Apple Final Cut Pro 5 - Manual (Page 1158)

Chapter 18 Compositing and Layering 371
II
Using Video and Graphics Clips With Alpha Channels
Ordinary video clips have three channels of information for red, green, and blue. An
alpha channel contains additional information that defines areas of transparency in the
clip or image. If you import a QuickTime movie or a still image from another
application, or one 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 it against any clips appearing in video tracks
underneath it.
Note: Alpha channels work the same in video and still graphics media files.
Types of Alpha Channels Recognized in Final Cut Pro
Final Cut Pro recognizes three kinds of alpha channels. A fourth state indicates there’s
no alpha channel available; you can choose “None/Ignore” to ignore an existing alpha
channel. The kind of alpha channel that Final Cut Pro determines a clip is using appears
in a pop-up menu in the Item Properties window for the clip. There is also an option to
reverse the available alpha channel.
The pop-up menu in the
Alpha column lets you
choose the type of
alpha channel.
Click in this column to
reverse the alpha
channel.