Rotoscope a mask’s shape
You can keyframe a mask’s control points to animate a changing shape over time. 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 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 masks or holdout mattes to improve the effect. (For more information on creating garbage masks or holdout mattes using masks, see Crop unwanted background areas using a garbage mask and Restore part of a foreground image using a holdout mask.)
The following task demonstrates how to use a mask to isolate an object in a clip in order to apply separate effects to the isolated object and its background.
The process used to animate both masks and shapes is identical. To see an example of shape animation, see Keyframe shape control points. For more information on keyframing in general, see Keyframing overview.
Rotoscope a subject by animating a mask
Choose View > Resolution > Full to ensure that you’re 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.
Move to the first frame where the mask animation should begin, then draw a mask that accurately isolates the subject.
For details on how to draw a complex mask, see Draw complex shapes and masks.
When you’re finished drawing the first mask, click the Record button in the transport controls (or press A) to turn on keyframe recording.
Move to the first frame of the Timeline where you want to change the shape of the mask, then drag the mask’s control points in the Canvas.
Note: If the mask is deselected, you must select it in the Layers list so its control points become visible in the Canvas. Make sure that the Show/Hide Masks button is selected in the Layers list so masks are visible.
Continue moving the playhead and dragging control points.
One imprecise rule of thumb is to move to the frame that’s halfway between any two keyframed mask shapes and make new adjustments. Continue keyframing shape changes at the halfway point between every two keyframes until the mask accurately follows the motion of the subject. For irregularly shaped objects or objects with complex motion, don’t be surprised if you need to add a large number of keyframes. Nobody ever said that rotoscoping was fast!
Every time you move the playhead to a new frame and make a change to the mask’s shape, a keyframe is created in that shape’s Shape Animation channel in the Keyframe Editor. If you move the playhead directly on top of a previously existing keyframe, you change the mask’s shape at that point without creating a keyframe.
When you’ve finished animating the mask, click the Record button again (or press A) to turn off keyframe recording.