3D Compositing with Action > Action: Masks >
About Masks

A GMask object is an animatable spline-based object that you draw directly in an Action scene that allows you to apply transparency effects to the scene. A mask can be applied to all media by default, or limited to selected media. Depending on what you want to accomplish, you can create a transparency that reveals the background image, or hide a portion of the matte source or a front source.You can move the mask in the 3D environment using the Axis object.