<div dir="ltr">Thank you for your response.<div>I am studying the PDF you sent.</div><div>Could I ask you more if I have more questions?</div><div><br></div><div>Thanks again.</div><div>Best regards,</div><div>Knight.</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">2020년 7월 28일 (화) 오전 7:48, <<a href="mailto:daala-request@xiph.org">daala-request@xiph.org</a>>님이 작성:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Send daala mailing list submissions to<br>
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Today's Topics:<br>
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1. Re: Can I implement entropy of AV1 encoder without<br>
back-propagation of precarry buffer? (Timothy B. Terriberry)<br>
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<br>
----------------------------------------------------------------------<br>
<br>
Message: 1<br>
Date: Mon, 27 Jul 2020 14:54:59 -0700<br>
From: "Timothy B. Terriberry" <<a href="mailto:tterriberry@mozilla.com" target="_blank">tterriberry@mozilla.com</a>><br>
To: Jarek Duda <<a href="mailto:dudajar@gmail.com" target="_blank">dudajar@gmail.com</a>>, <a href="mailto:daala@xiph.org" target="_blank">daala@xiph.org</a><br>
Subject: Re: [daala] Can I implement entropy of AV1 encoder without<br>
back-propagation of precarry buffer?<br>
Message-ID: <<a href="mailto:3c41971f-b803-7d3b-1b02-5bfa7488b20d@xiph.org" target="_blank">3c41971f-b803-7d3b-1b02-5bfa7488b20d@xiph.org</a>><br>
Content-Type: text/plain; charset="utf-8"; Format="flowed"<br>
<br>
Jarek Duda wrote:<br>
> I don't know the details, but might be the one to blame: sounds like an <br>
> artifact of being able to switch between RC (range coder) and rANS <br>
> during development (I think Alex Converse was responsible), while <br>
> finally RC was used.<br>
<br>
No, this was actually a totally unrelated software optimization. See <br>
Section 5.2 of Thomas Davies' 2008 paper "Speeding Up Arithmetic Coding <br>
and Decoding in the Schroedinger Implementation of Dirac." I can't <br>
actually find a copy online anymore to link, so I've attached it.<br>
<br>
> W dniu 27.07.2020 o 11:07, Knight Kim pisze:<br>
>> Hello,<br>
>><br>
>> I am implementing entropy HW for AV1 video codec.<br>
>> I want to make the HW with the smallest precarry buffer.<br>
>> Currently, aom encoder is scanning reverse direction adding precarry <br>
>> to previous buffer point.<br>
>> It means that I should keep all bitstreams in a tile, and scan all the <br>
>> bitstreams then generate it.<br>
>> At HW point of view, this kind of operation is quite inefficient.<br>
>> Is there any way to implement the entropy encoder without precarry <br>
>> back-propagation?<br>
>> Can I determine the maximum length of back-propagation?<br>
<br>
In general there is no limit to how far a carry can propagate. The <br>
traditional way to handle this (described, e.g., by witten, Neal, and <br>
Cleary in their 1987 Communications of the ACM paper, "Arithmetic Coding <br>
for Data Compression") is to maintain a counter storing how many <br>
outstanding 1 bits a carry *could* propagate through, in lieu of <br>
actually outputting those bits. Only once the final value of those bits <br>
is determined would you actually flush them to the buffer. We don't do <br>
this in modern software because it puts expensive branches in an inner loop.<br>
<br>
In practice you still need to have some maximum in order to know how <br>
many bits to use for the counter, but it could be as large as the <br>
maximum number of output bits for a tile, in theory (although this is <br>
exceedingly unlikely). In practice H.264's CABAC and other entropy <br>
coders (VP8, VP9, etc.) all require the exact same carry-propagation <br>
behavior, so whatever you are doing to deal with it for those codecs <br>
should apply just as easily for AV1.<br>
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