[XviD-devel] GME and Block ME

Christoph Lampert chl at math.uni-bonn.de
Sat Apr 19 14:15:11 CEST 2003


On Sat, 19 Apr 2003, Michael Niedermayer wrote:

> Hi
> 
> On Saturday 19 April 2003 08:56, Marco Al wrote:
> > From: "Michael Niedermayer" <michaelni at gmx.at>
> >
> > > yes sounds better, ill change ffmpeg/mencoder docs to that :)
> >
> > Hey, since you are reading this a question :) Does your code do an optimal
> > search or an M-best search?
> optimal, except that only 0, level, level-1 are checked, IIRC additionaly 
> checking level-2 had only a few byte lower filesize at constant quant, the 
> level-2 code is there and outcommented if u want to try
> 
> for h263 quantizers a larger level difference should not be needed
> the dequantized level stepsize is 2*qp and so for level-3 (!= 0) we need to 
> gain at least ((2.5*2*qp)^2 - (0.5*2*qp)^2)/lambda bits where lambda is 
> 0.85*qp^2 as recommanded by some doc

Hi, thanks for explainations, and btw. the lambda values would have
been my next question anyway: ffmpeg's mpegvideo.c reads 

   lambda= (qscale*qscale*64*105 + 64)>>7; //FIXME finetune

and 

   int score= distoration + length[UNI_AC_ENC_INDEX(run,level)]*lambda;

which I read as  lambda = 64*105/128 * qscale^2  

Where does the extra factor of 64 come from (or 16 perhaps, considering 
stepsize = 2*qscale) ?

gruel







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