Apple Final Cut Express HD - Manual (Page 799)

Chapter 54 Compositing and Layering 799
IX
Importing Clips With Alpha Channels
Some clips and images you import may have alpha channels. Ordinary video clips
have three channels of information for the red, green, and blue channels respectively.
An alpha channel contains additional information that defines areas of transparency
in the movie.
When you import a QuickTime movie or an image file, its alpha channel is immediately
recognized by Final Cut Express HD. After you edit this clip into a sequence,
Final Cut Express HD uses the alpha channel for compositing against background
layers. You can turn off alpha channels if you decide not to use them.
Important: When you import a clip with an alpha channel that contains no data, the
clip appears white. To display the clip properly, change the alpha channel for the clip to
None (see “
Changing a Clip’s Alpha Channel Type ,” next).
Changing a Clip’s Alpha Channel Type
There are several reasons why you might want to change a clip’s recognized type of
alpha channel in Final Cut Express HD:
 Sometimes Final Cut Express HD misinterprets the alpha channel when it imports a
clip, which can result in “fringing” around the edges of solid objects. For example, if a
clip with a black alpha channel is incorrectly identified as having a straight alpha
channel, a black fringe appears around the edges of the imported image.
 Some editing and compositing applications output alpha channels that are the
reverse of what Final Cut Express HD uses. When this happens, what’s supposed to
be transparent is solid, and what’s supposed to be solid is transparent.
In all these cases, you can change or reverse the alpha channel setting.
Foreground
graphic
Alpha
channel
Background
graphic
Final
composite