Re: [Cellar] Barry Leiba's Discuss on draft-ietf-cellar-ffv1-17: (with DISCUSS and COMMENT)

Michael Niedermayer <michael@niedermayer.cc> Tue, 26 January 2021 18:46 UTC

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Date: Tue, 26 Jan 2021 19:45:57 +0100
From: Michael Niedermayer <michael@niedermayer.cc>
To: Barry Leiba <barryleiba@computer.org>
Cc: cellar-chairs@ietf.org, Michael Richardson <mcr+ietf@sandelman.ca>, derek.buitenhuis@gmail.com, pb@das-werkstatt.com, draft-ietf-cellar-ffv1@ietf.org, Codec Encoding for LossLess Archiving and Realtime transmission <cellar@ietf.org>, The IESG <iesg@ietf.org>, Dave Rice <dave@dericed.com>
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Subject: Re: [Cellar] Barry Leiba's Discuss on draft-ietf-cellar-ffv1-17: (with DISCUSS and COMMENT)
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On Tue, Jan 26, 2021 at 12:54:08PM -0500, Barry Leiba wrote:
> Where are we on this?  I had thought you had the token to try to move
> the notation to pseudocode, but given the time lag I'm not sure we're
> all in agreement about what the next step is and who has it to take.
> So let's sync up and try to get this done.

I thought derek or someone else was working on the range coder description / pseudocode
because of: https://github.com/FFmpeg/FFV1/issues/231#issuecomment-697328963
But iam starting to have the feeling i have mixed this up a bit

