[Ffmpeg-devel] improving encoding (possibly big perceptual gains)

Michael Niedermayer michaelni
Mon Jan 2 19:37:40 CET 2006


On Wed, Dec 28, 2005 at 04:09:32PM -0500, Rich Felker wrote:
> with all the recent discussions over improving lavc, i've got some
> ideas. mainly i'm worried that psnr is very misleading when trying to
> improve motion estimation decisions since often a particular motion
> vector will give better psnr but much worse actual quality due to loss
> of small detail and mud/blocking.
> because of this, i'd like to suggest that lavc be extended to measure
> the 'uqi' (universal [image] quality index) proposed by the paper at
> this site:
> http://www.cns.nyu.edu/~zwang/files/research/quality_index/demo_lena.html
> i think the images there speak for themselves regarding how stupid
> psnr is as a judge of quality.
> however, printing statistics isn't much use if you can't actually
> improve encoding. so the main idea is too add a *cmp function to
> measure the uqi and use this for motion estimation and optimal
> quantization and qprd, etc. maybe also the ratecontrol engine itself.
> this should do a much better job of telling when it's ok to use poor
> quantization, zero ac coeffs, etc. and when it will look like shit
> (like the last lena picture ;).

theres a problem with using this quality meassure for any decission,
look at it, it pretty much ignores brightness changes, not only will
that ignore flickering/blinking but if done per block brightness will
be off per block this will not look good at all -> PSNR is still the
better cmp function for these decissions, maybe it should be done in
dct domain and different frequencies should be weighted differently
maybe a closer to 0 error should be considered less wrong then a farther
away from zero error ...



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