[FFmpeg-user] Why are these different?

Mark Filipak markfilipak.imdb at gmail.com
Sun Feb 11 23:01:37 EET 2024

-- 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 

> 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.

> 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.

More information about the ffmpeg-user mailing list