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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 vari‐
       ables 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 con‐
		   tains 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 sub‐
	   array 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 posi‐
	   tioning 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 cover‐
	   age 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's 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't 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 hasn't 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 Martin_Hosken@sil.org. See Font::TTF::Font for copyright
       and licensing.

perl v5.8.8			  2005-10-06		    Font::TTF::GPOS(3)
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