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Animating a mask


    You can change all of a layer's Mask property 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 control point at the initial keyframe as the first vertex and "numbers" each successive control point in ascending order from the first vertex. After Effects then assigns the same numbers to the corresponding control points at all successive keyframes. After Effects interpolates the movement of each control point from its initial position at one keyframe to the position of the correspondingly numbered control point at the next keyframe. At any time during an animation, you can designate another control point as the first vertex; this causes After Effects to renumber the control points 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 marker 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. For example, for information on setting Mask Opacity, see Adjusting the opacity of a mask.
  5. Set an initial keyframe. For information on setting keyframes, see Setting keyframes.
  6. Move the current-time marker to the time where you want to add the second keyframe.
  7. Change the value for the mask property.
  8. Repeat steps 7 and 8 as many times as you want to add more keyframes.

    Note: By default, when you add a control point to a mask, the new point appears on the mask throughout the mask's duration but reshapes the mask only at the time it was added. When you delete a control point from a mask at a specific point in time, the point is deleted from the mask throughout the mask's duration. Prevent After Effects from adding and deleting control points throughout the mask's duration by choosing Edit > Preferences > General and deselecting Preserve Constant Vertex Count When Editing Masks.

To designate another control point 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 marker to the point where you want to designate a new first vertex.
  3. Select the control point you want to designate as the first vertex. (See Selecting masks and control points for information on selecting a control point.)
  4. Choose Layer > Mask > Set First Vertex.
  5. Note: The control point designated as the first vertex appears slightly larger than the other control points in the Composition window.