Animating a mask

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. (See Understanding 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 as After Effects now maps the new vertex numbers to the corresponding "old" vertex numbers still saved at successive keyframes.

To animate a mask property:

1. In the Timeline or Composition window, 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. For information on setting a mask value, see the corresponding section for the value you want to set.
5. Set an initial keyframe.
6. Move the current-time indicator to the time where you want to add the second keyframe.
7. Change the value for the mask property.

Repeat steps 7 and 8 as many times as you want to add more keyframes.

For further information, see Adjusting the opacity of a mask or Setting keyframes.

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.

To designate another vertex as the first vertex:

1. Create an animated mask shape, as described in the previous procedure.
2. In the Timeline window, move the current-time indicator to the point where you want to designate a new first vertex.
3. Select the vertex you want to designate as the first vertex.
4. Choose Layer > Mask > Set First Vertex.

For further information, see Selecting masks and vertices.

Note: The vertex designated as the first vertex appears slightly larger than the other vertices in the Composition window.