Re: RTL PUA?

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
井作恆
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Received on Tue Aug 23 2011 - 16:01:25 CDT