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

[Guillaume POIRIER]

Hi,
Le ven 17/09/2004 à 18:37, Loren Merritt a écrit :
> On Fri, 17 Sep 2004, Guillaume POIRIER wrote:
> > Now, this analysis is only based on quants.
> 
> Quants are a pretty coarse measure. The actual parameter output by 
> rate control is lambda (lagrange multiplier), which is continuous. so you 
> might have a frame that wanted quant=1.6 (rounded to 2) on one pass, and 
> 2.4 (rounded to 2) on the other. But you can't see the actual lambda 
> unless you modify mencoder to output another divx2pass.log on the second 
> pass (attached).

First, I'd like to thank you for the enlightenment.
Then, just for the sake of curiosity, I skimmed though the paper
"Lagrange multiplier selection in hybrid video coder control"... I don't
have a strong knowledge in video encoding, but it allowed me to
- first understand what those damn Lagrange multiplier that I headed
about on several places do. If I understood correctly, the constrain in
video encoding is the target size (or the average bit-rate), so Lagrange
multipliers seem to be a good was to optimize the usage of bits...
Right?,

- then, it allowed me to understand why 3-pass is an interesting idea to
dig.... But I'm wondering... Wouldn't it be nice, instead of having a
1st pass that outputs quantizers, to have it output "Lagrange 
multipliers" too? Please forgive me if that sound like a dumb question,
I'm just trying to understand how it works.

> > 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)
> 
> I would use mean squared difference (possibly with a bias to care more 
> about low-PSNR frames),

That's what I did. Great minds think alike! ;-) Ok, that's a bit
pretentious..


> and an acceptable difference might be the same 
> order of difference as between a 2 pass and a 3 pass encode, whatever 
> number that happens to be.

Ok... I haven't investigate 3-pass mode, but when I have more time, 
I plan to experience with it.
Am I right to assume that, from a user point of view, it just works like
a 2-pass encode? (you just have to run 3 times the encode, incrementing
vpass each time, and adding more cpu-hungry options?)


> > ... 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% ?
> 
> The question is, using the same total amount of time, can you improve 
> quality by using turbo on the first pass and slower settings on the second 
> pass? I'm betting yes.

That's my goal, and I think I fulfilled it quite ok... ;-) but I'm
interested in what the user feedbacks might be.

Regards,
Guillaume