PDF::API2::Basic::TTF::GSUB offers module support for the GSUB table in conjunction with TTOpen. It handles the GSUB subtables in relation to Ttopen tables. Due to the variety of different lookup types, the data structures are not all that straightforward, although I have tried to make life easy for myself when using this!
The structure of a GSUB table is the same as that given in PDF::API2::Basic::TTF::Ttopen. Here we give some of the semantics specific to GSUB lookups.
This is a string taking one of 4 values indicating the nature of the information in the ACTION array of the rule:
The action contains a string of glyphs to replace the match string by
The action array contains a list of lookups and offsets to run, in order, on the matched string
The action array is an unordered set of optional replacements for the matched glyph. The application should make the selection somehow.
The action array is empty (in fact there is no rule array for this type of rule) and the ADJUST value should be added to the glyph id to find the replacement glyph id value
This indicates which type of information the various MATCH arrays (MATCH, PRE, POST) hold in the rule:
The array holds a string of glyph ids which should match exactly
The array holds a sequence of class definitions which each glyph should correspondingly match to
The array holds offsets to coverage tables