QuickTime 4 Reference

Zero-Source Effects

Effects that don't require a source, such as the fire or cloud effect, are free-standing special effects that can be added anywhere in a movie. Just set the offset and duration of the effects track to the part of the movie where you want the effect to appear. If there is already an active video track at that point in the movie, you control the interaction between the video track and the effects track in the usual ways: putting one track in front of the other, using an alpha channel to allow one track to be partly visible through the other, and so on.

Figure 1 An effect track overlaying a video track