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 𐐖𐐱𐑌 𐐐. 𐐖𐐩𐑍𐐿𐐮𐑌𐑆 jenk...@apple.com
