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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