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

[Michael Niedermayer]

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 ...


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

I know you won't believe me, but the highest form of Human Excellence is
to question oneself and others. -- Socrates
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