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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