[Ffmpeg-devel] [RFC] ratecontrol buffer size magic

[Michael Niedermayer]

Hi

On Thu, Apr 12, 2007 at 11:07:10AM +0200, Baptiste Coudurier wrote:
> Hi
> 
> Michael Niedermayer wrote:
> > Hi
> > 
> > On Wed, Apr 11, 2007 at 01:47:55PM +0200, Baptiste Coudurier wrote:
> >> Hi
> >>
> >> Michael Niedermayer wrote:
> >>> Hi
> >>>
> >>> On Thu, Apr 05, 2007 at 06:51:00PM +0200, Baptiste Coudurier wrote:
> >>>> Hi,
> >>>>
> >>>> I stumbed accross a weird behaviour of the CBR ratecontrol lately,
> >>>> basically doing:
> >>>>
> >>>>  ffmpeg -i file.mpg -vcodec mpeg2video -minrate 30000k -maxrate 30000k
> >>>> -b 30000k -intra -flags +ildct+low_delay -dc 10 -flags2
> >>>> +ivlc+non_linear_q -ps 1 -qmin 1 -qmax 12 -top 1 -vglobal 4 -acodec
> >>>> pcm_s16be -f mov -bufsize 1200000 -rc_init_occupancy 1200000 cbr.mov
> >>>>
> >>>> Constraints are so in order to achieve vbv delay as 3600.
> >>>> Now ffmpeg will only give me qp as 24. While tweaking I changed some
> >>>> magic in ratecontrol.c (see patch attached) and now qp have decent/good
> >>>> values.
> >>>>
> >>>> I dig into svn history and found out that r2601 bumped */2 to */3 to
> >>>> improve CBR ratecontrol. Can anyone explain in detail why that magic
> >>>> does improve behaviour ?
> >>> yes, your vbv buf size is VERY significantly too small it can barely
> >>> hold a single frame, increase it by a factor of 100 ...
> >>>
> >>> i hope theres no spec which mandates such nonsense parameters ...
> >>>
> >>>
> >>>> Can that parameter be user setable ? 
> >>> if you make it user settable ...
> >> Is something like that ok ?
> > 
> > yes except that name of the variable and the comment
> 
> +
> +    /**
> +     * Ratecontrol available buffer space coefficient applied when
> choosing qp.

does anyone understand what this variable does based on this description?
no?
if no then patch rejected

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Opposition brings concord. Out of discord comes the fairest harmony.
-- Heraclitus
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