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
on page 424.
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
” on page 440.
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
” on page 443.