[FFmpeg-user] Problem concatenating segments

[Yellow Penguin]

Try set-pts=0 as a filter

On Fri, Jan 24, 2020 at 5:24 PM George R. Welch <george at grwelch.com> wrote:

> Greetings,
>
> I get video files from another division.  It is my task to cut out a
> part of the video, and replace it with a part I create.  The videos I
> receive are h264 with aac audio.  It is very strongly hoped that this
> can be done without re-encoding the parts of the original which we keep.
>
> I have been using ffmpeg for cutting up the file I receive, and I've
> been using the concat demuxer to concatenate the pieces of the original
> with the part I make.  This has worked well (over a dozen times) until
> recently.
>
> In order for concat to work correctly, I take care to make sure the
> video I make fits the one I am inserting it into (same frame rate, refs,
> etc).  I also take care to only cut the original movie at I frames.
>
> I have run into trouble with the most recent job.  When I concatenate
> the parts, there is a slight glitch when it transitions from the part I
> made to the other.
>
> As far as I can tell, the only thing that is different from the previous
> runs is that the file I am given is encoded with B frames, whereas that
> had never happened before.
>
> An oddity is that when I cut the original video, the timestamp does not
> begin at zero:
>
> $ ffmpeg -i input.mkv -ss [start-time] -t [duration] -c copy part1.mkv
> [seems to work]
>
> Now look at the result:
>
> $ ffprobe -hide_banner -show_frames -select_streams v part1.mkv
>
> Here is the first frame:
>
> [FRAME]
> media_type=video
> stream_index=0
> key_frame=1
> pkt_pts=1968
> pkt_pts_time=1.968000
> pkt_dts=1968
> pkt_dts_time=1.968000
> best_effort_timestamp=1968
> best_effort_timestamp_time=1.968000
> ...
>
> I don't understand why the first frame does not start at zero.  I have
> tried adding "-seek_timestamp 1" but it has no effect.
>
> When I use concat to place this part after my video, then there is a
> glitch at the transition.
>
> However, if I change the flag to the input:
>
> $ ffmpeg -ss [start-time] -i input.mkv -t [duration] -c copy part1.mkv
>
> then ffmpeg generates some errors:
>
> [matroska @ 0x7ffadb028600] failed to avoid negative pts -58 in stream 1.
> Try -avoid_negative_ts 1 as a possible workaround.
>
> Repeating the command with -avoid -avoid_negative_ts 1 still generates
> the same message, and the output is the same.
>
> Now when I look at the first frame it does start at zero, but there is
> still a glitch at the transition after concatenating.
>
> Does anyone have any suggestions?  Is it impossible to concat segments
> with B frames?
>
> Thank you in advance,
>
> --George
>
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