Apple Final Cut Pro 7 - Manual (Page 1213)

Using Video and Graphics Clips with Alpha Channels
Ordinary video clips have three channels of information for red, green, and blue. An alpha
channel is an additional channel that defines areas of transparency in the clip or image.
If you import a QuickTime movie or a still image from another application, or a file that’s
been created by a 3D animation program, it may have an alpha channel in addition to
its color channels. An alpha channel is a grayscale channel where levels of white and
black determine varying degrees of transparency applied to the color channels during
compositing.
When you import a QuickTime movie or an image file, its alpha channel is recognized
immediately. When you edit the movie or image file into a sequence, Final Cut Pro uses
the alpha channel to composite the movie or image file against any clips appearing in
video tracks below it.
Note: Alpha channels work the same in video and still-image media files.
Alpha
channel Background
graphic Foreground
graphic Final
composite
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” ).
Working with Clips That Have Alpha Channels
You can edit clips that have alpha channels into your sequence as you would any other
clip, but you must make sure that a clip with an alpha channel is on a higher-numbered
video track than any clips that are supposed to be beneath it.
1213 Chapter 71 Compositing and Layering