Provided by: libfont-ttf-perl_1.06-1_all 
NAME
Font::TTF::Ttopen - Opentype superclass for standard Opentype lookup based tables (GSUB
and GPOS)
DESCRIPTION
Handles all the script, lang, feature, lookup stuff for a Font::TTF::Gsub/Font::TTF::Gpos
table leaving the class specifics to the subclass
INSTANCE VARIABLES
The instance variables of an opentype table form a complex sub-module hierarchy.
Version
This contains the version of the table as a floating point number
SCRIPTS
The scripts list is a hash of script tags. Each script tag (of the form
$t->{'SCRIPTS'}{$tag}) has information below it.
OFFSET This variable is preceded by a space and gives the offset from the start of
the table (not the table section) to the script table for this script
REFTAG This variable is preceded by a space and gives a corresponding script tag to
this one such that the offsets in the file are the same. When writing, it is
up to the caller to ensure that the REFTAGs are set correctly, since these
will be used to assume that the scripts are identical. Note that REFTAG must
refer to a script which has no REFTAG of its own.
DEFAULT This corresponds to the default language for this script, if there is one, and
contains the same information as an itemised language
LANG_TAGS
This contains an array of language tag strings (each 4 bytes) corresponding to
the languages listed by this script
$lang Each language is a hash containing its information:
OFFSET This variable is preceded by a a space and gives the offset from
the start of the whole table to the language table for this
language
REFTAG This variable is preceded by a space and has the same function as
for the script REFTAG, only for the languages within a script.
RE-ORDER This indicates re-ordering information, and has not been set. The
value should always be 0.
DEFAULT This holds the index of the default feature, if there is one, or
-1 otherwise.
FEATURES This is an array of feature tags for all the features enabled for
this language
FEATURES
The features section of instance variables corresponds to the feature table in the
opentype table.
FEAT_TAGS
This array gives the ordered list of feature tags for this table. It is used
during reading and writing for converting between feature index and feature
tag.
The rest of the FEATURES variable is itself a hash based on the feature tag for each
feature. Each feature has the following structure:
OFFSET This attribute is preceded by a space and gives the offset relative to the
start of the whole table of this particular feature.
PARMS If FeatureParams are defined for this feature, this contains a reference to
the corresponding FeatureParams object. Otherwise set to null.
LOOKUPS This is an array containing indices to lookups in the LOOKUP instance variable
of the table
INDEX This gives the feature index for this feature and is used during reading and
writing for converting between feature tag and feature index.
LOOKUP
This variable is an array of lookups in order and is indexed via the features of a
language of a script. Each lookup contains subtables and other information:
OFFSET This name is preceded by a space and contains the offset from the start of the
table to this particular lookup
TYPE This is a subclass specific type for a lookup. It stipulates the type of
lookup and hence subtables within the lookup
FLAG Holds the lookup flag bits
FILTER Holds the MarkFilteringSet (that is, the index into GDEF->MARKSETS) for the
lookup.
SUB This holds an array of subtables which are subclass specific. Each subtable
must have an OFFSET. The other variables described here are an abstraction
used in both the GSUB and GPOS tables which are the target subclasses of this
class.
OFFSET This is preceded by a space and gives the offset relative to the
start of the table for this subtable
FORMAT Gives the sub-table sub format for this GSUB subtable. It is
assumed that this value is correct when it comes time to write the
subtable.
COVERAGE Most lookups consist of a coverage table corresponding to the
first glyph to match. The offset of this coverage table is stored
here and the coverage table looked up against the GSUB table
proper. There are two lookups without this initial coverage table
which is used to index into the RULES array. These lookups have
one element in the RULES array which is used for the whole match.
RULES The rules are a complex array. In most cases, each element of the
array corresponds to an element in the coverage table (governed by
the coverage index). In a few caess, such as when there is no
coverage table, then there is considered to be only one element in
the rules array. Each element of the array is itself an array
corresponding to the possibly multiple string matches which may
follow the initial glyph. Each element of this array is a hash
with fixed keys corresponding to information needed to match a
glyph string or act upon it. Thus the RULES element is an array of
arrays of hashes which contain the following keys:
MATCH This contains a sequence of elements held as an
array. The elements may be glyph ids (gid), class
ids (cids), or offsets to coverage tables. Each
element corresponds to one glyph in the glyph
string. See MATCH_TYPE for details of how the
different element types are marked.
PRE This array holds the sequence of elements
preceding the first match element and has the same
form as the MATCH array.
POST This array holds the sequence of elements to be
tested for following the match string and is of
the same form as the MATCH array.
ACTION This array holds information regarding what should
be done if a match is found. The array may either
hold glyph ids (which are used to replace or
insert or whatever glyphs in the glyph string) or
2 element arrays consisting of:
OFFSET Offset from the start of the
matched string that the lookup
should start at when
processing the substring.
