Chapter 6 Using Primatte 193
To create a holdout matte:
1 Add a second QuickPaint node (a RotoShape node would also do the job), and connect
the node to the fourth input (holdOutMatte) of the Primatte1 node.
Note: To identify inputs on the top of a node, position the pointer over the input and
read its name in the Info field at the bottom-right portion of the interface.
2 Using the brush in the QuickPaint node, paint a holdout matte along the line under the
woman’s arm.
3 In the Primatte1 parameters, set the hMatteChannel masking channel for the
holdout matte.
In this case, hMatteChannel can be set to any of the channels—r, g, b, or a—because
the paint strokes of the QuickPaint node appear in all channels. However, if you use an
image that does not have an alpha channel, or you want to use a specific channel,
make sure the channel you want to use is selected for the hMatteChannel.
A final parameter that helps with multiple masks is the arithmetic parameter. The
arithmetic parameter indicates how the Primatte key interacts with the alpha channel
of the foreground.