ADOBE AFTER EFFECTS 7.0
User Guide
266
Animating masks
About mask animation
You can change all of a layer’s Mask property’s values—Mask Shape, Mask Feather, Mask Opacity, or Mask
Expansion—over time by using keyframes.
To animate a mask shape, After Effects designates the topmost vertex at the initial keyframe as the
first vertex
and
numbers each successive vertex in ascending order from the first vertex. After Effects then assigns the same numbers
to the corresponding vertices at all successive keyframes. After Effects interpolates the movement of each vertex
from its initial position at one keyframe to the position of the correspondingly numbered vertex at the next
keyframe. At any time during an animation, you can designate another vertex as the first vertex; this causes After
Effects to renumber the vertices of the shape you assigned a new first vertex, and the mask animates differently
because After Effects then maps the new vertex numbers to the corresponding old vertex numbers still saved at
successive keyframes.
See also
“About keyframes” on page 192
To create an initial mask shape keyframe
❖
Select one or more masks, and then do one of the following:
•
Choose the Add Mask Shape Keyframe command.
•
Click the stopwatch icon next to the Mask Shape property for each mask in the Timeline panel.
Note:
You can quickly set an initial Mask Shape keyframe for all masks in selected layers by pressing Shift+Alt+M
(Windows) or Shift+M (Mac OS).
To animate a mask property
1
In the Timeline or Composition panel, select the mask that you want to animate.
2
Move the current-time indicator to the time where you want to begin the animation.
3
Expand the Mask properties and locate the property that you want to change.
4
Set a value for the mask property.
5
Set an initial keyframe.
6
Move the current-time indicator to the time where you want to add the next keyframe.
7
Change the value for the mask property.
8
Repeat steps 6 and 7 for any other keyframes that you want to add.
Note:
By default, when you add a vertex to a mask, the new vertex appears on the mask throughout the mask’s duration
but reshapes the mask only at the time it was added. When you delete a vertex from a mask at a specific point in time,
the vertex is deleted from the mask throughout the mask’s duration. Prevent After Effects from adding and deleting
vertices throughout the mask’s duration by choosing Edit > Preferences > General (Windows) or After Effects >
Preferences > General (Mac OS), and deselecting Preserve Constant Vertex Count When Editing Masks.
