Masking & Mask Layers
A mask layer is used to let Flash know which parts of a layer you would like to show.
1. Copy the mask.fla file to your user space and then launch Flash and open the document.
2. There are 2 layers in this document. The bottom layer is the background picture. The top layer is going to be our mask layer. The background image has been imported to the library and a symbol has been created to act as a mask.
3. Run the animation as it is. You should only see the head part of the image on the background.
4. Now to change things a little. Remove all of the frames from the mask layer. Add a keyframe to the start and place the mask symbol somewhere on the layer.
5. Add a blank keyframe to frame 60 of the mask layer. Move the mask to another place on the stage. Create a motion tween to make the mask move smoothly across the stage. Different parts of the background are revealed as the mask moves across the stage.