Apple Final Cut Pro 7 - Manual (Page 1225)

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 Filters.” )
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.
Alpha Channels and Key, Matte, and Mask Filters
Key, matte, and mask filters all create or modify the alpha channel of the clip to which
they’re applied. A clip’s alpha channel defines areas of transparency within that clip.
Key filters generally create new alpha channel information and are useful in situations
where the foreground subject is moving or has a complex or changing shape. For more
information about keying, see
“Using Keying to Isolate Foreground Elements.”
Matte filters can create alpha channel information, but they can also be used to add to
or subtract from alpha channel information that has already been applied to the clip.
Matte filters are useful when you want to simply isolate a region of the frame, or when
you’re cutting out a static foreground object with a relatively simple shape. For more
information about using mattes, see
“Using Mattes to Add or Modify Alpha Channels.”
Masks are most useful when you’re copying an alpha channel (static or in motion) from
one clip to another, although masks can also change the quality of a clip’s alpha channel,
letting you feather out edges, for example. For more information about using masks, see
“Using Masks to Replace or Modify Alpha Channels.”
For information about enabling or disabling alpha channels in sequence settings, see
“Confirming Alpha Channel Status.”
1225 Chapter 72 Keying, Mattes, and Masks