[theora] NHW Image codec - improvement of precision

Raphael Canut nhwcodec at gmail.com
Wed Nov 21 07:52:13 PST 2012


I have just re-tested my codec with the YCbCr color space, used in JPEG for
example, and that's right that YCbCr is better, more accurate (than
YUV).Files are 10% larger, but the results are better with more
precision.Maybe I should use this color space by default? I have just added
a YCbCr version on my demo page (this version can be compiled from the
source code by selecting the YCbCr color space, and with removing the x298
multiply for Y in the decoder...).Any comment is welcome.
For the frequency prediction and the small wavelet coefficients, it is
certainly a wrong approach to try to predict specifically small frequency
coeffs (wavelets, DCT,...) one by one, and that's certainly why I couldn't
have any result..., and a better (common) approach is certainly to predict
a block of coeffs (freq) or pixels, and then study the variance of the
residuals (prediction errors), -and if the variance of the residuals is
smaller than the variance of the original block, then prediction is good-.I
will have to study it, but it is rather complex...

Anyway, any comment on this new version is very welcome.

Many thanks again,

2012/7/25 Raphael Canut <nhwcodec at gmail.com>

> Hi again,
> I have improved precision of my codec (on the encoder and decoder).I have
> also improved the -h1 quality setting.-Source code and binaries at
> http://nhwcodec.blogspot.com/ -.
> I am still trying to improve precision of my codec, with keeping my
> low-complexity (fast) approach.
> I do not totally use the reference (and impressive) block prediction with
> different modes + residual coding scheme, but I just apply residual coding
> on the first order wavelet "image" (for example for a 512x512 image, I just
> code the errors on the 256x256 wavelet "high-resolution" part), hence the
> lack of precision.I try to compensate it with a little more neatness of the
> wavelet 5/3 filterbank, but that's right that precision seems visually more
> important.
> Any opinion on this approach or on the codec in general is very welcome.
> Many thanks again,
> Raphael
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