Apple Motion 4 - Manual (Page 1179)

Garbage Masks
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.
You can also use a garbage mask to conceal parts of the background that are too difficult
to key without the loss of foreground detail.
To create a garbage mask
µ
Select a keyed layer and use one of the mask tools to draw a mask around the foreground
subject.
Note: The garbage mask must be animated if the subject is moving.
By default, the mask is set to Add, and crops out everything outside the mask, while
leaving the transparent areas within the mask alone.
For more information about using Motion’s keying filters, see Keying Filters .
Holdout Masks
Sometimes, while pulling a key, you lose part of the image you’re trying to keep. This can
happen when the color of the subject’s clothing is too close to the color of the background
being keyed out, or when you need to use keying values that are too aggressive. In this
example, very small parts of the lioness’s head and face are getting removed with the
key, rather than just the blue sky.
1179 Chapter 17 Using Shapes, Masks, and Paint Strokes