NAME

       Font::TTF::GPOS - Support for Opentype GPOS tables in conjunction with TTOpen

DESCRIPTION

       The GPOS table is one of the most complicated tables in the TTF spec and the corresponding
       data structure abstraction is also not trivial. While much of the structure of a GPOS is
       shared with a GSUB table via the Font::TTF::Ttopen

INSTANCE VARIABLES

       Here we describe the additions and lookup specific information for GPOS tables.
       Unfortunately there is no one abstraction which seems to work comfortable for all GPOS
       tables, so we will also examine how the variables are used for different lookup types.

       The following are the values allowed in the ACTION_TYPE and MATCH_TYPE variables:

       ACTION_TYPE
           This can take any of the following values

           a       The ACTION is an array of anchor tables

           o       Offset. There is no RULE array. The ADJUST variable contains a value record
                   (see later in this description)

           v       The ACTION is a value record.

           p       Pair adjustment. The ACTION contains an array of two value records for the
                   matched two glyphs.

           e       Exit and Entry records. The ACTION contains an array of two anchors
                   corresponding to the exit and entry anchors for the glyph.

           l       Indicates a lookup based contextual rule as per the GSUB table.

       MATCH_TYPE
           This can take any of the following values

           g       A glyph array

           c       An array of class values

           o       An array of coverage tables

       The following variables are added for Attachment Positioning Subtables:

       MATCH
           This contains an array of glyphs to match against for all RULES. It is much like
           having the same MATCH string in all RULES. In the cases it is used so far, it only
           ever contains one element.

       MARKS
           This contains a Mark array consisting of each element being a subarray of two
           elements:

           CLASS   The class that this mark uses on its base

           ANCHOR  The anchor with which to attach this mark glyph

           The base table for mark to base, ligature or mark attachment positioning is structured
           with the ACTION containing an array of anchors corresponding to each attachment class.
           For ligatures, there is more than one RULE in the RULE array corresponding to each
           glyph in the coverage table.

       Other variables which are provided for informational purposes are:

       VFMT
           Value format for the adjustment of the glyph matched by the coverage table.

       VFMT2
           Value format used in pair adjustment for the second glyph in the pair

   Value Records
       There is a subtype used in GPOS tables called a value record. It is used to adjust the
       position of a glyph from its default position. The value record is variable length with a
       bitfield at the beginning to indicate which of the following entries are included. The
       bitfield is not stored since it is recalculated at write time.

       XPlacement
           Horizontal adjustment for placement (not affecting other unattached glyphs)

       YPlacement
           Vertical adjustment for placement (not affecting other unattached glyphs)

       XAdvance
           Adjust the advance width glyph (used only in horizontal writing systems)

       YAdvance
           Adjust the vertical advance (used only in vertical writing systems)

       XPlaDevice
           Device table for device specific adjustment of horizontal placement

       YPlaDevice
           Device table for device specific adjustment of vertical placement

       XAdvDevice
           Device table for device specific adjustment of horizontal advance

       YAdDevice
           Device table for device specific adjustment of vertical advance

       XIdPlacement
           Horizontal placement metric id (for Multiple Master fonts - but that is all I know!)

       YIdPlacement
           Vertical placement metric id

       XIdAdvance
           Horizontal advance metric id

       YIdAdvance
           Vertical advance metric id

CORRESPONDANCE TO LAYOUT TYPES

       Here is what is stored in the ACTION_TYPE and MATCH_TYPE for each of the known GPOS
       subtable types:

                       1.1 1.2 2.1 2.2 3   4   5   6   7.1 7.2 7.3 8.1 8.2 8.3
         ACTION_TYPE    o   v   p   p  e   a   a   a    l   l   l   l   l   l
         MATCH_TYPE             g   c                   g   c   o   g   c   o

METHODS

   read_sub
       Reads the subtable into the data structures

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

   $t->out_sub
       Outputs the subtable to the given filehandle

   $t->read_value($format, $base, $lookup, $fh)
       Reads a value record from the current location in the file, according to the format given.

   $t->read_delta($offset, $base, $lookup, $fh)
       Reads a delta (device table) at the given offset if it hasn not already been read.  Store
       the offset and item in the lookup cache ($lookup->{' CACHE'})

   $t->read_anchor($offset, $base, $lookup, $fh)
       Reads an Anchor table at the given offset if it has not already been read.

   $t->fmt_value
       Returns the value format for a given value record

   $t->out_value
       Returns the output string for the outputting of the value for a given format. Also updates
       the offset cache for any device tables referenced.

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.