SetMovieGWorld Can Slow You Down

Dispatch 9

Applications use SetMovieGWorld to associate a QuickTime movie with a drawing destination called a GWorld. Most applications only call SetMovieGWorld once, to indicating where they want the movie to be drawn when the movie is first created. Some applications, however, display a single movie in several different windows. These applications often call SetMovieGWorld more often than necessary, in order to ensure that the movie is set to the correct destination. Unfortunately, changing the destination GWorld that a movie is drawing to is a relatively expensive operation.

When the movie's GWorld is set, internally QuickTime needs to rebuild its drawing pipeline. This can require reallocating QuickTime's internal buffers and renegotiating the drawing pipeline with Image Decompressors. In many cases, particularly if a hardware based Image Decompressor is involved, this can be a non-trivial operation. As a result, calling SetMovieGWorld can result in the overall processing time used by QuickTime increasing considerably. Note that the work in rebuilding the drawing pipeline is not actually done in SetMovieGWorld, but during the next call to MoviesTask. This means that if profiling of calls to QuickTime will only indicate that MoviesTask is taking longer than necessary, not that SetMovieGWorld is the cause of the problem.

One of the common mistakes applications make is to call SetMovieGWorld between calls to MoviesTask. In this case, the GWorld being set with SetMovieGWorld is always the same. It might seem reasonable in this case to expect QuickTime to ignore the call to SetMovieGWorld as the GWorld isn't really being changed. However, because of the way GWorlds are allocated, it is possible that after disposing one GWorld and allocating another, that the new GWorld will have the same address but different properties as the disposed GWorld. For this reason, QuickTime can't ignore this case of the same GWorld being set in SetMovieGWorld. Therefore, applications concerned with playback and rendering performance must take care only to call SetMovieGWorld when necessary.

See Also

Inside Macintosh: QuickTime - Movie Toolbox

Change History

4/16/98 - jph - First published