1Font::TTF::GPOS(3) User Contributed Perl Documentation Font::TTF::GPOS(3)
2
6 Font::TTF::GPOS - Support for Opentype GPOS tables in conjunction with
7 TTOpen
8
10 The GPOS table is one of the most complicated tables in the TTF spec
11 and the corresponding data structure abstraction is also not trivial.
12 While much of the structure of a GPOS is shared with a GSUB table via
13 the Font::TTF::Ttopen
14
16 Here we describe the additions and lookup specific information for GPOS
17 tables. Unfortunately there is no one abstraction which seems to work
18 comfortable for all GPOS tables, so we will also examine how the
19 variables are used for different lookup types.
20 The following are the values allowed in the ACTION_TYPE and MATCH_TYPE
22 variables:
23 ACTION_TYPE
25 This can take any of the following values
26 a The ACTION is an array of anchor tables
28 o Offset. There is no RULE array. The ADJUST variable
30 contains a value record (see later in this description)
31 v The ACTION is a value record.
33 p Pair adjustment. The ACTION contains an array of two value
35 records for the matched two glyphs.
36 e Exit and Entry records. The ACTION contains an array of two
38 anchors corresponding to the exit and entry anchors for the
39 glyph.
40 l Indicates a lookup based contextual rule as per the GSUB
42 table.
43 MATCH_TYPE
45 This can take any of the following values
46 g A glyph array
48 c An array of class values
50 o An array of coverage tables
52 The following variables are added for Attachment Positioning Subtables:
54 MATCH
56 This contains an array of glyphs to match against for all RULES. It
57 is much like having the same MATCH string in all RULES. In the
58 cases it is used so far, it only ever contains one element.
59 MARKS
61 This contains a Mark array consisting of each element being a
62 subarray of two elements:
63 CLASS The class that this mark uses on its base
65 ANCHOR The anchor with which to attach this mark glyph
67 The base table for mark to base, ligature or mark attachment
69 positioning is structured with the ACTION containing an array of
70 anchors corresponding to each attachment class. For ligatures,
71 there is more than one RULE in the RULE array corresponding to each
72 glyph in the coverage table.
73 Other variables which are provided for informational purposes are:
75 VFMT
77 Value format for the adjustment of the glyph matched by the
78 coverage table.
79 VFMT2
81 Value format used in pair adjustment for the second glyph in the
82 pair
83 Value Records
85 There is a subtype used in GPOS tables called a value record. It is
86 used to adjust the position of a glyph from its default position. The
87 value record is variable length with a bitfield at the beginning to
88 indicate which of the following entries are included. The bitfield is
89 not stored since it is recalculated at write time.
90 XPlacement
92 Horizontal adjustment for placement (not affecting other unattached
93 glyphs)
94 YPlacement
96 Vertical adjustment for placement (not affecting other unattached
97 glyphs)
98 XAdvance
100 Adjust the advance width glyph (used only in horizontal writing
101 systems)
102 YAdvance
104 Adjust the vertical advance (used only in vertical writing systems)
105 XPlaDevice
107 Device table for device specific adjustment of horizontal placement
108 YPlaDevice
110 Device table for device specific adjustment of vertical placement
111 XAdvDevice
113 Device table for device specific adjustment of horizontal advance
114 YAdDevice
116 Device table for device specific adjustment of vertical advance
117 XIdPlacement
119 Horizontal placement metric id (for Multiple Master fonts - but
120 that is all I know!)
121 YIdPlacement
123 Vertical placement metric id
124 XIdAdvance
126 Horizontal advance metric id
127 YIdAdvance
129 Vertical advance metric id
130
132 Here is what is stored in the ACTION_TYPE and MATCH_TYPE for each of
133 the known GPOS subtable types:
134 1.1 1.2 2.1 2.2 3 4 5 6 7.1 7.2 7.3 8.1 8.2 8.3
136 ACTION_TYPE o v p p e a a a l l l l l l
137 MATCH_TYPE g c g c o g c o
138
140 read_sub
141 Reads the subtable into the data structures
142 $t->extension
144 Returns the table type number for the extension table
145 $t->out_sub
147 Outputs the subtable to the given filehandle
148 $t->read_value($format, $base, $lookup, $fh)
150 Reads a value record from the current location in the file, according
151 to the format given.
152 $t->read_delta($offset, $base, $lookup, $fh)
154 Reads a delta (device table) at the given offset if it hasn not already
155 been read. Store the offset and item in the lookup cache ($lookup->{'
156 CACHE'})
157 $t->read_anchor($offset, $base, $lookup, $fh)
159 Reads an Anchor table at the given offset if it has not already been
160 read.
161 $t->fmt_value
163 Returns the value format for a given value record
164 $t->out_value
166 Returns the output string for the outputting of the value for a given
167 format. Also updates the offset cache for any device tables referenced.
168
170 Martin Hosken <http://scripts.sil.org/FontUtils>.
171
173 Copyright (c) 1998-2016, SIL International (http://www.sil.org)
174 This module is released under the terms of the Artistic License 2.0.
176 For details, see the full text of the license in the file LICENSE.
177perl v5.36.0 2022-07-22 Font::TTF::GPOS(3)