How do I count the frames in an MPEG movie?

Q:  I'm using the Movie Toolbox function GetMovieNextInterestingTime with QuickTime 6 to count the frames in an MPEG movie but it does not seem to work. The first call to this function just returns a sample time of zero and a duration which is equal to the duration of the movie. Do I have to write special code for MPEG movie tracks?

A: GetMovieNextInterestingTime works for all kinds of tracks. However, you should avoid using the nextTimeMediaSample flag when searching for frames, because some media types store many frames in a single media sample (for example, MPEG and Flash do this). If you are searching for distinct frame times, pass the nextTimeStep flag to GetMovieNextInterestingTime.

Also, for MPEG media there is a bug (r. 3236091) in QuickTime 6 which requires you to first task the movie before calling GetMovieNextInterestingTime to count the frames in this manner. Here's a code snippet showing how it's done:

Listing 1  Counting the frames of an MPEG movie.

#include <QuickTime/QuickTime.h>

long GetFrameCount (Movie theMovie)

{

    long        frameCount = 0;

    TimeValue   curMovieTime;

    if (theMovie == NULL) goto bail;

    // due to a bug in QuickTime 6 we

    // must task the movie first

    MoviesTask( theMovie, 0 );

    curMovieTime = 0;

    while( curMovieTime >= 0 )

    {

        GetMovieNextInterestingTime(theMovie,

                                    nextTimeStep,

                                    0, NULL,

                                    curMovieTime,

                                    fixed1,

                                    &curMovieTime,

                                    NULL );

        frameCount++;

     }

    // there's an extra time step at the end

    // of the movie

    frameCount--;

bail:

    return(frameCount);

}

Be aware, also, that tweening can be used to describe transformations that occur smoothly across a continuum of time rather than at discrete moments. For example, a wipe effect between two video sources will produce a distinct output image at every moment between its start and end times. In such a situation, the question, "How many frames are there?" isn't very helpful, since the honest answer is: "As many as you'd like."

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