# Font::TTF::GPOS - phpMan

## 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.