Image masks overview
Another way to create transparency in a layer is by using image masks. An image mask creates transparency in a layer by deriving an alpha channel from another layer, such as a shape, text, movie, or still image.
Note: You can use masks and image masks together.
The power of image masks is that they do not have to be drawn or animated.
You can add filters to image masks to further manipulate the transparency effect. You may be able to improve an image mask by using color correction filters such as Brightness, Contrast, and Gamma to manipulate the effect, enhancing or reducing detail to create the transparency you need. For more information, see About applying filters or behaviors to masks.
You can also apply behaviors to image masks to create animated transparency effects. For more information, see About applying filters or behaviors to masks.