Apply behaviors to masks – Apple Motion 5.1.1 User Manual
Page 896
Chapter 20
Shapes, masks, and paint strokes
896
Apply behaviors to masks
You can apply behaviors to masks. When you apply a behavior to a mask, the mask is animated
like any other layer. However, because the mask only affects the transparency in a layer, and not
the position, rotation, or scale of the layer, the result is similar to that of a “traveling matte,” where
the mask moves in the layer it is applied to, hiding and showing different areas of the image as
it moves.
Apply a behavior to a mask
Do one of the following:
m
Drag a behavior from the Library onto a mask in the Layers list or Timeline.
m
Select a mask in the Layers list or Timeline, then choose a behavior from the Add Behavior
pop-up menu in the toolbar.
The behavior appears nested underneath the mask to which it’s applied.
Keyframe a mask’s shape for animation and rotoscoping
You can keyframe a mask’s animation parameter to change its 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 end
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 or holdout
mattes to improve the effect. (For more information on creating garbage and holdoutmattes or
masks using masks, see
on page 509 and
on page 511.)
All mask animation is stored in the Shape Animation channel. It’s worth noting that animated
masks trigger the same motion blur as any other keyframed parameter in Motion. For example,
if you animate a layer’s position so it moves really fast, the layer is blurred when you enable
motion blur. If you animate a layer’s mask so it also moves fast, the edges of the mask are blurred
as well. This is important because an animated mask’s blur should match any blur present in the
foreground subject.
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