[FFmpeg-user] Ticket#5936: Purple haze on ffmpeg mpeg4 decoding

[Toerless Eckert]

Btw:

Looking at: https://github.com/FFmpeg/FFmpeg/commits/master/libavcodec/mpegvideo_motion.c

 It seems that every second commit in 2016/2015 changes the code back and force between the "good old" code (no purple haze) - eg: with variable "uvbuf" and the "new" code - eg: with variable ubuf/vbuf. There is also 39a2d3288e82e4e576c03efb32179ef5a19fff50 that pulled out the code into a separate function (using the good old code), but that fix also vanished later on. Without an explanation why (that i could find).

Quite confusing.

On Sat, Nov 12, 2016 at 07:27:53AM +0100, Toerless Eckert wrote:
> Thanks a lot, Andy
> 
> Mind explaining how you did that trick with git bisect ? 
> 
> Just verified:
> 
> Applying c5fc8ae12622a507d7b9ee30ddcd3734e6de6b1d to a working older version
> of ffmpeg introduces the purple haze.
> 
> Reverting c5fc8ae12622a507d7b9ee30ddcd3734e6de6b1d even in latest 3.2 makes
> purple haze go away.
> 
> Of course, i have no idea how/what type of problem with vsynth3-wmv2.avi
> the fix was meant to correct. Just looking at the clip i couldn't figure it out.
> 
> Coulnd't make heads or tails out of the fate testing for vsynth3-wmv2, with
> 3.2, the output i see is:
> 
>     tests/data/fate/vsynth3-wmv2.rep 
>       vsynth3-wmv2:0::
> 
> and with e12622a507d7b9ee30ddcd3734e6de6b1d, it shows:
> 
> vsynth3-wmv2:0::KysgZW5jX2Rl..... (and so on, long).
> 
> 
> On Thu, Nov 10, 2016 at 06:33:28PM +0000, Andy Furniss wrote:
> > Toerless Eckert wrote:
> > >Just opened https://trac.ffmpeg.org/ticket/5936
> > 
> > >git diff 03bb99ae1a99fa315621308b885a8fc70702c9bc 9236f7b5a23b4907f7b2bf6346ecd88e6d76f1e0
> > >produce 700k diff and i can not figure out what a likely cause is ;-(
> > 
> > Git bisect - can sometimes be tricky, but this was easy (he says
> > hoping he found the right commit).
> > 
> > c5fc8ae12622a507d7b9ee30ddcd3734e6de6b1d is the first bad commit
> > commit c5fc8ae12622a507d7b9ee30ddcd3734e6de6b1d
> > Author: Michael Niedermayer <michaelni at gmx.at>
> > Date:   Fri Jun 13 01:45:23 2014 +0200
> > 
> >     avcodec/mpegvideo: fix edge emulation with uvlinesize below 25
> > 
> >     Fixes fate-vsynth3-wmv2
> > 
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> > 
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> 
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> tte at cs.fau.de
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