Re: RTL PUA?

Philippe Verdy <verdy_p_at_wanadoo.fr> wrote:

>> The computing order of features should not then be:
>> - BiDi algorithm for reordering grapheme clusters
>> - font search and font fallback (using cmap)
>> - GSUB (lookups of ligatures or discretionary glyph variants)
>> - GPOS

>> but really:

>> - font lookup and font fallback (using cmap)
>> - GSUB (lookups of ligatures or discretionary glyph variants)
>> - BiDi algorithm for reordering glyphs representing the grapheme
>> clusters or ligatured grapheme clusters
>> - GPOS

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.

JH

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