26 July 2010
For spotlight effects, wipes, and transitions, use a mask layer in Adobe Flash Professional 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. Use shape tweening for a filled shape used as a mask; use motion tweening for a type object, graphic instance, or movie clip. 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.
You can also create a mask layer from a movie clip using ActionScript. A mask layer created with ActionScript can be applied only to another movie clip. See Using movie clips as masks in the ActionScript 2 documentation for Flash CS4.
Note: The 3D tools cannot be used on objects that are on mask layers, and layers containing 3D objects cannot be used as mask layers. For more information about the 3D tools, see the section titled 3D graphics in the Flash documentation.
To learn more about working with masks, watch the video titled Creating and animating masks.