Font::TTF::Silf

Section: User Contributed Perl Documentation (3)
Updated: 2020-07-28
Page Index
 

NAME

Font::TTF::Silf - 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

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.


 

Index

NAME
DESCRIPTION
INSTANCE VARIABLES
@opcodes
read
out
$t->minsize()
AUTHOR
LICENSING