Apple Final Cut Pro 5 - Manual (Page 1190)

Chapter 19 Keying, Mattes, and Masks 403
II
13 Finally, you’re ready to insert the background image. Make sure that the clip you’re
editing into your sequence to use as the background image appears in a video track
below the foreground clip being keyed.
14 To really make the foreground and background look as if they were shot together, it
may be necessary to perform additional compositing steps such as blurring the
background to make it look out of focus, color correcting the foreground subject to
have the same color temperature, or using motion effects to move the foreground
subject. Even with a successful key, these are the details that make a composite look
like a finished shot.
The background image now
appears.
The background image is
placed below the foreground
clip that was keyed.