Re: [mpeg2-dev] A funny stream

[Michel L. <wa...@zo...>]

On Tue, Nov 06, 2001 at 06:45:41PM +0700, Peter Gubanov wrote:

> Don't forget about quantizer and saturation. I'm not sure there are
> no bugs in the encoder. I guess the inverse quantizers are slightly
> different, and this iduces errors %-) But you are right, equal
> routines produce equal rounding errors, so the result in both
> decoders should be equal.

Yes, normally the only possible difference should be in the IDCT, so
any differences should be either due to a bug or to IDCT differences.

> >This weekend I've been looking at an ffmpeg bug related to IDCT and I
> >couldnt put my finger on it. They use our IDCT and for some streams
> >(basically one stream that has very little I pictures, I think one I
> >every 100 or 132 pictures) they get bad error accumulation, in the
> >form of horizontal stripes and/or color drift. Someone came up with a
> >different IDCT routine, that doesnt behave much better than ours in
> >the IEEE tests (ok, about 5-10% better), but does not show up this
> >error accumulation. This is all in their divx codec. But I dont
> >understand: according to the IEEE tests our worst mean square error is
> >0.01, and our worst mean error (which is what should really matter for
> >error accumulation) is 0.0026. So even if you multiply these by 100 or
> >132, you really shouldnt get anything noticeable at all !!!!! somehow
> >it seems that the stastistical tests do not match the behaviour of
> >their divx codec, and I can not understand why at all.

> Are you sure they haven't changed anything else except the idct? They 
> can add error in motion compensation, block adding, etc. Stripes don't 
> appear suddenly after 132 frames. And if there is a problem with idct, 
> they should appear on luma much earlier.

its not suddenly - they appear progressively, and then disappear
suddenly (probably at the next I frame).

> I'd like to take a look at this first before I say anything.

Download mplayer: http://www.MPlayerHQ.hu/homepage/dload.html

Download streams from
ftp://mplayerhq.hu/MPlayer/samples/ffmpeg-bugs/HorizontalStripesDivX.avi
and
ftp://mplayerhq.hu/MPlayer/samples/Divx4-bugs/Lorenna_McKennit-Mummers_Dance-Mononoke_Hime-gabucino.avi

run with mplayer -vc ffdivx foo.avi

apply the attached patch and compare

HorizontalStripes has (faint, but still visible, and statistically
they shouldnt be visible) stripes, and Lorenna has weird colors at
some points.

-- 
Michel "Walken" LESPINASSE
Is this the best that god can do ? Then I'm not impressed.