Re: [mpeg2-dev] A funny stream

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

On Tue, Nov 06, 2001 at 05:52:50PM +0700, Peter Gubanov wrote:
> >Oh ? weird. How does thar error look like, I suppose this is
> >high-frequency noise ?

> Not only HF - the picture becomes lighter and lighter, color shifts to 
> blue. Actually, this depends on the content, so the pciture may go 
> darker and red on different material. I guess this is natural, because 
> we use statistically accurate iDCT and MC, and rounding errors become 
> visible after 100 iterations on the same macroblock.

OK...

Well I still dont understand, because you said you use the same IDCT
in the encoder and the decoder, and in that case errors should not
accumulate... basically the prediction frames should be bitwise
identical in the encoder and the decoder and so there is no reason to
get error accumulation... is there ? Also given our statistics in the
IEEE tests, I dont see that the errors should be visible after 100
frames. In my maths the errors should still be a deviation of about 1
level of gray after 100 frames - not anything noticeable really.

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.

Any idea about this ?

Puzzled,

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