Provided by: libfont-ttf-perl_1.06-2_all bug

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.