There are times when you need to combine two clips to create an image with bits of both. Compositing refers to combining parts of two or more video clips into a single image.
There are a number of ways to combine video images in Final Cut Pro:
Transitions: All video transitions involve combining the end of one clip with the start of a second clip, which results in a combination of the two clips appearing at the same time during the transition.
Keying: All keys involve compositing a foreground image over a background image.
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.
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.