QuickTime for Java: A Developer's Notebook

Moving Frame by Frame

One popular feature for playback is the ability to step forward exactly one frame. It turns out to be trickier than one might initially expect: it’s not like there’s a Movie.nextFrame( ) method. Indeed, a Movie might not have a video track at all, if it represents an MP3 or some other audio-only media. So, finding the next frame requires being a little smarter about looking inside the Movie’s structure.

How do I do that?

This example builds on the earlier BasicQTButtons code. In this example, the implementations of the forward and back buttons are altered so that instead of changing the play rate, they change the current time to be the next frame before or after the current time. For space, this example shows only the changes from the original BasicQTButtons.

This example needs to import quicktime.std to use StdQTConstants , and quicktime.std.clocks for some time-related classes. It also adds an instance variable visualTrack, which is found with the following call:

theMovie = m;
// find video track
visualTrack =
  m.getIndTrackType (1,
                     StdQTConstants.visualMediaCharacteristic,
                     StdQTConstants.movieTrackCharacteristic);

If a visual track isn’t found, the revButton and fwdButton are disabled later in the constructor.

Finally, a new implementation of actionPerformed() does the frame-step logic when the revButton or fwdButton is clicked:

if (e.getSource( ) = = revButton) { TimeInfo ti = visualTrack.getNextInterestingTime ( StdQTConstants.nextTimeMediaSample, theMovie.getTime( ...