[MPlayer-users] MPlayer-1.0pre4 bug with divx4 multipass filter

[Vladimir Mosgalin]

On Mon, 13 Sep 2004, D Richard Felker III wrote:

DRFI>> I've never encountered that bug in the wild, but...
DRFI>
DRFI>Because no real release group is so stupid as to encode width%16!=0...

Really? 640x360 is still more popular than 640x352, I guess...

DRFI>> There's your problem. After a little fiddling, I can reproduce your
DRFI>> bug with:
DRFI>> video resolution not divisible by 16,
DRFI>
DRFI>704 _is_ divisible by 16 though. And....if Vladimir is such a
DRFI>stickler for quality, I can't imagine he would encode with a bad
DRFI>width...

I do. Actually, I encoded with width divisible only by 4 in the past,
and had absolutely no problems with it. Slightly increased filesize?
Well, I write 2 2Gb avi files onto 4.38Gb media, so I have 400Mb left
just in case, also can be used for soundtracks & stuff. And since not
every film requires 2Gb, I have enough space left. And no, I'm not
feeling like fitting 3 films on the same media. Quality dvd+rw disc
costs $2, why should I have a headache about space?

DRFI>> BTW, multiples of 16 (not just 8) really are a good idea, and are very 
DRFI>> cheap compared to everything else you could do to improve video quality: 
DRFI>> At a given PSNR (and I see no reason why this comparison shouldn't be 
DRFI>> adequately measured by PSNR), A mpeg4 video with both dimensions 
                                            note this  ^^^^^^
DRFI>> non-mod16 takes 5-10% higher bitrate than one cropped, downscaled, or
                                       and this ^^^^^^^^^^^^^^
DRFI>> even upscaled to the nearest mod16.
DRFI>> The penalty is about half that if only one dimension is non-mod16.
DRFI>> Even with 4mv and a resolution divisible by 8, you don't get much of 
DRFI>> that back (though it depends on the content).
DRFI>> Maybe I'll get around to posting graphs/writeups of my mod16 benchmarks 
DRFI>> sometime...
DRFI>
DRFI>Hahaha, wow, I'm glad to finally have some quantitative figures on
DRFI>this. The 5-10% figure is huge...I'd like to see some info about test
DRFI>methodology.

His methods must be incorrect (well, not really, but they aren't showing
the real situation, because this is comparsion of the worst case with
the best case). With the method "encode with qp=2 and compare the
difference in filesize" I get insignificant difference. Up to 1%, maybe.
Of course, _both_ files were cropped correctly and usually one of the
dimensions was multiply of 16. The other was, say, mod8. Or even only
mod4.

-- 

Vladimir