Apple Motion 5.1.1 - Manual (Page 510)

Chapter 13 Keying 510
Important: When using masks with keying, apply any masks after applying the keyer.
The second step in many keying shots is to create a garbage mask to crop out unwanted objects
in the shot that can’t be keyed, such as the edge of a blue screen stage, lighting rigs, or tape that
appears in the background.
Keyed image Original image
You can also use a garbage mask to conceal parts of the background too difficult to key without
the loss of foreground detail.
Create a garbage mask
m Select a keyed layer and use a mask tool to draw a mask around the foreground subject.
Note: The garbage mask must be animated if the subject is moving.
By default, the Mask Blend Mode is set to Add, and crops out everything outside the mask, while
leaving the transparent areas in the mask alone.
To crop out areas within masks, as shown in the following image, choose Subtract from the Mask
Blend Mode pop-up menu.
For more information about creating masks, see Masks and transparency overview on page 888.
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