Masking allows you to hide part of an object or text. It can be used for making a spotlight effects.
Start a new document with stage size 550 by 200 with any background color.
Rename Layer 1 Mask by double clicking the works Layer 1 and typing in the text.
Draw a circle any color on the far left side of the document. We will eventually mask the circle so the color does not matter:
Click the circle with the selection tool . Convert the circle to a symbol by clicking Modify>Convert to Symbol. By converting to a symbol Flash groups the shape. In order to create a motion tween objects must be grouped.
Name the symbol circle_mc and click motion clip as the type.
Right-click the circle instance on the Stage and choose the option to Create Motion Tween in the menu that appears.
Notice that the Timeline is automatically extended to Frame 24 and the red marker (the current frame indicator, also known as the playhead) moves to Frame 24. This indicates that the Timeline is prepared for you to edit the ending location of the symbol and create a one-second animation—assuming that you haven't changed default frame rate for the project from 24 fps in the Property inspector. A span of 24 frames is the equivalent of one second at a rate of 24 fps.
While the playhead is still on Frame 24 of the Timeline, select the circle instance on the Stage and drag it to the right, just past the Stage area. This step of relocating the instance creates a tweened animation. On Frame 24, notice the diamond-shaped dot (known as a property keyframe) that appears.
On the Stage, also notice the motion guide that indicates the circle's path of motion between Frame 1 and Frame 24.
In the Timeline, drag the red playhead back and forth from Frame 1 to Frame 24 to preview the animation; this is known as scrubbing the Timeline.
Right click in frame 48 and select Insert Keyframe and then select Position.
Your timeline should now look like this:
Click in frame 48 and move the circle back to the left side of the stage:
Lock the mask layer by clicking the dot below the lock on the timeline:
Add a new layer by clicking the new layer button on the Timeline:
Name this layer text. Make sure you are in the first frame of the text layer. Using the text tool , insert some text into the first frame of this layer. I recommend a thick font like Arial Black, size 48. Select the Free Transform tool and enlarge the text to fit the stage.
Make sure you don't make it larger than the height of your circle!!
The last step is to create the mask. Make sure the text layer is the top layer!!
Right-click on the top layer (the area shown above outlined in red with the word Text) and select Mask from the menu. Play the movie to see the results.
Make sure you publish your movie then save it!!