In Action, 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.
When creating a mask for a stereo object, you can apply it to both eyes simultaneously. You can change the camera visibility options to apply a mask to only the Left Eye or Right Eye. Use the Visibility box to set the camera visibility options. To set up stereo objects in Action, see Starting a Stereoscopic Session.