For certain narrated slide shows and software demos, you sometimes have pauses where your image doesn't change, and other points where changes are happening quickly. If you set your frame rate high enough to capture all the changes, you find that many frames are duplicates--frames that take up precious bytes if you need to conserve space (e.g. if you intend to publish on the web, or want to fit the file on a floppy disk). What you need is to have different frames stay up for a different amount of time.

This week, we'll assume that you're starting with an existing QuickTime movie that has repeat frames in it. This may happen if you record screen actions using a tool such as MotionWorks CameraMan or Strata Instant Replay.You can use ConvertToMovie (available at the QuickTime FAQ software page) to stretch out the duration of frames that are subsequently duplicated, and eliminate the duplicates.

  1. Open ConvertToMovie.

    An Open file dialog will appear with a prompt to pick a file.

  2. Pick the movie file in which you want duplicates removed.

  3. In the compression dialog that appears, pick Animation as the compressor. 256 Colors should be sufficient for a screen recording. Make sure Quality is set to Most. Set your frame rate to be the same as the original movie. To get the smallest file size, set the key frame number to be greater than the number of frames in your movie; quality will be just as good as with more frequent key frames.

  4. In the dialog that next appears, make sure to select "No duplicate differences". (The other choices are up to you, but you probably want to use the settings as below.)

  5. The next dialog is a standard save dialog. Indicate where you'd like your file saved and click Save.

    The movie will be converted frame-by-frame; if you watch in the lower left corner of the window where the number of the frame it's working on is indicated, you'll notice that some frame numbers are skipped.

  6. You can click Quit Convert to Movie when you get the dialog with that choice (as well as "Convert another file(s)" and "Append another file(s)).

    It will take a few more seconds as the movie is flattened and rechunked.

To see your resulting movie, you can open it in MoviePlayer. If you use the step buttons to step through frame-by-frame, you'll see that the playhead jumps forward different amounts for different frames. You should also notice that this new file is smaller than the original.

If you want an even smaller file (and 256 Colors is ok), use ConvertToMovie on the movie in which you just removed the duplicate frames, this time choosing the Graphics compressor and leaving Frames per second blank. You don't have to select "No duplicate differences" this time. (Unfortunately, you can't use the Graphics compressor in the original pass through ConvertToMovie when you eliminate duplicate frames.)




copyright 1996-97  Judith L. Stern and Robert Lettieri,
QuickTime, the QuickTime logo, and Macintosh are registered trademarks of Apple Computer Inc