[MPlayer-dev-eng] [PATCH] further dvr-ms playback improvements

[Trent Piepho]

On Wed, 27 Sep 2006, John Donaghy wrote:
> I've attached an improved patch that works by continuously
> recalculating the  'average frame time' rather than simply reading it
> from the container. The reason for doing it this way is that the value
> in the container is wrong in at least one example I have (it may well
> be wrong for all HD recordings).

+ asf->avg_vid_frame_time = (float)((double)time*0.001f/(double)asf->vid_frame_ct);

Since avg_vid_frame_time is a double, what is the point of the float cast?
Also, floating point constants are type double by default.  Just use this:

asf->avg_vid_frame_time = time*0.001/asf->vid_frame_ct;

> This approach works well for all the examples I have as long as the
> user doesnt seek too early  - ie before an accurate average has been
> calculated. Once a seek is performed we no longer know the frame
> number and so cant calculate the average frame time. There may be ways
> to improve upon this and I'm open to suggestions.

Try printing out avg_vid_frame_time and vid_frame_ct for each frame and
then graphing them.  That should give you a good idea of how long it takes
to converge on the correct value.

If there is a seek before this occurs, reset the averager to the current
frame and start over (you could try to keep the data already collected, but
it doesn't seem worth the trouble).

It would look something like this:

if (asf->vid_frame_ct == -1) {
    // Seek has reset averaging data, this frame is new start point
    asf->start_time = time;
    asf->vid_frame_ct = 0;
} else if (asf->vid_frame_ct >= 0) {
    // Estimate average frame time by dividing the total time between our
    // averaging start frame and this frame by the number of frames.
    // If each frame time has an maximum error of e, then the max error
    // of the estimate is 2 * e / vid_frame_ct.
    asf->vid_frame_ct++;
    asf->avg_vid_frame_time = (time - asf->start_time)*0.001/asf->vid_frame_ct;
}
// else, avg_vid_frame_time is already estimated, do nothing

And in the seek function:

if (asf->vid_frame_ct < XXXX) {
    // Not enough frame to have an accurate estimate of average frame time
    asf->vid_frame_ct = -1;  // -1 == start over
} else {
    // Estimate is good enough, don't bother calculating a new one
    asf->vid_frame_ct = -2;  // -2 == finished estimating
}

For extra credit, keep using the old average after a seek until the new
average is based on more samples than the old one.