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

[Michael Niedermayer]

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 buffer space available coefficient used when
> +     * encoding with a buffer size.
> +     * - encoding: Set by user.
> +     * - decoding: unused
> +     */
> +    float rc_buffer_avail_coeff;
>  } AVCodecContext;

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

There will always be a question for which you do not know the correct awnser.
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