[NTG-context] GSUB LookupType 6 in fea file not working

Paolo Brasolin paolo.brasolin at gmail.com
Wed Jan 6 17:28:32 CET 2016


Hello.

I am trying to add some custom ligatures to a font using a feature file.
Everything works just fine, until I try what is called a chained
contextual substitution, i.e. a GSUB lookup of type 6.
I am studying the specs from
http://www.adobe.com/devnet/opentype/afdko/topic_feature_file_syntax.html#5.f

A simple example.fea that does not work for me is:


languagesystem dflt dflt;

lookup FOOBAR {
  sub B by Q ;
} FOOBAR;

feature foba {
  sub [A] B' lookup FOOBAR;
} foba ;


I'd expect it to replace B with Q only when preceeded by A.
The same rule expressed with a lookup of type 8 works just fine:


languagesystem dflt dflt;

feature foba {
  sub [A] B' by Q;
} foba ;


Ultimately I would need to use multiple inline lookups, something like
  sub [A]' B' lookup FOO C' lookup BAR;

Are inline lookups unsupported by the engine?
That would seem strange to me.
I'm probably missing some point, but I'm unable to find more documentation.
Am I using the wrong syntax?

Thanks to anyone who can shed some light.


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