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Chapter 12 Using Shapes and Masks 1101
Applying Behaviors to Masks
You can apply behaviors directly to masks. When you apply a behavior to a mask, the
mask is animated like any other layer. However, since the mask only affects the
transparency within a layer, and not the position, rotation, or scale of the layer itself, the
result is similar to that of a “traveling matte,” where the mask moves within the layer to
which it’s applied, hiding and showing different areas of the image as it moves.
To apply a behavior to a mask, do one of the following:
m Drag a behavior from the Library directly onto a mask in the Layers tab or Timeline.
m Select a mask in the Layers tab 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 in order to isolate
it from the background. The end result is similar to a bluescreen or greenscreen effect.
Why would you bother? In a wide variety of situations, keying is either 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” on page 1108.)
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 that it moves really fast, the
layer is blurred when you enable motion blur. If you animate a layer’s mask so that it
also moves fast, the edges of the mask are blurred as well. This is important as an
animated mask’s blur should ideally match any blur that’s present in the foreground
subject itself.
For more information about motion blur, see “Render Settings Tab of the Project
Properties Dialog” on page 187.