Re: RTL PUA?

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.

J.

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