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Font::TTF::GPOS(3)    User Contributed Perl Documentation   Font::TTF::GPOS(3)

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.

perl v5.24.1			  2016-08-03		    Font::TTF::GPOS(3)

NAME | DESCRIPTION | INSTANCE VARIABLES | CORRESPONDANCE TO LAYOUT TYPES | METHODS | AUTHOR | LICENSING

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