Fwd: Proposed Draft UTR #53, Unicode Arabic Mark Ordering Algorithm Now Available for Public Review

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Behnam Esfahbod

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Oct 3, 2017, 1:51:14 PM10/3/17
to Persian Computing
Hi P-C'ers,

Here's a proposed draft Unicode Technical Report related to Perso-Arabic script.

Sharing here for your information and any discussion.

Any formal comment or feedback should be submitted online at <http://www.unicode.org/review/pri359/>.


Yours,
-Behnam



---------- Forwarded message ----------
From: <announ...@unicode.org>
Date: Mon, Sep 25, 2017 at 10:35 AM
Subject: Proposed Draft UTR #53, Unicode Arabic Mark Ordering Algorithm Now Available for Public Review
To: announ...@unicode.org


The Unicode Consortium has released Proposed Draft Unicode Technical Report #53, Unicode Arabic Mark Ordering Algorithm. This UTR describes an algorithm for determining correct rendering of Arabic combining mark sequences.

The combining classes of Arabic combining characters in Unicode are a mixture of special classes for specific marks plus two more generalized classes for all the other marks. For many years this has resulted in inconsistent rendering for sequences with multiple combining marks such as:

[ Arabic Image ]

The algorithm described in this UTR provides a method to reorder Arabic combining marks in order to accomplish the following goals:

  • The inside-out rendering rule will display combining marks in the expected visual order.
  • Ensure identical display of canonically equivalent sequences.
  • Provide a mechanism for overriding the display order in exceptional cases.

The document is in “Proposed Draft” state, and made available for public review and comment. Information about this type of document can be found on the About Unicode Technical Reports page.

For information about how to discuss this Public Review Issue and how to supply formal feedback, please see the PRI #359 page.

http://blog.unicode.org/2017/09/proposed-draft-utr-53-unicode-arabic.html





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Behnam Esfahbod | بهنام اسفهبد
http://behnam.es/
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Soroush Rabiei

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Oct 4, 2017, 5:13:47 AM10/4/17
to Persian Computing
That's interesting. I wonder if this proposal covers "Language" or not. For example we use always write kasra below letter in Persian like this:

but we write it below shadda  in Arabic:

We used to handle this differences in OTF (substitution tables with rules on .lang IIRC). Now will this feature change (or become easier to implement) if this proposal is accepted? 

Behnam Esfahbod

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Oct 4, 2017, 12:37:16 PM10/4/17
to Soroush Rabiei, Persian Computing
Hi Soroush,

This specific case has been discussed in this Section 5.3 Kasra and kasra-like characters:

UAOA is able to handle the special ligation of kasra and kasra-like characters which are ligated with a shadda or hamza in some styles and appear just below them instead of below the base letter; they still logically follow the shadda or hamza.

It says "UAOA is able to handle", meaning that it supports both cases.

Basically, what happens with and without UAOA is that your font logic will decide how to show a sequence that has both shadda and kasra.


-b



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Behnam Esfahbod | بهنام اسفهبد
http://behnam.es/
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Behnam Rassi

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Oct 4, 2017, 5:55:04 PM10/4/17
to Soroush Rabiei, Persian Computing, Behnam Esfahbod
I think if the font contains dual marks as pre-combined glyphs, then the OT instructions should be able to handle these cases. For example the dual mark shadda and kasra can be localized for Arabic language.
If there is no pre-combined dual marks, then the sequence of the marks has greater importance for glyph positioning (GPOS) of the marks in general. For example shadda sits above fatha if fatha is typed first (which is wrong and will look like shadda-kasra Arabic style). The Persian style positioning of kasra and shadda will not be affected by the sequence of typing but having them positioned as a dual mark above the baseline will require special OT instructions for localized Arabic language and special provision about the sequence of the marks.

Behnam

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