Thanks

> 
> Thanks,
> Barry
> 
> On Fri, Dec 4, 2020 at 1:34 PM Michael Niedermayer
> <michael@niedermayer.cc> wrote:
> >
> > On Thu, Dec 03, 2020 at 06:46:48PM -0500, Dave Rice wrote:
> > > Hi Barry,
> > > Thanks so much for your comments.
> > >
> > > > On Dec 2, 2020, at 5:43 PM, Barry Leiba <barryleiba@computer.org> wrote:
> > > >
> > > > OK, Dave, here's where I am with 3.8.1.x:
> > > >
> > > > — Section 3.8.1 —
> > > >
> > > >   CABAC was replaced by a Range coder based on an algorithm defined by
> > > >   G. Nigel N. Martin in 1979 [range-coding].
> > > >
> > > > Keep in mind as you read my comments below that I’m not versed in this
> > > > field at all, I am not familiar with Range coding, and can’t easily
> > > > look up the reference.  Ha… but after typing that, I just did: I found
> > > > it here:
> > > > http://sachingarg.com/compression/entropy_coding/range_coder.pdf
> > > >
> > > > Maybe it would help to add that URI to the reference?  Anyway, again,
> > > > this isn’t my area, so it’s valid to say, for any of this below, that
> > > > someone actually implementing this would already understand these
> > > > things.
> > >
> > > We had some discussion on this at https://github.com/FFmpeg/FFV1/pull/219 and  considered adding a reference to http://www.compressconsult.com/rangecoder/#literature or https://web.archive.org/web/*/http://www.compressconsult.com/rangecoder/#literature. A reason against is that these are unofficial sources of this knowledge and thus perhaps not ideal to use for the citation.
> > >
> > > I did just check to see if other RFCs cite this and found that RFC6716 cites http://en.wikipedia.org/w/index.php?title=Range_encoding&oldid=509582757 as an informative reference.
> > >
> > > For IETF is there a preference for which of these citations if any to use?
> > >
> > > > — Section 3.8.1.1 —
> > > >
> > > >   To encode binary digits efficiently a Range coder is used.  C_(i) is
> > > >   the i-th Context.  B_(i) is the i-th byte of the bytestream. b_(i) is
> > > >   the i-th Range coded binary value, S_(0, i) is the i-th initial
> > > >   state.  The length of the bytestream encoding n binary symbols is
> > > >   j_(n) bytes.
> > > >
> > > > Two things here:
> > > >
> > > > Editorial: I think it would be easier to read this as an item list
> > > > (what do you think?):
> > > >
> > > > NEW
> > > >   To encode binary digits efficiently a Range coder is used.
> > > >     - C_(i) is the i-th Context
> > > >     - B_(i) is the i-th byte of the bytestream
> > > >     - b_(i) is the i-th Range coded binary value
> > > >     - S_(0, i) is the i-th initial state
> > > >     - j_(n) is the length of the bytestream encoding n binary symbols
> > > > END
> > >
> > > Locally, I’ve tested converting this into a definition list per https://tools.ietf.org/html/rfc7991#section-2.20, which rendered in the text output as:
> > >
> > >    C_(i)  the i-th Context.
> > >    B_(i)  the i-th byte of the bytestream.
> > >    b_(i)  the i-th Range coded binary value.
> > >    S_(0, i)  the i-th initial State.
> > >    j_(n)  the length of the bytestream encoding n binary symbols.
> > >
> > > > Substantive: It would also help to add to that list L_(i), l_(i),
> > > > R_(i), r_(i), and t_(i), so things are complete.  Maybe also S_(x, i),
> > > > where x is not 0.
> > >
> > > Nudging Michael Niedermayer and Derek Buitenhuis on this for help. Some guess work but if I understand correctly.
> > >
> > > L_(i) is the Low value of the Range
> > > l_(i) is a temporary storage variable used in assigning values to L_(i)
> > > R_(i) is the Range value
> > > r_(i) is a temporary storage variable used in assigning values to R_(i)
> > > t_(i) is a temporary storage variable used in assigning values to R_(i)
> > > S_(x, i) is the ‘x, 1’-th State.
> > >
> > > >   S_(i + 1, C_(i)) =  zero_state_(S_(i, C_(i)))  AND
> > > >              l_(i) =  L_(i)                      AND
> > > >              t_(i) =  R_(i) - r_(i)              <==
> > > >              b_(i) =  0                          <==>
> > > >              L_(i) <  R_(i) - r_(i)
> > > >
> > > > I still don’t understand how to read this.
> > >
> > > I should note that these equation appear a bit differently in the HTML version of the draft where they are depicted in SVG images of LaTeX equations. In the SVG, the equation all exists on one line; however, in the plain text output it doesn’t fit in the line width so we wrap the output a bit for cosmetics. If more clear we could re-arrange the line breaks, such as:
> > >
> > >    (
> > >      S_(i + 1, C_(i)) =  zero_state_(S_(i, C_(i)))  AND
> > >      l_(i) =  L_(i)                                 AND
> > >      t_(i) =  R_(i) - r_(i)
> > >     ) <== ( b_(i) = 0 ) <==> ( L_(i) <  R_(i) - r_(i) )
> > >
> > > > Perhaps explain verbally
> > > > what it’s supposed to mean, and I can help figure out some words to
> > > > add that will help others?
> > >
> > > b_(i) = 0 and L_(i) <  R_(i) - r_(i) would imply each other as well as implies the other group of three assignments.
> > >
> > > > I don’t know what the “AND” and “<==“ and
> > > > “<==>” on the right are trying to tell me (I get what “A <== B” means,
> > > > but I don’t know how to interpret “<==“ at the end of the line, for
> > > > example).
> > >
> > > It is only a line break, if not for the line width limitation this would be:
> > >
> > > S_(i + 1, C_(i)) =  zero_state_(S_(i, C_(i))) AND l_(i) =  L_(i) AND t_(i) =  R_(i) - r_(i) <== b_(i) =  0 <==> L_(i) <  R_(i) - r_(i)
> > >
> > > which is closer to how it looks in the SVG HTML version.
> > >
> > > > Do others who haven’t worked on this understand it?  It’s quite
> > > > possible that it’s just me, in which case I’ll happily step aside.
> > > > But I’m also happy to try to make it more understandable.
> > >
> > > Yes, please more understandability. :)
> > >
> > > >   S_(i + 1, k) = S_(i, k) <== C_(i) != k
> > > >
> > > >     Figure 13: The "i+1,k"-th State is equal to the "i,k"-th State if
> > > >         the value of "k" is unequal to the i-th value of Context.
> > > >
> > > > The caption on this does help, so now I see what it’s getting at!  But
> > > > I would *never* understand it without the caption.  (1) The
> > > > right-to-left conditional is odd to me.  (2) I’m not used to that sort
> > > > of conditional being used as an “if-then” statement.  I’m curious why
> > > > you didn’t write this whole section in pseudo-code, and perhaps it’s
> > > > another artifact of my wading into a field that’s foreign to me.  Had
> > > > Figure 13 been written like this, I would understand it perfectly:
> > > >
> > > >   if ( C_(i) != k ) then { S_(i + 1, k) = S_(i, k) }
> > >
> > > Yes, in pseudo-code that is the same.
> > > Michael Niedermayer, any objection to converting some of the math to pseudocode?
> >
> > Iam perfectly fine with changing the notation so it is easier to understand.
> >
> > One disadavantage of pure pseudocode could be that it may be hard to describe
> > encoding in such a way that it describes not one bitstream but a set of
> > bitstreams which are guranteed to be decoded correctly.
> > That is if the encoder side is described by pseudocode.
> >
> > If the approuch of trying to describe the bitstream by the non pseudocode
> > notation is disliked. Then maybe the decoder side can be described by
> > normative pseudocode and the encoder side by informative pseudocode
> > so that the encoder side would be "one possible" solution but not the only
> > one.
> > We could of course also describe both by strict normative pseudocode but
> > that would be different from what the original intend was
> >
> > None of this is specific to the quoted line of code above of course
> >
> > I had not imagined that the notation would lead to so much confusion,
> > its definitly important that the text can be understood and implemented
> > by the average target audience, if the notation causes problems here it
> > certainly should be improved ...
> >
> > Thanks
> >
> > [...]
> > --
> > Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
> >
> > There will always be a question for which you do not know the correct answer.
> 
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-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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