iMovie '09 & iDVD: The Missing Manual

Picture-in-Picture (PiP)

Picture-in-Picture is the effect that lets TV junkies watch two channels at once. The football game fills the big screen, but 60 Minutes plays in a small inset window in the corner.

POWER USERS' CLINIC: Using a Key Frame

If iMovie knows exactly what your green-screen background looks like, shadows and all, it can do a much better job of creating the effect, because it knows exactly what elements to remove.

That's why, when you film your actors in front of a green screen, it's a good idea to include (in the same shot, without stopping the camera) some video of the motionless green background all by itself, with the actors out of the frame.

(Why is it so important to shoot the empty-background video without turning off the camera? Because if you stop the camera, you'll get separate clips in iMovie—and you can't merge Event clips in iMovie. The moving-actors video and the empty-background video must be part of the same clip.)

Then, once in iMovie, open the Inspector window. Turn on "Subtract last frame".

That doesn't mean iMovie is going to delete the last frame. It means, instead, "Subtract this image when superimposing the green-screen video." That is, by comparing the moving-actor footage with the still-background footage, iMovie knows what's background and what's actor. (This background-only portion of the clip doesn't have to appear in your project. As long as it was there at the end of the original, untrimmed clip, iMovie can use the "Subtract last frame" ...