# phpman > perldoc > Font::TTF::GSUB

## NAME
    [Font::TTF::GSUB](https://www.chedong.com/phpMan.php/perldoc/Font%3A%3ATTF%3A%3AGSUB/markdown) - 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](https://www.chedong.com/phpMan.php/perldoc/Font%3A%3ATTF%3A%3ATtopen/markdown). 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.