[luatex] Opentype init/medi/fina.

[Paul Isambert]

Hello all,

  The init, medi and fina tags in OT fonts set variants of glyphs 
according to position (used by arabic fonts or scripts). Now you'd think 
it's a contextual substitution.

However, Microsoft 
(http://www.microsoft.com/typography/otspec/features_fj.htm#init) says 
those features should be implemented as single substitutions (GSYB 
lookup type 1), and that "the application is responsible for noting word 
boundaries".

Now, in my (ideal) view of opentype, tags are just pointers to lookups, 
and only lookup types decide how things are implemented, so that you can 
ignore the tag once you have the lookup. But this is not the case here: 
the tag modifies how the lookup should be implemented (so that a simple 
substitution should be implemented as a contextual one).

So: have I misunderstood something? Or: should I correct my view of tags 
and lookups (so that tags can't be ignored)? And in the latter case, any 
idea why init/medi/fina weren't implemented as contextual subtitutions 
in the first place (as are e.g. word-final variants in Minion)?

Best,
Paul