For spotlight effects and transitions, use a mask layer to create a hole through which underlying layers are visible. A mask item can be a filled shape, a type object, an instance of a graphic symbol, or a movie clip. Group multiple layers under a single mask layer to create sophisticated effects.
To create dynamic effects, animate a mask layer. For a filled shape used as a mask, use shape tweening; for a type object, graphic instance, or movie clip, use motion tweening. When using a movie clip instance as a mask, animate the mask along a motion path.
To create a mask layer, place a mask item on the layer to use as a mask. Instead of having a fill or stroke, the mask item acts as a window that reveals the area of linked layers beneath it. The rest of the mask layer conceals everything except what shows through the mask item. A mask layer can contain only one mask item. A mask layer cannot be inside a button, and you cannot apply a mask to another mask.
To create a mask layer from a movie clip, use ActionScript. A mask layer created with ActionScript can be applied only to another movie clip.
The 3D tools cannot be used on objects on mask layers and layers containing 3D objects cannot be used as mask layers. For more information about the 3D tools, see 3D graphics.
You can use mask layers to reveal portions of a picture or graphic in the layer below. To create a mask, you specify that a layer is a mask layer, and either draw or place a filled shape on that layer. You can use any filled shape, including groups, text, and symbols, as a mask. The mask layer reveals the area of linked layers beneath the filled shape.
Right-click (Windows) or Control‑click (Macintosh) the mask layer’s name in the Timeline, and select Mask. A mask layer icon indicates the mask layer. The layer immediately below it is linked to the mask layer, and its contents show through the filled area on the mask. The masked layer name is indented, and its icon changes to a masked layer icon.