Flash CS3 Tutorial
How to Create a Flash Mask Layer
To use a layer mask, you first define a shape on the mask layer. Directly below the mask layer is a second layer with your content. The content is only visible in areas defined by your shape on the mask layer.
Make sure the timeline/layers panel is open by choosing Window > Timeline.
Add a rectangular object to the workspace with the Rectangle tool (r key).
Name the layer with the rectangle, content.
Add a new layer directly above the content layer and name it mask.
Add some text to the mask layer that covers the rectangle. The text will work as a cookie cutter to modify the rectangle shape.
Right-click the mask name of the mask layer and choose Mask from the menu.
Mask Object Surprises
To mask with multiple objects, it is best to convert all objects to Merge Shapes. A good rule of thumb is to always use Merge Shapes for your mask.
Another option is to select your mask objects and choose Modify > Combine Objects > Union. If you forget these rules, generally only one of your mask objects is used in the masking process.
Animating a Mask
Consider changing the mask shape over time with a Shape Tween.