The mask tracker transforms a mask so that it follows the motion of an object (or objects) in a movie. You usually create and use masks to hide clips from the final output, select a part of the image or video to apply effects, or combine clips from different sequences.
To use the mask tracker, select a mask and click the Mask Path setting under the mask. Right-click the selected mask and select Track Mask to begin tracking a mask.
When a mask is selected, the Tracker panel switches to mask tracking mode, and displays the following controls:
If the Tracker panel is not displayed, select a mask and go to Animation > Track Mask to display the panel. You can also context-click a mask and choose Track Mask from the context menu.
The result of using the mask tracker can be seen in the application of keyframes for the Mask Path property; the mask shape matches the transformations tracked in the layer, depending on the type of method that is chosen.
The mask tracking analysis searches for content inside the mask. Use the Mask Expansion property to expand or contract the area of the mask.