[FFmpeg-user] cutting mp4 with ffmpeg

[Andrew Randrianasulu]

On Sat, Jan 13, 2024 at 10:27 PM Mark Filipak
<markfilipak.imdb at gmail.com> wrote:
>
> On 13/01/2024 12.50, Andrew Randrianasulu wrote:
> > On Sat, Jan 13, 2024 at 8:57 PM Def Etienne <shaker.doc at gmail.com> wrote:
> -snip-
> >> ffmpeg -i ~/6029.mp4  -copyts  c:v copy -c:acopy -t 64.600 dev/shm/6029_cut.mp4
>
> According to Devon Heitmueller -- thanks again, Devon -- '-copyts' and '-muxdelay 0' are _both_ needed.
>
> '-copyts' -- before the input -- bypasses a routine that apparently is named "tsfixup".
>
> '-muxdelay 0' -- after the input -- bypasses another ts fixup that defaults to adding 0.7 seconds to
> the timestamps.
>
> Exactly why ffmpeg does those fixups is a mystery.
>
> I can confirm that when cutting off the beginning:
> ffmpeg -copyts -ss ... -i ... -c copy -muxdelay 0 ...
> works. It preserves the timestamps (DTS, PTS).
>
>
> Regarding cutting off the ending:
> ffmpeg -to ... -i ... -c copy ...
> I am trying to determine the behavior of '-to'. Is it PTS? Or is it ticks (i.e., duration in
> timebase units)? And does ffmpeg do fixups for '-to' clauses, also? It appears there are such '-to'
> fixups.
>
>
> Also be aware that MPV rounds time, and rounds to milliseconds whereas ffmpeg truncates time and
> truncates frame numbers, DTSs, and PTSs. So times from MPV may miss an I-frame in ffmpeg. What I do
> is use MPV times to find approximate frame number. Then I use '-vf showinfo' to find exact frame
> number. Then I compute exact DTS and exact PTS from exact frame number. Then I convert DTS and PTS
> to nanoseconds in the '-ss' clause.

I added ticket about ffprobe/vf_showinfo displaying truncated duration:

https://trac.ffmpeg.org/ticket/10799#ticket


>
> Why does 'ffmpeg -ss' use times instead of relative timebase ticks (i.e. durations) or absolute DTSs
> and PTSs? Your guess is as good as mine, but it is a super PITA.
>
> Why does ffmpeg apply ts fixups? Your guess is as good as mine, but it is a super PITA.
>
>
> Also, be aware that in this listing:
>
> > [Parsed_showinfo_0 @ 0xee202580] n:1612 pts:5803200 ...
>
> the reported PTS is _not_ PTS. It is ticks (i.e., duration in timebase units) that is marked "pts"
> but isn't -- another super PITA.
>
>
> Also, remuxing from M2TS to MP4 for example, applies more unknown fixups even when they have the
> exact same timebase -- another super PITA -- so don't change packet type until the very end of your
> timming and concatenating.
>
> --Mark.
>
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