[FFmpeg-devel] [PATCH] avcodec/svq1: fix interframe mean VLC symbols

[Peter Ross]

On Mon, Oct 17, 2022 at 05:04:29AM +0200, Andreas Rheinhardt wrote:
> Peter Ross:
> > Fixes ticket #128.
> > 
> > The SVQ1 interframe mean VLC symbols -128 and 128 are incorrectly swapped
> > in our SVQ1 implementation, resulting in visible artifacts for some videos.
> > This patch unswaps the order of these two symbols.
> > 
> > The most noticable example of the artiacts caused by this error can be observed in
> > https://trac.ffmpeg.org/attachment/ticket/128/svq1_set.7z '352_288_k_50.mov'.
> > The artifacts are not observed when using the reference decoder
> > (QuickTime 7.7.9 x86 binary).
> > 
> > As a result of this patch, the reference data for the fate-svq1 test
> > ($SAMPLES/svq1/marymary-shackles.mov) must be modified. For this file, our
> > decoder output is now bitwise identical to the reference decoder. I have
> > tested patch with various other samples and they are all now bitwise identical.
> 
> Seems like this is not the only test whose reference needs to be
> updated. There are also the fate-vsynth%-svq1 tests.

Right, the encoder. This change will cause previous videos created by the FFmpeg
encoder to playback *with* artifacts in newer builds of FFmpeg with this patch
applied. There is not much we can do about this, since there isn't place for a
version string in the SVQ3 bitstream (unlike MPEG-4 etc).

-- Peter
(A907 E02F A6E5 0CD2 34CD 20D2 6760 79C5 AC40 DD6B)
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