Font::TTF::GPOS(3p)

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.