Apple Motion 3 - Manual (Page 1111)

Chapter 12 Using Shapes and Masks 1111
Make sure that the mask is entirely within the subject being keyed.
Note: You may need to animate the holdout mask 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.
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.
Final composite The mask on the lioness’s head creates a clean
key by removing the “dirt” left by the original
key.