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

Michael Niedermayer michaelni
Wed Apr 13 22:18:22 CEST 2005

```Hi

On Wednesday 13 April 2005 20:53, Yartrebo wrote:
> On Wed, 2005-04-13 at 12:13 +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
> >
> > [...]
>
> I've got another idea too ... writing either a brute force, a hill
> climbing, or a genetic algorithm (depending just how big the problem
> space is

its as big as you like, you could certainly consider any sequence of
operations which transform a 2d array into another :)
or limit yourself to separable transforms (1d vertical, 1d horizontal)
linear ones
symmetric ones
compact (finite number of non zero coefficients)
length 9/7
4 lifting steps where each is symmetric and uses only the adjacent 2 samples
...

> and what rules I can figure out by hand) to find the simplest
> lift implementation for a given set of inputs and outputs (fastest to
> compute, for inputs limited to a certain # of bits, and operations
> bounded to 32 bit intermediates).
>
> Maybe the algorithm could even find a better implementation that the
> current one.
>
> Any comments on this approach?

iam not sure you have already seen it but, if not, maybe you want to look at
the wave.c file i posted here, a few days/weeks ago

[...]
--
Michael

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

```