5.5

851Motion User Guide
Use keyframes to rotoscope a mask
Rotoscope a mask’s shape in Motion
You can keyframe a mask’s control points to animate a changing shape over time.
For example, you can animate a subtractive mask to change the shape of a hole in a
foreground object, allowing other layers in the background to show through.
A more conventional use of animated masks is to rotoscope a foreground subject.
Rotoscoping is the process of manually tracing a foreground subject to isolate it from
the background. The result is similar to a blue screen or green screen effect. Why would
you bother? In a wide variety of situations, keying is impractical or impossible if the shot
wasn’t well planned. Even for shots where keying is possible, manual rotoscoping is often
necessary to create garbage masks or holdout masks to improve the effect. (See Crop
background with a garbage mask in Motion and Restore foreground with a holdout mask in
Motion.)
The following task demonstrates how to use a mask to isolate an object in a clip in order to
apply separate effects to the isolated object and its background.
The process used to animate both masks and shapes is identical. To see an example of
shape animation, see Keyframe shape control points in Motion. For more information on
keyframing in general, see Intro to keyframing in Motion.