[x264-devel] [PATCH]adaptive lowpass

[Gabriel Bouvigne]

Loren Merritt a écrit :
> Is it better to multiply the coeffs by some fraction, rather than 
> subtracting a constant (in the case of deadzone) or increasing lambda (in 
> the case of trellis)? That wouldn't be strictly a lowpass, but it would 
> also reduce the bits in high frequencies by changing coefficient magnitude 
> rather than step size, and would be closer to RD optimal.

The problem is that one you go into the high Qp area, it's a bit hard to 
define a good distortion computation.
Ideally, you would have a psychovisual model that would be run first and 
which would determine good lambdas for the different frequency bands of 
a given block.

But it's still usually a good thing to reduce your frequency range in 
order to increase precision of encoded freqs, once you are into the 
visible/audible degradation area. (of course, this is not RD optimal if 
you consider the whole spectrum)
That's why every audio encoder is increasing its lowpass when you 
increase the compression ratio. Lowpass is better than a metallic 
wma-like sound. I think that this trade-off is also applicable in video.


>> So the *adaptive* LPF provides a means to trade off the two types of
>> distortions.  If the adaptive filter control is sound, this can be a
>> benefit.
> 
> It can adapt the amount of bits spent in high vs low freqs. But the CQM is 
> still constant, so the LPF vs quantization noise is still not adaptive.

Yes, you can not really decide locally about such trade-off. This is 
something that initially surprised me when going from audio to video.
Ideally, a video coding scheme would allow you to specify the index of 
the CQM used for the current MB. With classical vlc coding, that would 
probably kill efficiency, but with cabac that would not impact much 
entropy coding.


--
Gabriel
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