[FFmpeg-devel] [RFC]Improve adp detection

Michael Niedermayer michaelni at gmx.at
Wed Oct 30 13:06:44 CET 2013

On Wed, Oct 30, 2013 at 10:32:13AM +0000, Carl Eugen Hoyos wrote:
> Michael Niedermayer <michaelni <at> gmx.at> writes:
> > On Wed, Oct 30, 2013 at 08:39:34AM +0000, Carl Eugen Hoyos wrote:
> > > Michael Niedermayer <michaelni <at> gmx.at> writes:
> > > 
> > > > if you print them for the files we have do some of these 4bit
> > > > values never occur ?
> > > 
> > > I suspect only 0, 1, 2, 3 actually occur but that does not 
> > > help for the "000000000000000..." case or do I miss something?
> > 
> > if only 0123 occur then that could be checked for
> This would break (theoretical) files that the decoder accepts 
> without fixing anything.

the decoder accepts any input, even any that fails the current
probe function.

Also our decoder is not the official specification of this format,
just because it accepts something doesnt mean its actually valid

And being valid doesnt mean it actually occurs in the wild

ideally anything valid and anything decodeable should be probed with
high score but that doesnt work here as /dev/random is decodeable
and we dont know whats valid as we have no specification, so we can
only guess here

whats left is to try to detect what actually exists in the wild and
apply some common sense on top of that

the 4 bits select the predictor, only 4 predictors are defined in
the decoder. I suspect its unlikely that if a 5th value would occur
that it would just be treated as one of the 4. But thats a guess of
course ...

> > it doesnt fix the 0000000 issue
> Could that be intended behaviour?

i see nothing that would make the case of all header bytes being 0
to be non valid
in combination with the other bytes being 0 too though, 0 headers
might be non occuring with actual encoders

Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Into a blind darkness they enter who follow after the Ignorance,
they as if into a greater darkness enter who devote themselves
to the Knowledge alone. -- Isha Upanishad
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