[FFmpeg-user] problem with fast seeking and splitting

[Paul B Mahol]

On Mon, May 2, 2022 at 7:27 AM Alexander Solonsky via ffmpeg-user <
ffmpeg-user at ffmpeg.org> wrote:

> Dear Experts,
>
> I'm trying to split an input file into 2 second chunks, transcode and
> assemble using a concat muxer. And experiencing two problems:
> major one:
> 1) when using ffmpeg -ss 00:00:08 -t 2 -i <input_file> -c:v libx264 ....
> instead of seeking to second 8, ffmpeg seeks somewhere to second 15 or so.
> The first segments are correct, and then it start to go haywire.
> Using accurate seek ffmpeg  -i <input_file> -ss 00:00:08 -t 2 -c:v libx264
> ....  resolves the problem. But it is completely unusuble if you want to
> split the whole file.
> Minor one:
> 2) when concatenating the resulting files - there happens a noticeable
> break between the chunks. For both audio and video and is equal to 2
> additional frame durations (e.g. prev chunk ends at 1.981333 and the next
> chunk (after concat) while it should start at 2.023000 actually starts at
> 2.095744 for 24 fps video, and for audio prev chunk ends at 1.927256 and
> starts at 2.023220 while it should start at 1.950476).
>
> adding all additional flags like copyts, copytb, enctimebase, vsync
> doesn't change anything. And it's really curious when using the hls demuxer
> - it splits the file accurately and fast, without these breaks.
>
> Can you please help me fix those two problems or suggest to understand
> which command like I could use to reproduce splitting behavior from the HLS
> demuxer?
>

It can not be fixed. You can not seek and start decoding without key frames.


>
> Thanks,
> Alexander
> //////////////
>
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