[MPlayer-dev-eng] Re: [MP] Re: [MP] [PATCH] "turbo" mode

[Guillaume POIRIER]

Hi,

Le jeu 16/09/2004 à 23:34, Matthias Wieser a écrit :
> Am Mittwoch, 15. September 2004 22:01 schrieb Guillaume POIRIER:
> > How do I do that?
> 
> Add "psnr" to your lavc options. Then you will find a file named like 
> "psnr_342234.log". This file contains psnr numbers for each individual 
> frame. You could plot those numers, calculate differences between 
> different passes/options/... and so on.

Ok, I didn't think about having a look at the psnr log (though I always
add psnr to lavc options)
Having a quick look at the log files, on the one hand, I saw that around
2/3 of quantizers are the same, which seem to be a good point, on the
other hand pretty much all psnr per frames are different.
If I plot with main/TOOLS/plotpsnr.pl both psnr, the global psnr curve
is quite similar, although with "turbo" there some "outliers" (quants
are [2-8] with turbo, whereas without they are [2-7]).
Those outliers aren't many, but they exist, which means _if I understand
rate-control quite alright_ that at some point the rate-control had a
bitrate shortage, and had to raise some quants.

Now, this analysis is only based on quants.
If I want to do a mathematical analysis of the frame-by-frame psnr
difference, am I supposed to plot the difference, the absolute
difference, the mean absolute difference?? I'm quite puzzled here...
And if the mean absolute difference is relevant, what would be an
acceptable mean psnr difference?
(sorry if I ask too many questions, I just wanna to things right)


> > How may that be reverent ? Do you mean that given 
> > that the statistics are not as good, maybe say, first frames lack/have
> > too much bits?
> 
> Maybe a normally good looking scene gets even more bits. Then the PSNR of 
> the frames of that scene would be higher but there would be nearly no 
> visual difference. But those bits are not available anymore for scenes 
> that would need them. So maybe the mean PSNR would be the same but the 
> visual appearence could be worse.

Got that. I figured it out anyway, not looking at the video (I can't see
much difference) but just as I for me it just makes sense that the
statistics being gathered differently produce a slightly different
result.
... but is that all that important if the difference is very small?
Will the end-user notice such a small difference? I guess they are
likely to afford a slightly different PSNR if it reduce encoding time by
40-45% ?

Regards,
Guillaume
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