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

[Baptiste Coudurier]

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 ...

yes, point is to only have 1 frame at 25 fps in vbv I guess, for NTSC it
is 3003. I wish I could increase it, you get very good encoding even
with vbv 36000.

> i hope theres no spec which mandates such nonsense parameters ...

there is unfortunately... D10 ;)

>> Can that parameter be user setable ? 
> 
> if you make it user settable ...

ok.

-- 
Baptiste COUDURIER                              GnuPG Key Id: 0x5C1ABAAA
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