From: John H. Jenkins <>
Date: Tue, 23 Aug 2011 14:58:57 -0600

John Hudson 於 2011年8月23日 下午2:33 寫道:

> 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.

I'll toss in my obligatory, "That's how AAT does it" reference. It has advantages and disadvantages—but, as you say, OT would have to be heavily redesigned to do it.

John H. Jenkins
𐐖𐐱𐑌 𐐐. 𐐖𐐩𐑍𐐿𐐮𐑌𐑆
Received on Tue Aug 23 2011 - 16:01:25 CDT

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