Apple Motion 4 - Manual (Page 1181)

Make sure that the mask is entirely within the subject being keyed.
Note: The holdout mask must be animated if the subject is moving.
4 On the newly duplicated layer, delete the original keying filters since they’re unnecessary.
If you’ve used a Spill Suppressor filter, don’t delete it, since it’s probably changing the
color of the subject.
5 Feather the edge of the holdout mask you’ve just created, to make sure that it blends in
with the object you initially keyed.
In the following image, the mask on the lioness’s head creates a clean key by removing
the “dirt” left by the original key.
6 As an optional step, you can nest the originally keyed layer along with the holdout matte
you just created inside a dedicated group so that you can manipulate the entire subject
as a single object.
Performing Multiple Keys on a Single Subject
In some instances, it may be difficult to key an entire subject with a single keying filter
while retaining fine details. For example, uneven lighting or background color might
mean that the best settings to key a subject’s hair may not work elsewhere.
1181 Chapter 17 Using Shapes, Masks, and Paint Strokes