[FFmpeg-user] Why are these different?

[Paul B Mahol]

On Sun, Feb 11, 2024 at 10:01 PM Mark Filipak <markfilipak.imdb at gmail.com>
wrote:

> -- Forgive me if this is a duplicate (trouble with email client, again) --
>
> I think I can help out here, Vincent.
>
> On 11/02/2024 14.53, Vincent Deconinck wrote:
> > Hi,
> >
> > I will probably get flamed because I'm for jumping into this conversation
> > while being a complete newbie :-), but I think nobody replied with an
> > answer to Mark's question "why".
>
> Before "why" there's the question of "should". I backed off "why" and am
> now simply trying to
> determine whether '-ss'-before-'-i' _should_ be the same as
> '-i'-before-'-ss'.
>
> > All I know is the ffmpeg page on seeking -
> > https://trac.ffmpeg.org/wiki/Seeking - ends with a paragraph saying:
> > "Using -ss as input/output option *together with -c:v copy might not be
> > accurate* since ffmpeg is forced to only use/split on i-frames" (emphasis
> > mine)
>
> I split/trim _only_ on I-frames via '-ss value' in which 'value' has
> nanosecond resolution. I have
> confirmed that the trim PTSs are correct for '-ss'-before-'-i' but wrong
> by hundreds of frames for
> '-i'-before-'-ss'.
>
> > The page also states that:
> > "As of FFmpeg 2.1, when transcoding with ffmpeg (*i.e. not stream
> copying*),
> > -ss is now also "frame-accurate" even when used as an input option"
>
> I am not transcoding.
>

Codec copy with -ss before -i and after -i is not documented that it gives
different results
because once you do not decode everything from start you basically can not
expect to get
always (for any container and for any input) frame-accurate results in both
cases.


>
> > My understanding is thus that with input seeking, the requested start
> time
> > will not be satisfied without transcoding
>
> Your quotation is "when transcoding with ffmpeg". It apparently doesn't
> apply to "without transcoding".
>
> I asked
> Should Command 1 and Command 2 make the same outputs? "Yes" or "No"?
>
> Arguing (or even citing) the so-called documentation is pointless.
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