From: John Hudson <>
Date: Tue, 23 Aug 2011 13:33:47 -0700

Behdad Esfahbod wrote:

>> I can see the advantages of such an approach -- performing GSUB prior to BiDi
>> would enable cross-directional contextual substitutions, which are currently
>> impossible -- but the existing model in which BiDi is applied to characters
>> *not glyphs* isn't likely to change. Switching from processing GSUB lookups in
>> logical order rather than reading order would break too many things.

> You can't get cross-directional-run GSUB either way because by definition
> GSUB in an RTL run runs RTL, and GSUB in an LTR run runs LTR. If you do it
> before Bidi, you get, eg, kerning between two glyphs which end up being
> reordered far apart from eachother. You really want GSUB to be applied on the
> visual glyph string, but which direction it runs is a different issue.

Kerning is GPOS, not GSUB.

But generally I agree. My point was that Philippe's suggestion, although
it could be the basis of an alternative form of layout that might have
some benefits if fully worked out, is a radical departure from how
OpenType works.


Tiro Typeworks
Gulf Islands, BC
The criminologist's definition of 'public order
crimes' comes perilously close to the historian's
description of 'working-class leisure-time activity.'
  - Sidney Harring, _Policing a Class Society_
Received on Tue Aug 23 2011 - 15:38:51 CDT

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