Technical Q&A's

QTMTB 17 - Determining QuickDrawVideo Media Pixel Depth (1-May-95)

Q How do I get the pixel depth of the QuickTime video media for a given track?

A To find the video media pixel depth, retrieve the media's image description handle. You can use GetMediaSampleDescription to get it, but this routine needs both the video media and the track's index number. It's not obvious, but a media's type is identified by its media handler's type. Thus, you can walk through a movie's tracks by using its indexes until you find video media, at which point you have both the track index and video media.

The following sample code does the trick:

Media GetFirstVideoMedia(Movie coolMovie, long *trackIndex)
{
  Track    coolTrack = nil;
  Media    coolMedia = nil;
  long    numTracks;
  OSType  mediaType;
  numTracks = GetMovieTrackCount(coolMovie);
  for (*trackIndex=1; *trackIndex<=numTracks; (*trackIndex)++) {
    coolTrack = GetMovieIndTrack(coolMovie, *trackIndex);
    if (coolTrack) coolMedia = GetTrackMedia(coolTrack);
    if (coolMedia) GetMediaHandlerDescription(coolMedia,
      &mediaType, nil, nil);
    if (mediaType = VideoMediaType) return coolMedia;
  }
  *trackIndex = 0;  // trackIndex can't be 0
  return nil;      // went through all tracks and no video
}

short GetFirstVideoTrackPixelDepth(Movie coolMovie)
{
  SampleDescriptionHandle imageDescH =
    (SampleDescriptionHandle)NewHandle(sizeof(Handle));
  long    trackIndex = 0;
  Media    coolMedia = nil;
  coolMedia = GetFirstVideoMedia(coolMovie, &trackIndex);
  if (!trackIndex || !coolMedia) return -1;  // we need both
  GetMediaSampleDescription(coolMedia, trackIndex, imageDescH);
  return (*(ImageDescriptionHandle)imageDescH)->depth;
}

Note that QuickTime 2.0 has a new function called GetMovieIndTrackType that does most of the work described in the sample. GetMovieIndTrackType lets you search for all of a movie's tracks that share a given media type or media characteristic. See the QuickTime 2.0 SDK documentation for more details.

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