LOOKUP_INDEX The index to a lookup to be
acted upon on the match
string.
CLASS For those lookups which use class categories rather than glyph ids
for matching this is the offset to the class definition used to
categories glyphs in the match string.
PRE_CLASS This is the offset to the class definition for the before match
glyphs
POST_CLASS This is the offset to the class definition for the after match
glyphs.
ACTION_TYPE This string holds the type of information held in the ACTION
variable of a RULE. It is subclass specific.
MATCH_TYPE This holds the type of information in the MATCH array of a RULE.
This is subclass specific.
ADJUST This corresponds to a single action for all items in a coverage
table. The meaning is subclass specific.
CACHE This key starts with a space
A hash of other tables (such as coverage tables, classes, anchors,
device tables) based on the offset given in the subtable to that
other information. Note that the documentation is particularly
unhelpful here in that such tables are given as offsets relative
to the beginning of the subtable not the whole GSUB table. This
includes those items which are stored relative to another base
within the subtable.
METHODS
$t->read
Reads the table passing control to the subclass to handle the subtable specifics
$t->read_sub($fh, $lookup, $index)
This stub is to allow subclasses to read subtables of lookups in a table specific manner.
A reference to the lookup is passed in along with the subtable index. The file is located
at the start of the subtable to be read
$t->extension()
Returns the lookup number for the extension table that allows access to 32-bit offsets.
$t->out($fh)
Writes this Opentype table to the output calling $t->out_sub for each sub table at the
appropriate point in the output. The assumption is that on entry the number of scripts,
languages, features, lookups, etc. are all resolved and the relationships fixed. This
includes a LANG_TAGS list for a script, and that all scripts and languages in their
respective dictionaries either have a REFTAG or contain real data.
$t->num_sub($lookup)
Asks the subclass to count the number of subtables for a particular lookup and to return
that value. Used in out().
$t->out_sub($fh, $lookup, $index)
This stub is to allow subclasses to output subtables of lookups in a table specific
manner. A reference to the lookup is passed in along with the subtable index. The file is
located at the start of the subtable to be output
$t->dirty
Setting GPOS or GSUB dirty means that OS/2 may need updating, so set it dirty.
$t->maxContext
Returns the length of the longest opentype rule in this table.
$t->update
Perform various housekeeping items:
For all lookups, set/clear 0x0010 bit of flag words based on 'FILTER' value.
Sort COVERAGE table and RULES for all lookups.
Unless $t->{' PARENT'}{' noharmony'} is true, update will make sure that GPOS and GSUB
include the same scripts and languages. Any added scripts and languages will have empty
feature sets.
Internal Functions & Methods
Most of these methods are used by subclasses for handling such things as coverage tables.
copy($ref)
Internal function to copy the top level of a dictionary to create a new dictionary. Only
the top level is copied.
$t->read_cover($cover_offset, $lookup_loc, $lookup, $fh, $is_cover)
Reads a coverage table and stores the results in $lookup->{' CACHE'}, that is, if it has
not been read already.
ref_cache($obj, $cache, $offset [, $template])
Internal function to keep track of the local positioning of subobjects such as coverage
and class definition tables, and their offsets. What happens is that the cache is a hash
of sub objects indexed by the reference (using a string mashing of the reference name
which is valid for the duration of the reference) and holds a list of locations in the
output string which should be filled in with the offset to the sub object when the final
string is output in out_final.
Uses tricks for Tie::Refhash
out_final($fh, $out, $cache_list, $state)
Internal function to actually output everything to the file handle given that now we know
the offset to the first sub object to be output and which sub objects are to be output and
what locations need to be updated, we can now generate everything. $cache_list is an array
of two element arrays. The first element is a cache object, the second is an offset to be
subtracted from each reference to that object made in the cache.
If $state is 1, then the output is not sent to the filehandle and the return value is the
string to be output. If $state is absent or 0 then output is not limited by storing in a
string first and the return value is "";
$self->read_context($lookup, $fh, $type, $fmt, $cover, $count, $loc)
Internal method to read context (simple and chaining context) lookup subtables for the
GSUB and GPOS table types. The assumed values for $type correspond to those for GSUB, so
GPOS should adjust the values upon calling.
$self->out_context($lookup, $fh, $type, $fmt, $ctables, $out, $num)
Provides shared behaviour between GSUB and GPOS tables during output for context (chained
and simple) rules. In addition, support is provided here for type 4 GSUB tables, which are
not used in GPOS. The value for $type corresponds to the type in a GSUB table so calling
from GPOS should adjust the value accordingly.
BUGS
• No way to share cachable items (coverage tables, classes, anchors, device tables)
across different lookups. The items are always output after the lookup and repeated if
necessary. Within lookup sharing is possible.
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.