[Ffmpeg-devel] Bug in snow revision 1.50 - broken lossless mode

[Michael Niedermayer]

Hi

On Wednesday 13 April 2005 21:26, D Richard Felker III wrote:
> On Wed, Apr 13, 2005 at 12:13:53PM +0200, Michael Niedermayer wrote:
> > Hi
> >
> > On Wednesday 13 April 2005 06:03, Yartrebo wrote:
> > > Michael, those rescaled IDWT coefficients (revision 1.50) appear to
> > > have broken lossless mode quite bad. It's all fuzzy now and is similar
> > > in quality to quant=10 or 20. It happens for all inputs I've tried.
> >
> > the regression tests worked yesterday and they do test lossless with the
> > 5/3 wavelet
> > the 9/7 wavelet is with the scaling merged not bijective with fixed point
> > numbers, its bijective if rational or real numbers are used, but as the
> > 5/3 wavelet is superior for lossless (at least it was in my tests) thats
> > really irrelevant
> > but then again, dont hesitate to look at the lifting steps for the 9/7
> > wavelet, maybe you can come up with a better solution so that this
> > dissadvantage can be avoided
>
> is the problem just that we have too few bits? because fracbits count
> certainly be increased... i.e. we never seem to use more than 24 bits
> or so out of the 32...

seems you missunderstand how lossless wavelet compression works
the trick is to use 0 fractional bits, and no quantization or dequantization, 
now the new 9/7 wavelet does a rescaling step by 4/5 during encoding and 5/4 
during decoding which obviously wont be lossless that way, OTOH you can 
certainly use enough fractional bits and accurate enough quantization to 
reach lossless but that would need significantly more bits then 0 fractional 
bits without quantization

[...]
-- 
Michael

"nothing is evil in the beginning. Even Sauron was not so." -- Elrond