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PDF::API2::Basic::TTF::Ttopen
Opentype superclass for standard Opentype lookup based tables

PDF::API2::Basic::TTF::Ttopen - Opentype superclass for standard Opentype lookup based tables


NAME

PDF::API2::Basic::TTF::Ttopen - Opentype superclass for standard Opentype lookup based tables (GSUB and GPOS)


DESCRIPTION

Handles all the script, lang, feature, lookup stuff for a the PDF::API2::Basic::TTF::Gsub manpage/the PDF::API2::Basic::TTF::Gpos manpage 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 preceeded 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 preceeded 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 indices which index into the FEATURES instance variable of the table

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 preceeded 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 preceeded 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 preceeded 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 preceeding 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.




=head1 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

copy($ref)

Internal function to copy the top level of a dictionary to create a new dictionary. Only the top level is copied.


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 Martin_Hosken@sil.org. See the PDF::API2::Basic::TTF::Font manpage for copyright and licensing.

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