Font::TTF::Ttopen(3) User Contributed Perl Documentation Font::TTF::Ttopen(3)NAMEFont::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 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 corre‐
sponding 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 tags for all the
features enabled for this language
FEATURES
The features section of instance variables corresponds to the fea‐
ture 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 struc‐
ture:
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 subta‐
bles 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 stipu‐
lates 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 spe‐
cific. Each subtable must have an OFFSET. The other vari‐
ables 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 off‐
set 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 cor‐
rect when it comes time to write the subtable.
COVERAGE Most lookups consist of a coverage table corre‐
sponding to the first glyph to match. The off‐
set 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 cov‐
erage 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 pro‐
cessing 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 vari‐
able 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 ta‐
ble. 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 ta‐
ble 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, $index)
This stub is to allow subclasses to output subtables of lookups in a
table specific manner. A reference to the lookup is passed in along
with the subtable index. The file is located at the start of the sub‐
table to be output
Internal Functions & Methods
Most of these methods are used by subclasses for handling such things
as coverage tables.
copy($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, $offset)
Internal function to keep track of the local positioning of subobjects
such as coverage and class definition tables, and their offsets. What
happens is that the cache is a hash of sub objects indexed by the ref‐
erence (using a string mashing of the reference name which is valid for
the duration of the reference) and holds a list of locations in the
output string which should be filled in with the offset to the sub
object when the final string is output in out_final.
Uses tricks for Tie::Refhash
out_final($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 pro‐
vided 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 out‐
put after the lookup and repeated if necessary. Within lookup shar‐
ing is possible.
AUTHOR
Martin Hosken Martin_Hosken@sil.org. See Font::TTF::Font for copyright
and licensing.
perl v5.8.8 2005-10-06 Font::TTF::Ttopen(3)
