[daala] Curious about progress of codec
Jarek Duda
dudajar at gmail.com
Sun Apr 24 14:43:50 UTC 2016
Hi Aaron,
I had some discussion and analysis regarding their coding part, and
there is a few percents to easily squeeze there (5-10%).
In contrast to nasty CABAC scheme - start with binarization then try to
model these bits, Daala has very nice general scheme:
send to coding part sequence of (ID, symbol),
where symbol is from up to size 16 alphabet,
each ID is statistically modeled as independent random variable - hence
ID itself contains both data type and context (like neighborhood).
However, they have a few inefficiencies:
1) start each frame with flat uniform probability distribution (this is
the worst!),
2) use approximated multi-symbol range coder:
https://people.xiph.org/~tterribe/tmp/StuiverMoffat98-%20Piecewise%20Integer%20Mapping%20for%20Arithmetic%20Coding.pdf
the cost of this approximation is 1-3% ratio loss for binary alphabet:
https://dl.dropboxusercontent.com/u/12405967/Moffat.nb
3) use costly every symbol adaptation (mainly to handle the flat initial
distribution problem), but with
freq[s] = count[s] / total
poor type of adaptation: a symbol far in the past has the same influence
on the probability as the most recent symbols,
4) use fixed adaptation rate.
These can be improved by:
1) Start with a probability distribution characteristic for a given ID,
for example a fixed parametric, eventually modified somewhere in the
file (better behaves in the beginning and allows for more subtle
adaptation),
2) use an accurate entropy coder, like rANS in VP10,
3) use adaptation with exponential forgetting to make recent symbol more
important, like
for (int i = 1; i < m; i++) CDF[i] -= (CDF[i] - mixCDF[i]) >> rate;
where mixCDF is for the new part, can be tabled for symbol-wise
adaptation such that frequencies of not used symbol will drop to the
minimal nonzero frequency,
4) Allow for varying adaptation rate - for example some ID use more
static probability distribution, for some it is beneficial to allow
encoder to choose one of a few possible adaptation rates.
Kind regards,
Jarek Duda
On 16/04/24 05:36, Aaron Boxer wrote:
> Dear Daala-istas,
>
> I took a look at the PSNR and PSNR-HVS charts for daala vs H.264 and
> H.265.
> May I ask at what PSNR value you would consider Daala to be
> competitive with H.265 ? From the graph, it looks like you are
> asymptotically matching H.264 quality, but there is still significant
> difference with HEVC, and progress is flattening out.
>
> I don't mean to be that guy, but when do you think you will be able to
> meet your project goal of meeting or beating HEVC quality? I am asking
> because I think this is a great project, and want to see it beating
> out the $$-driven competition.
>
>
> Kind Regards,
> Aaron Boxer
>
>
>
>
>
>
>
>
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--
dr JarosÅ‚aw Duda
Institute of Computer Science and Computer Mathematics,
Jagiellonian University, Cracow, Poland
http://th.if.uj.edu.pl/~dudaj/
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