[FFmpeg-devel] [PATCH] Maximum bitrate FAQ

[Michael Niedermayer]

Hi

On Mon, Sep 03, 2007 at 09:14:29PM +0200, Stefano Sabatini wrote:
> On date Saturday 2007-09-01 15:08:29 +0200, Michael Niedermayer encoded:
> > Hi
> > 
> > On Sat, Sep 01, 2007 at 12:59:58PM +0200, Stefano Sabatini wrote:
> > > Hi all.
> > > 
> > > Suggested log: add a discussion for the FAQ: "How can I encode a video
> > > without going over a maximum bitrate?"
> > > 
> > > The given reply is likely somehow incorrect, so improvements and
> > > suggestions will be appreciated.
> > 
> > please test and mention -mbd rd -flags trell and large -lmax
> > 
> > (testing is so you see yourself what effect they have on the over/underflows
> > ... they should have a good one but i think the text will be better if the
> > author has used the options himself instead of repeating what someone else
> > says ...)
> 
> I tested -mb rd and -mb bits getting in some cases an output size 5%
> littler, with an encoding time increase of about 30%, in other cases a
> less significant improvement; I got similiar results with -flags trell.
> 
> For what regards -lmax: I really can't figure out what does it mean,
> I tried several values getting the best result (lower bitrate) at 10k,
> increasing -lmax the bitrate seems to grow when the quality seem to
> stay the same.
> 
> I feel like I'd have to understand libavcodec/ratecontrol.c ;-).

well ...
here are some actual examples to show the effect and as startpoints
for experimentation

ffmpeg -f rawvideo -s cif -i foreman_cif -b 50k -bt 100k -g 300 -maxrate 70k -bufsize 100k test.avi
117 x [mpeg4 @ 0xXXXXXXXX]rc buffer underflow
frame=  300 fps=116 q=31.0 Lsize=     147kB time=12.0 bitrate= 100.4kbits/s    
video:134kB audio:0kB global headers:0kB muxing overhead 9.471236%


ffmpeg -f rawvideo -s cif -i foreman_cif -b 50k -bt 100k -g 300 -maxrate 70k -bufsize 100k -mbd rd -flags trell -lmax 100*QP2LAMBDA test.avi
no buffer underflows
frame=  300 fps= 52 q=45.1 Lsize=      86kB time=12.0 bitrate=  58.7kbits/s    
video:73kB audio:0kB global headers:0kB muxing overhead 17.369228%

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

Republics decline into democracies and democracies degenerate into
despotisms. -- Aristotle
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