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Old 26th June 2018, 02:03   #11  |  Link
TD-Linux
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Quote:
Originally Posted by blurred View Post
Daala range coder using 16 multiplications per symbol has won with rANS using 1 multiplication per symbol, and ~7x faster implementations: https://sites.google.com/site/powturbo/entropy-coder

Do anybody know why the slower and more costly one was chosen?
Firstly, the AV1 range coder only uses 1 multiplication per CDF entry, the 16 is the "worst case" (keep in mind that they can be done in parallel, e.g. with SIMD, so it's actually better to use more than less as the multiply is the cheapest part in software). Secondly, the difference is nowhere near 7x when we benchnmarked the two - rANS was faster, but by a factor of about 2. However, the requirement to buffer and reverse the symbols was unfortunately insurmountable.

Also keep in mind that AV1 adjusts the probabilities on a per-symbol basis. The entropy coder CDFs are designed to make adapting the probabilities very fast (with only adds and shifts). This puts some constraints on the design that don't exist in the linked benchmark (which uses fixed probabilities as far as I can tell).

Last edited by TD-Linux; 26th June 2018 at 02:06.
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