[XviD-devel] Qpel bug - please help...

[Radek Czyz]

Hey everyone.

I've been tracking qpel artifacts ever since qpel appeared - but with
no success.
Let me explain: qpel gets decoded wrong by different decoders. It's
difficult to tell how wrong, but it's mostly visible when new
keyframe changes the picture.

I've hacked motion estimation to create a clip when the problem is
clearly visible.
Please load a short clip from http://noa.sm.pl/~syskin/test.avi .
Second row of macroblocks always has the vectors of (2, 0) (in qpel
resolution). Second colum of macroblocks always has (0,2). Diagonal
macroblocks have (2,2).

This is what happens: if xvid decodes the clip, it's ok.
If ffdshow decodes the clip, we can see that all 3 types of
interpolation are decoded completely wrong, which causes artifacts
over time (note: xvid iDCT, but it's the same with other iDCTs, so
it's not an iDCT problem).
Now the funny thing: if DivX5 (version 5.01, I don't have any other)
decodes the clip, only HV interpolation (diagonal macroblocks) are
wrong - H and V are correct. (?)

I'd really appriciate if someone could test decoding with more
decoders - reference ones would be great. I really don't know if it's
xvid's problem or ffdshow's and divx5's.... But it's a problem, I'm
sure of it.

Thanks,
Radek