NAME

       Font::TTF::GSUB - Module support for the GSUB table in conjunction with TTOpen

DESCRIPTION

       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!

INSTANCE VARIABLES

       The structure of a GSUB table is the same as that given in Font::TTF::Ttopen.  Here we give some of the
       semantics specific to GSUB lookups.

       ACTION_TYPE
           This is a string taking one of 4 values indicating the nature of the information in the ACTION array
           of the rule:

           g       The action contains a string of glyphs to replace the match string by

           l       The action array contains a list of offsets and lookups to run, in order, on the matched
                   string

           a       The action array is an unordered set of optional replacements for the matched glyph. The
                   application should make the selection somehow.

           o       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

           r       The action array is a list of replacement glyphs in coverage order. This ACTION_TYPE is used
                   only for Type 8 Reverse Chaining lookups which, by design, are single glyph substitution.

       MATCH_TYPE
           This indicates which type of information the various MATCH arrays (MATCH, PRE, POST) hold in the
           rule:

           g       The array holds a string of glyph ids which should match exactly

           c       The array holds a sequence of class definitions which each glyph should correspondingly match
                   to

           o       The array holds offsets to coverage tables

CORRESPONDANCE TO LAYOUT TYPES

       The following table gives the values for ACTION_TYPE and MATCH_TYPE for each of the 12 different lookup
       types found in the GSUB table definition:

                       1.1 1.2  2  3  4  5.1 5.2 5.3  6.1 6.2 6.3  8
         ACTION_TYPE    o   g   g  a  g   l   l   l    l   l   l   r
         MATCH_TYPE                   g   g   c   o    g   c   o   o

       Hopefully, the rest of the uses of the variables should make sense from this table.

METHODS

   $t->read_sub($fh, $lookup, $index)
       Asked by the superclass to read in from the given file the indexth subtable from lookup number lookup.
       The file is positioned ready for the read.

   $t->extension
       Returns the table type number for the extension table

   $t->out_sub($fh, $lookup, $index)
       Passed the filehandle to output to, suitably positioned, the lookup and subtable index, this function
       outputs the subtable to $fh at that point.

AUTHOR

       Martin Hosken <http://scripts.sil.org/FontUtils>.

LICENSING

       Copyright (c) 1998-2016, SIL International (http://www.sil.org)

       This module is released under the terms of the Artistic License 2.0.  For details, see the full text of
       the license in the file LICENSE.