man Font::TTF::Silf (3): The main Graphite table

DESCRIPTION

The Silf table holds the core of the Graphite rules for a font. A Silf table has potentially multiple silf subtables, although there is usually only one. Within a silf subtable, there are a number of passes which contain the actual finite state machines to match rules and the constraint and action code to be executed when a rule matches.

INSTANCE VARIABLES

Version

Silf table format version

Compiler

Lowest compiler version necessary to fully support the semantics expressed in this Graphite description

SILF

An array of Silf subtables

maxGlyphID

The maximum glyph id referenced including pseudo and non glyphs

Ascent

Extra ascent to be added to the font ascent.

Descent

Extra descent to be added to the font descent. Both values are assumed to be positive for a descender below the base line.

substPass

Pass index into PASS of the first substitution pass.

posPass

Pass index into PASS of the first positioning pass.

justPass

Pass index into PASS of the first justification pass.

bidiPass

Pass index of the pass before which the bidirectional processing pass will be executed. 0xFF indicates that there is no bidi pass to be executed.

Flags

A bitfield of flags:

    0 - Indicates there are line end contextual rules in one of the passes

maxPreContext

Maximum length of a context preceding a cross line boundary contextualisation.

maxPostContext

Maximum length of a context following a cross line boundary contextualsation.

attrPseudo

Glyph attribute for the actual glyph id associated with a pseudo glyph.

attrBreakWeight

Glyph attribute number of the attribute holding the default breakweight associated with a glyph.

attrDirectionality

Glyph attribute number of the attribute holding the default directionality value associated with a glyph.

JUST

The may be a number of justification levels each with their own property values. This points to an array of hashes, one for each justification level.

attrStretch: Glyph attribute number for the amount of stretch allowed before this glyph.
attrShrink: Glyph attribute number for the amount of shrink allowed before this glyph.
attrStep: Glyph attribute number specifying the minimum granularity of actual spacing associated with this glyph at this level.
attrWeight: Glyph attribute number giving the weight associated with spreading space across a run of glyphs.
runto: Which level starts the next stage.

numLigComp

Number of initial glyph attributes that represent ligature components

numUserAttr

Number of user defined slot attributes referenced. Tells the engine how much space to allocate to a slot for user attributes.

maxCompPerLig

Maximum number of components per ligature.

direction

Supported directions for this writing system

CRIT_FEATURE

Array of critical features.

scripts

Array of script tags that indicate which set of GDL rules to execute if there is more than one in a font.

lbGID

Glyph ID of the linebreak pseudo glyph.

pseudos

Hash of Unicode values to pseduo glyph ids.

classes

This is an array of classes, each of which is an array of glyph ids in class order.

PASS

The details of rules and actions are stored in passes. This value is an array of pass subobjects one for each pass.

flags

This is a bitfield:

    0 - If true, this pass makes no change to the slot stream considered as a sequence of glyph ids.
        Only slot attributes are expected to change (for example during positioning).

maxRuleLoop

How many times the engine will allow rules to be tested and run without the engine advancing through the input slot stream.

maxRuleContext

Number of slots of input needed to run this pass.

maxBackup

Number of slots by which the following pass needs to trail this pass (i.e. the maximum this pass is allowed to back up).

numRules

Number of action code blocks, and so uncompressed rules, in this pass.

numRows

Number of rows in the finite state machine.

numTransitional

Number of rows in the finite state machine that are not final states. This specifies the number of rows in the fsm element.

numSuccess

Number of success states. A success state may also be a transitional state.

numColumns

Number of columns in the finite state machine.

colmap

A hash, indexed by glyphid, that gives the fsm column number associated with that glyphid. If not present, then the glyphid is not part of the fsm and will finish fsm processing if it occurs.

rulemap

An array of arrays, one for each success state. Each array holds a list of rule numbers associated with that state.

minRulePreContext

Minimum number of items in a rule's precontext.

maxRulePreContext

The maximum number of items in any rule's precontext.

startStates

Array of starting state numbers dependeing on the length of actual precontext. There are maxRulePreContext - minRulePreContext + 1 of these.

ruleSortKeys

An array of sort keys one for each rule giving the length of the rule including its precontext.

rulePreContexts

An array of precontext lengths for each rule.

fsm

A two dimensional array such that $p->{'fsm'}[$row][$col] gives the row of the next node to try in the fsm.

passConstraintLen

Length in bytes of the passConstraint code.

passConstraintCode

A byte string holding the pass constraint code.

constraintCode

An array of byte strings holding the constraint code for each rule.

actionCode

An array of byte strings holding the action code for each rule.

@opcodes

Each array holds the name of the opcode, the number of operand bytes and a string describing the operands. The characters in the string have the following meaning:

    c - lsb of class id
    C - msb of class id
    f - feature index
    g - lsb of glyph attribute id
    G - msb of glyph attribute id
    l - lsb of a 32-bit extension to a 16-bit number
    L - msb of a 32-bit number
    m - glyph metric id
    n - lsb of a number
    N - msb of a 16-bit number
    o - offset (jump)
    s - slot reference
    S - slot attribute id
    v - variable number of following arguments

read

Reads the Silf table into the internal data structure

out

Outputs a Silf data structure to a font file in binary format

$t->minsize()

Returns the minimum size this table can be. If it is smaller than this, then the table must be bad and should be deleted or whatever.

AUTHOR

Martin Hosken <http://scripts.sil.org/FontUtils>.

LICENSING

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.