What Are Masks and How Are They Used?
A mask is an image that is used to create areas of transparency in another image. For
example, the luma in one clip can be used to create transparency in another clip. You
can also assign the alpha channel of one clip to a completely different clip. (For more
information about alpha channels, see “
Alpha Channels and Key, Matte, and Mask
,” next.) Using additional mask filters, you can further modify the resulting regions
of transparency—widening, narrowing, or feathering them as needed. Clips used to
create masks can be in motion, creating a moving area of transparency.