Final Cut Pro X: Compositing overview
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.