L2/11-286

Date: Thu, 21 Jul 2011 14:15:50 -0400
From: Behdad Esfahbod <behdad@behdad.org>
Subject: Syriac shaping fixes and clarifications


A few months ago I implemented Syriac shaping in HarfBuzz and during the
process I identified a few items that can be improved in the Unicode Syriac
Shaping text:

1) The Syriac Shaping section talks about "dalath and rish" without defining
exactly which characters are meant.  I suggest qualifying this as
Joining_Group=Dalath_Rish.  This would match what Uniscribe does.  In the same
vein, I suggest qualifying references to Alaph with Joining_Group=Alaph.

2) Make a note that when applying the Syriac shaping rules (R1, R2, R3),
Joining_Type=Transparent characters are skipped.  That is, note that the R1
rule from Arabic Shaping has precedence over the Syriac shaping rules.

3) Perhaps rename the Syriac R1, R2, R3 rules to something that does not
conflict with the Arabic Shaping rules.  Maybe call them R1.1, R1.2, R1.3 to
make it clear that they fit right after Arabic R1.

4) The Unicode Syriac Shaping has rules that depend on "word breaking character"s:

      An alaph that has a non-left-joining character to its right, except
      for a dalath or rish, and a word breaking character to its left will
      take the form of A_fn.

That can be hard to implement since 1) there's no such thing as "word breaking
character" in Unicode, there is the Unicode Text Segmentation Algorithm, 2)
shaping engines typically don't have access to the word-boundaries as
determined by the Text Segmentation Algorithm.  I suggest replacing that with
something like this:

      An alaph that has a non-left-joining character to its right, except
      for a dalath or rish, and a non-joining character or the end of the
      text to its left will take the form of A_fn.

Ie. rely on the fact that in this context, roughly any "word breaking
character" is non-joining and vice versa.

[The full writeup and sample fonts are at: http://behdad.org/syriac/]

behdad