[FFmpeg-user] concat loses 2 frames or freezes up video

[Nicolas George]

Le septidi 17 frimaire, an CCXXIII, S Andreason a écrit :
> Test #1. When I concat trimmed clips made with ffmpeg-0.5, it loses
> or drops 2 frames from the first file. Playback has a jump over the
> lost frames, and a frame by frame count reveals 28 frames in the
> second as indicated by the time position as reported by mplayer on
> screen (pressing [o]) instead of the 30 before and after.

As you noticed, the file cut with ffmpeg-0.5 does not have the exact audio
duration: its video stream is exactly 13 seconds long, but the audio is
13.056 seconds.

When concatenating, the extra 0.056 seconds are taken into account, shifting
the audio and video of the second segment. This produces output with
variable frame rate at the stitching point. The MOV muxer in FFmpeg is said
to handle VFR badly, so you might want to check with a better format. And
anyway, a small stutter is to be expected when playing frame by frame with
omitted frames.

> Test #2. When I concat a trimmed clip with one not trimmed, still
> having the encoding and metadata from the camera, then at the joint
> the video freezes up, and pressing the right arrow (in mplayer) to
> advance the time position, reveals it thinks the video is very long
> now.

You can see in ffprobe that the original file has time base 1/600 for the video
stream while the t2-13 has time base 1/30. Files with different time bases
are not supported currently, there is still work to get it right.

> Test #3. When I trim a clip with newest ffmpeg, then the concat
> operation output has many errors displayed.
> Playback is okay until the joint, when video continues at an
> accelerated rate without any audio, and time position stays stuck.
> After a while the audio resumes in bursts followed by more silence,
> so it stutters badly.

I suspect the problem is the same as test #2: I can see none of these errors
if I try to concatenate c13-3 with itself instead of with the untrimmed
file.

Regards,

-- 
  Nicolas George
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