5

Applying 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.
To apply a behavior to a mask
Do one of the following:
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Drag a behavior from the Library onto a mask in the Layers list or Timeline.
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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.
Keyframing a Mask’s Shape for Animation and Rotoscoping
You can keyframe a mask’s animation parameter to change its shape over time. This can
serve many purposes. 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 holdout mattes or masks using masks, see Using
Masks to Aid Keying Effects.)
All mask animation is stored in the Shape Animation parameter. 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 ideally match any blur present in the foreground subject.
To rotoscope a subject by animating a mask
1 Choose View > Resolution > Full to ensure that you are viewing the Canvas at full
resolution.
Important: If the Canvas resolution is not set to full, the outlines of objects and images
may shift slightly. As a result, masks created to trace a subject at less than full resolution
may not be accurate.
1207Chapter 20 Using Shapes, Masks, and Paint Strokes