Inside Macintosh: QuickTime Reference
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Once the movie has been constructed, the SMIL importer relies heavily on the Movie Media Handler to handle the runtime details. For example, the SMIL importer does not "know" the actual dimensions of the movie because it doesn't instantiate the media. Thus, the spatial layout may need to be adjusted when the actual dimensions are known. The Movie Media Handler takes care of this by using the spatial composition information that the SMIL importer put into the movie media's sample data (taken from the SMIL document's layout section).
Similarly, the duration of the media may not be known. A SMIL document does not have to -- and usually does not -- specify the duration of a media element. Instead, it describes the temporal relationships between elements (i.e., play this element after this other element). The Movie Media Handler adjusts the temporal layout of the movie by editing it at runtime. As with spatial layout changes, the temporal changes are made based on information taken from the SMIL document's body and placed in the movie media sample.
Inside Macintosh: QuickTime Reference
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