[FFmpeg-user] Problem concatenating segments

[George R. Welch]

Thank you for the suggestion.

As I understand it, filtering and stream copy cannot be used together.  
Am I wrong?  As I said, I must not transcode the pieces of the original 
video file I am using.

If I could transcode the original, this process would be easy.

--George

On 1/25/20 1:10 PM, Yellow Penguin wrote:
> 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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