Apple Final Cut Pro X (10.0.9) - Manual (Page 352)

Chapter 12 Keying and compositing 352
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Alpha channels: Many computer-generated video clips have an alpha channel—a built-in
mask—that defines precisely the areas of the clip that are composited over a background
clip. These require no key setup and generally just automatically work. Many effects, such as
Transform and Distort; generators, such as Shapes and Timecode; and titles use alpha channels.
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Compositing: Each video clip in the Timeline has a set of Compositing settings. These are used
when you want to combine clips without using the key effects.
Use alpha channels
Alpha channels provide an easy way to composite one image over another. Using an application
like Motion, an Apple application designed to work with Final Cut Pro, you can create still images
and video clips that have alpha channels. Alpha channels can be thought of as having a built-in
keyer—when you use the key effects you are creating a mask that is used in the same way a clip
with an alpha channel is used for compositing.
Below is an example of a clip with an alpha channel, showing the image that fills the
alpha channel.
Below is the alpha channel image—the white areas define the video areas that are composited
over the background. The gray areas (which define the shadow in this example) define parts of
the image that are partially transparent.