[FFmpeg-user] 2nd pass has more frames than 1st pass (99021)2kbits/s dup=27 drop=0 speed=4.76x

[Carl Eugen Hoyos]

Am Mo., 30. Nov. 2020 um 08:57 Uhr schrieb Mathieu Malaterre
<mathieu.malaterre at gmail.com>:

> > > $ ffmpeg -y -i input.avi -c:v libx264 -b:v 1321894 -pass 1 -an -f null
> > > /dev/null && \
> > > ffmpeg -y -i input.avi -c:v libx264 -b:v 1321894 -pass 2 -c:a aac -b:a
> > > 128k fixed.mp4
> >
> > This cannot work (and this is not related to the fact that you are not
> > using a version of FFmpeg that we support here).
> >
> > In some cases - if you are lucky - there is a crash and no output file
> > is written, in other cases, a "broken" output file is written, broken in
> > the sense that two-pass encoding did not work the way it is supposed
> > to (more bitrate was given to frames that do not need it).
> >
> > Do not use -an on the first pass and add "-vsync cfr" to the first pass.
> > (Other workarounds are possible.)
>
> Confirmed. The above suggestion does fix the symptoms.

I would say it fixes the cause.

> Did you base your suggestion on:
>
> "Error while decoding stream #0:1: Invalid data found when processing input" ?

No, I know that there are many guides suggesting to use -f null for two-pass
encoding with FFmpeg where the second pass outputs to mov, these guides
are typically broken.

> > For future questions: Do not cut FFmpeg's console output, only
> > post ffprobe output if you see an issue with it.
> >
> > Not really related:
> > > Does it make sense to simply copy/paste the existing bit_rate to
> > > re-encode it ?
> >
> > No, to the best of my knowledge, it makes no sense whatsoever.
> >
> > Very generally you either want a specific output quality, in this
> > case specifying a bitrate and two-pass encoding make no
> > sense.
> > Or you have a particular size requirement for the output file,
> > then bitrate and two-pass encoding make sense.
>
> Are you saying that taking the original file size as size requirement
> is the whole issue ?

Partly.
If you need the original file size (but for example another codec) then
two-pass encoding should provide the best quality (if done correctly).

But this is an extremely unlikely use case since DVD's are less
popular to store videos. Nowadays, what you (likely) want is to
specify an output quality (how depends on the encoder) and let
the encoder decide how to produce the quality you requested.

Carl Eugen