[FFmpeg-user] cutting mp4 with ffmpeg

[Mark Filipak]

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.

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.