Creating movie clip masks

You can use a movie clip as a mask to create a hole through which the contents of another movie clip are visible. The mask movie clip plays all the frames in its Timeline, just like a regular movie clip. You can make the mask movie clip draggable, animate it along a motion guide, use separate shapes within a single mask, or resize a mask dynamically. You can also use ActionScript to turn a mask on and off.

You cannot use a mask to mask another mask. You cannot set the _alpha property of a mask movie clip. Only fills are used in a movie clip that is used as a mask; strokes are ignored.

To create a mask:

On the Stage, select a movie clip to be masked.
In the Property inspector, enter an instance name for the movie clip, such as image.
Create a movie clip to be a mask. Give it an instance name in the Property inspector, such as mask.
The masked movie clip will be revealed under all opaque (nontransparent) areas of the movie clip acting as the mask.
Select Frame 1 in the Timeline.
Open the Actions panel (Window > Development Panels > Actions) if it isn't already open.
In the Actions panel, enter the following code:
```
image.setMask(mask);
```

For detailed information, see MovieClip.setMask().