Font::TTF::Ttopen(3pm) User Contributed Perl Documentation Font::TTF::Ttopen(3pm)
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 This is an unused offset to the parameters for each feature
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
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. Each element of the array corresponds to an element in the coverage table (governed
by the coverage index). If 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
script's LANG_TAGS list 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
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 $lookup->{' CACHE'}, that is, if it hasn't been read already.
ref_cache($obj, $cache, $offset)
Internal function to keep track of the local positioning of subobjects such as coverage and class definition tables, and their offsets.
What happens is that the cache is a hash of sub objects indexed by the reference (using a string mashing of the reference name which is
valid for the duration of the reference) and holds a list of locations in the output string which should be filled in with the offset to
the sub object when the final string is output in out_final.
Uses tricks for Tie::Refhash
out_final($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
o 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 Martin_Hosken@sil.org. See Font::TTF::Font for copyright and licensing.
perl v5.10.1 2011-02-25 Font::TTF::Ttopen(3pm)