[issue1081] lavc mepg2video buffer underflows: reproduced and pinpointed

[Francois Visagie]

Francois Visagie <francois.visa...@gmail.com> added the comment:

On Sun, May 24, 2009 at 2:22 AM, Michael Niedermayer <
iss...@roundup.ffmpeg.org> wrote:

>
> Michael Niedermayer <michae...@gmx.at> added the comment:
>
> [...]
> > > > COMMAND LINE 1 RESULTS
> > > > ----------------------
> > > > No "impossible bitrate constraints, this will
> > fail"
> > > > warning
> > > > Buffer underflow
> > >
> > > > Video Peak 9845 Average 8398
> > >
> > > What tool was used to find these 2 values?
> >
> > "MPEG Bitrate Viewer" (bitrate.exe) 1.5.054
>
> Considering that google shows zero hits for
> "mpeg bitrate viewer" "video buffer verifier"
>
> I have no faith that this tool implements a VBV, and if it does not, its
> output would be wild guesses and meaningless

For this encode it reports "VBV buffer size: 112".

<< snip >>

> also id like to repeat that the target dvd option enables a maxrate and
> that your adding of a maxrate command line option really is just changing
> its value not adding a limit where none was before.

I could find no description of what effects the ...dvd options have, which
is why to be safe I specify my own upper limits. Including -muxrate 10080000
to ensure total bitrate (including mux overhead even) remain within the
maximum total bitrate mentioned at http://en.wikipedia.org/wiki/Mpeg-2 and
elsewhere.

When pal-dvd has been specified, what are its effects and what if any
additional options are required to ensure or improve DVD-Video compliance?
Or can you point me to comprehensive documentation on this?

Thank you,
Francois

_____________________________________________________
FFmpeg issue tracker <iss...@roundup.ffmpeg.org>
<https://roundup.ffmpeg.org/roundup/ffmpeg/issue1081>
_____________________________________________________

On Sun, May 24, 2009 at 2:22 AM, Michael Niedermayer <iss...@roundup.ffmpeg.org> wrote:

Michael Niedermayer <michae...@gmx.at> added the comment:

[...]
> > > COMMAND LINE 1 RESULTS
> > > ----------------------
> > > No "impossible bitrate constraints, this will
> fail"
> > > warning
> > > Buffer underflow
> >
> > > Video Peak 9845 Average 8398
> >
> > What tool was used to find these 2 values?
>
> "MPEG Bitrate Viewer" (bitrate.exe) 1.5.054

Considering that google shows zero hits for
"mpeg bitrate viewer" "video buffer verifier"

I have no faith that this tool implements a VBV, and if it does not, its
output would be wild guesses and meaningless

 

For this encode it reports "VBV buffer size: 112".

 

[...]
> > >
> > > WHAT WE KNOW SO FAR
> > > -------------------
> > >
> > > a) Buffer underflows seem to occur more severely
> with "high
> > quality"
> > > encodes that involve e.g. -mbd 2 - trellis. Both
> average and peak
> > > bitrate are also higher.
> >
> > > b) Hervé found that reducing -buf_size avoids
> excessive
> > bitrate, but
> > > at the expense of DVD-Video compliance of course.
> It's
> > unclear whether
> > > this was in the presence of the -maxrate option.
> >
> > what i know is that you guys dont know too much.
>
> That is quite correct in my case :-).
>
> > a increase in bufsize breaks compliance, a decrease
> does not
> > normally also iam not convinced that there is any
> bug in
> > libav* at all at least you have not provided a non
> > contradictionary case that produced any issues
> >
> > [...]
> > > CLOSE
> > > -----
> > >
> > > I am under lots of pressure to finish my current
> DVD magnum
> > opus soon,
> > > and if there's any prospect of investigating and
> hopefully
> > fixing this
> > > bug in the near future, please let me know where I
> can help with
> > > testing and/or more information.
> >
> > given that you managed to feed ffmpeg with
> contradictionary
> > parameters the buffer underflows you report are just
> a case
> > of garbage in, garbage out.
> >
> > If you can provide a minimal command line with non
> > contradictionary paremeters (you need a gap between
> muxrate
> > and the sum of all video and audio max rates), how
> large that
> > gap has to be, i dont know but not 0, and also not
> 0.1%,
> > mpeg-ps is a mildly bloated format it does need some
> space
> > for its bloat!
>
> Yes.
>
> The very compelling evidence is on my hard drive. It
> involves an encode with more real-world bitrates:
>
> ffmpeg -i "Movie.avi.37.avs" -target pal-dvd -muxrate
> 10080000 -acodec ac3 -ab 128000 -flags ildct+ilme -g
> 15 -vcodec mpeg2video -b 6300000 -pass
> 1 "Movie.avi.37.mpg"
> ffmpeg -i "Movie.avi.37.avs" -target pal-dvd -muxrate
> 10080000 -acodec ac3 -ab 128000 -flags ildct+ilme -g
> 15 -vcodec mpeg2video -b 6300000 -pass
> 2 "Movie.avi.37.mpg"
>
> That encode works.
>
> When I add -maxrate 9800000, the average and peak
> video bitrates shoot up and (on pass 2) buffer
> underflows result.
>
> Much more believable evidence? ;-)

nope,
10080k - 128k - 9800k = 152k
152k / 10080k = ~ 0.015 that is 1.5%

1.5% might simply not be enough for mpeg-ps overhead

<< snip >>

 

also id like to repeat that the target dvd option enables a maxrate and
that your adding of a maxrate command line option really is just changing
its value not adding a limit where none was before.

 

I could find no description of what effects the ...dvd options have, which is why to be safe I specify my own upper limits. Including -muxrate 10080000 to ensure total bitrate (including mux overhead even) remain within the maximum total bitrate mentioned at http://en.wikipedia.org/wiki/Mpeg-2 and elsewhere.

 

When pal-dvd has been specified, what are its effects and what if any additional options are required to ensure or improve DVD-Video compliance? Or can you point me to comprehensive documentation on this?

 

Thank you,

Francois