consider the following images;
(ffdshow's mv representation [find it under "Visualizations" in ffdshow], divx3 left, xvid right)
now read this
http://forum.doom9.org/showthread.php?postid=411000&highlight=more+sophisticated+creation+of+motion+vectors#post411000
(highlighted part is important)
also notice the following post by temporance; where he says
"the fix is probably through motion estimation, but my only idea is to add a bias which favors MV (0, 0)."
the ffvfw and divx3 (and ms-mpeg4 codecs in general) codecs "favour MV(0, 0)" when they "see" noise.
(btw. i wish i could say the same for wmv9, but my recent tests show that wmv9 will act more like xvid, ie make a mess if source is noisy...i have also spotted something that reminded me of divx3's "shit"..it appeared on subtitles...but blocks were smaller (probably 4x4)...i'll post wmv samples (soon) to illustrate this)
mugfunky also took a simillar guess when he said
"ffvfw appears to not consider noise in motion estimation"
http://www.hydrogenaudio.org/forums/index.php?showtopic=16297&st=25&p=184832&#entry184843
so the problem with xvid and noise is this;
xvid is mistaking noise for a movement, so it's trying to use mv's to describe the noise, and that's a nonsense, which results in "erroneous motion" aka "swimming(floating) walls" and "tearing motion"
(i was reffering to it as "ME(motionestimation) bug", "jelly-waving", "evaporating effect" and narler called it "heat haze"...many names for same problem;
next time you watch xvid encoding of a source with some noise, or you just see wall moving, you'll know what it was.)
as mug suggested, solution to this problem lays in applying an option of (for example) "ffvfw ME" in xvid encoder, or indeed "divx3 ME" (divx3 is even better than ffvfw as ffvfw is fooled by noise sometimes; divx3 is not)
that way, xvid would become true universal mpeg4 codec which would blow anything else out there.(the stuff like qpel and other advanced tools would beat divx3 with ease...same as they beat it now on clean sources)
today, only way to "solve" this, is to turn off xvid's ME alltogether (by setting the "motion search precision" to "0"), but..heh...although there won't be any swimming (all mv's are (0,0) that way), you won't like the image quality.
mpeg's do need motionestimation/compensation, you know.
/ivo
edited(21 Feb 2007); missing pics...uploaded again...and again;
http://img72.imageshack.us/my.php?image=frame541bd2.jpg
http://img72.imageshack.us/my.php?image=frame551ap1.jpg
http://img376.imageshack.us/my.php?image=frame561rl6.jpg
http://img225.imageshack.us/my.php?image=frame571ac7.jpg
http://img225.imageshack.us/my.php?image=frame581fx1.jpg
let's see who fails first, imageshack, or arachnotron..
) )