Manual

Chapter 55 Keying, Mattes, and Masks 807
IX
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 luminance 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,” which follows. 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.
Keying 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 808.
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” on page 822.
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 824.