QuickTime 4 Reference

Sources Other Than Video Tracks

Any track that produces video output, such as a sprite track, a text track, or another effect track, can be used as the source for an effect. Generally speaking, you use these track types as sources in the same way you use a video track, but some special considerations apply.

If you use sprite tracks as sources, and the sprites can move as a result of user interaction, it may be difficult to accurately predict what a transition effect will look like when it executes at run time.

If you "stack" effects, by using an effects track as a source, the target system must have enough speed to render the source frames for the original effect, then render the effect that is acting as a source, then render the stacked effect, while maintaining a reasonable frame rate. For example, the target system might need to decode a pair of Sorenson frames from two video source tracks, apply a cross-fade transition effect between them, and then apply a ripple effect to the final output. This will only give good results on a fast system.

Similarly, you can use an effect as the source for a sprite track, making the cloud effect a sprite, for example. Build the effect description and sample description as described later in this section, then add the effect description to the sprite track as a media sample, just as you would add an ordinary video sample.