Apple Motion 4 - Manual (Page 1168)

2 Move to the first frame where the mask animation should begin, then draw a mask that
accurately isolates the subject.
3 When you’re finished drawing the first mask, enable Record (press A).
Record button
4 Move to the first frame of the Timeline where you want to change the shape of the mask,
then make any necessary changes to its control points.
In this example, the best place to start when rotoscoping the dolphin is the frame in
which all the fins are showing.
Note: For some tips on rotoscoping, see Rotoscoping Tips .
When rotoscoping a moving subject, it’s a good idea to play through the clip and examine
its direction and speed to get a sense of how the mask needs to be animated. In particular,
you may find it helpful to place markers indicating frames where the subject or camera
stops, changes direction, or changes speed, since these are likely candidates for your first
pass of keyframed mask changes.
1168 Chapter 17 Using Shapes, Masks, and Paint Strokes