[FFmpeg-devel] [PATCH] AAC Pulse data boundaries

[Michael Niedermayer]

On Sun, Sep 14, 2008 at 10:48:17PM -0400, Alex Converse wrote:
> On Sun, Sep 14, 2008 at 8:55 PM, Michael Niedermayer <michaelni at gmx.at>wrote:
> 
> > On Mon, Sep 15, 2008 at 12:36:21AM +0100, Robert Swain wrote:
> > > 2008/9/14 Alex Converse <alex.converse at gmail.com>:
> > > > Currently the AAC pulse_data tool can try to apply pulses to offsets
> > outside
> > > > of the boundaries of the spectral coefficients. This leads to a crash
> > > > (SIGSEGV) with a stream generated wit the following command:
> > > > zzuf -s16 -r0.001:0.1 -b1193- <
> > > > mpeg4audio-conformance/compressedMp4/al04_08.mp4 > fuzzed.mp4
> > > >
> > > > Attached is a patch to clip pulse positioning to 1023.
> > >
> > > The code changes look OK but is clipping the best solution to this or
> > > should those pulses rather be discarded? Michael?
> > >
> > > It would seem to me that if there are multiple pulses, clipping them
> > > could lead to multiple pulses being applied to position 1023. Is a
> > > spike in the spectral data better than no spike or vice versa? My
> > > guess is that ignoring incorrectly positioned pulses may be a better
> > > option.
> >
> > Well as you ask :)
> > Bitstream errors should cause the data until the next resync possibility
> > to be discarded. It also should discard some of the pevious data as the
> > bitstream error likely happened earlier.
> > In practice that likely means replacing the current frame by silence, that
> > is before the windowing. Or maybe to duplicate the last frame.
> >
> 
> I was under the impression that the MPEG-4 Visual decoder accepts all kinds
> of things that aren't quite legal.

There are 2 different things
1. invalid bitstreams due to buggy encoders
2. invalid bitstreams due to bit/byte/block/sector/packet damage.

Until we have seen a buggy encoder encoding invalid pulses its IMO
reasonable to assume that no such encoder exists, if one does we
will get a bugreport and can adapt our decoder.
And if needed use error_recognition to decide where to draw the line
between points 1 and 2.


[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The greatest way to live with honor in this world is to be what we pretend
to be. -- Socrates
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