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

[Michael Niedermayer]

Hi

On Thursday 14 April 2005 00:11, D Richard Felker III wrote:
> On Wed, Apr 13, 2005 at 10:00:00PM +0200, Michael Niedermayer wrote:
> > 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
>
> couldn't you scale by 8/5 and 5/8 instead so that the mapping would be
> injective?

yes, but it would make the other lifting steps slightly more complex, and 
increase the number of bits needed in the lowpass band by 1 per 
decomposition, which not only risks overflows and also isnt too good lossless 
compression wise

[...]
-- 
Michael

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