Stairstepping artifact in XviD

[Raptus]

Hello,

I noticed an unexpected stairstepping artifact in XviD encoded movies, especialy visible on diagonals. Its patern is regular, repeating every 8 pixels (both horizontal and vertical), so my guess is that its somehow connected to macroblocks, though I dont know if on the encoding or the decoding part. During normal filtered playback its very hard to spot it, but when looking at it unfiltered with 2x zoom (in VDub) its quite visible. Here is a cropped example: http://www.sknop.net/files/xvidstairstepping.jpg

The movie was encoded using DVD2AVI + VFAPI + VirtualDub, XviD (koepis latest unstable), H.263, quant.2, B-frames 2, VHQ 1, chroma opt., no GMC/Qpel/Trellis quant/luma.
For some time I blamed poor resizing algorythms, but VDubs output is smooth, and when using DivX5.1 as codec with same settings, there is no artifact.

Any ideas?

[Gaia]

Little tip, use search. I found 22 threads with word "stairstepping" and there is even more discussion about this problem.

[Raptus]

Don't be fooled by my post count.

The topics you were refering to are about resizing/interlacing, wich is not the cause of the problem I'm experincing; its not related to the source but to the codec. You would´ve known that if You read my post carefully. Please take your time to take a look at the example I posted. Enlighten me if there is indeed a topic about THIS problem, or even better, a solutions for it.

thanx in advance.

[Selur]

just guessing: could it be a color conversion Problem?
Did you try to open your input file directly via VirtualDubMod and encode the specific scene ? (just to be sure it's not vfapi's 'fault')

Cu Selur

[Koepi]

I'd suggest trying a plain YV12 process chain - vfapi is "forbidden" for high quality for ages now ;)

It can also be that your avisynth script uses SimpleResize - that gave me such artefacts sometimes as well. Try a different resizer in that case.

Best regards
Koepi

[Raptus]

Thanx for the answers but I figured it out by my self just now.
I forgot I had installed uManiacs latest build, which seemed to be the problem. Switched back to koepis and everythings fine :)

koepi, i bow to your wisdom ;) , but could you point out why exactly vfapi is "forbidden"? I thought colorspace conversion wasn't recomended because of speed. I am aware of the fact that colorspace conversion is in fact lossy, but I thought it was a negligible loss.

btw, i'm not using any AVS, doing all the processing in VDub. Go ahead, brand me as a heretic :D

[Tommy Carrot]

I've also experienced this problem. It is not related to resizing (it can happen in direct avi->avi transcoding, without any filter). The "stairstepping" occurs on macroblock borders.

EDIT: i've used umaniac's build too, not tried with dev-api4 yet.

[mf]

The problem lies in the latest api3 build. I have it too, there's no problem with ffdshow or anything, only vfw. So there's nothing to worry about, really, just don't re-encode from such output.