[theora] NHW Image codec - lower quality settings

[Raphael Canut]

Hello,

Just a quick message, I have improved the -l3 lower quality setting.I have
increased the quantization to 0.8 and decrease a little the denoising, and
so to save 1Ko in average, I have added a 8x8 block variance
analysis.Blocks that have a low variance are averaged to save bits.The -l3
lower quality setting is a little better now.

For the -h3 higher quality setting, yes for now I do not compress a small
data packet (but same compression scheme as for the other residual datas),
so file size will be smaller for -h3 setting.Residual coding on higher
wavelet orders seems ok now for the higher quality settings.Do not hesitate
to give me your opinion.

The most difficult is still the lower quality settings and increasing
compression (-l3 and below), -l2 could be ok? For these lower settings, I
would like to increase the denoising ("edge-preserving" denoising) -to
increase the compression- and keep a good sharpness.Would this
approach sound acceptable? For now (and for minimal extra computational
cost), I have the wavelet coefficients, and also data of the pre-processing
function (segmentation kernel), to perform the denoising for the lower
quality settings, but it is not that easy...

Many thanks,
Raphael
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