File: GTF.txt

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= Graphite Table Format =

:docinfo:
:numbered:

== Introduction ==

The Graphite font table format is structured in order that a Graphite binary
description may be incorporated into a TrueType font. Thus the binary format
uses the TrueType table structure, identically to how it is used in a TrueType
font. The only difference between using an external file containing Graphite
binary information in tables, and inserting the binary information into tables
in the font is that tables are considered local to their file and are considered
to override those found in the font file. This allows there to be multiple,
independent descriptions held in separate files. Those independent descriptions
would have to be merged, in a way described in this document, if they were to be
held together in the same font file or binary file.

The description consists of a set of table descriptions. The format of a file
follows that of a TrueType font containing only those tables pertinent to the
description (i.e. for a separate binary description, those tables listed here).

As is standard for all TrueType tables, the data is in big-endian format
(most significant byte first).

== Version ==

This file describes version 4.0 of the Graphite font table specification.
Modifications from previous versions are indicated in the “Version notes” column
of the various tables.

== Tables ==

This document describes several additional TrueType table types. The “Silf” and
“Sile” tables are unique to the needs of Graphite, whilst “Gloc” and “Glat”
provide an extended glyph attribute mechanism. The “Feat” table is based very
closely on the GX “feat” table. (If necessary the tables could be restructured
to be stored inside the single “Silf” table.) In addition, use is made of the
“name” table type.

This version of the Graphite format includes the ability to compress “Glat” and
“Silf” tables, the extensions to those table provide a compression scheme field
permitting up to 32 compression schemes. Currently only 2 schemes are defined:
0 – no compression, and 1 – an LZ4 block decompressor. This is not the LZ4
framing format just the inner block level format without any checksum.

All compressed tables have the same form the original table's 32bit version
number followed by a 32 bit compression header. This consists of the top 5 bits
for the scheme and 27 remaining bits for the uncompressed table size. This is
then followed by the compression scheme's data.

[[comp_table]]
.Compressed table
[cols="3,3,8,3",options="unbreakable,header,compact",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type     |Name            |Description                          |Version notes

|FIXED    |version         |Uncompressed Table version number    |
|ULONG:5  |scheme          |Compression scheme must not be 0     |5.0 – added
|ULONG:27 |full_size       |Size of uncompressed table           |5.0 – added
|BYTE[]   |compressed_data |Compression scheme data              |5.0 – added
|===============================================================================

The uncompressed form is the complete table including the version number but
with the scheme always set to 0. The remaining 27 bits are available to the
uncompressed table.

=== Glat ===

The Glat table type is used for storing glyph attributes. Each glyph may be
considered to have a sparse array of, at the most, 65536 16-bit signed
attributes. The Glat table is the mechanism by which they are stored.

The Glat table consists of a table header and an array of Glat_entry items. Two
formats for the Glat table are typically used. Most fonts will use a version 2
table without Octabox metrics. Those few fonts that have collision avoidance
support, will use a version 3 table.

.Glat version 2
[cols="3,3,8,3",options="unbreakable,header,compact",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type         |Name       |Description                          |Version notes

|FIXED        |version    |Table version: 00030000              |4.0 – 00020000
|Glat_entry[] |entries    |Glyph attribute entries              |
|===============================================================================


.Glat version 3
[cols="3,3,8,3",options="unbreakable,header,compact",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type            |Name      |Description                        |Version notes

|FIXED           |version   |Table version: 00030000            |5.0 – 00030000
|ULONG:5         |scheme    |Compression scheme must be 0       |5.0 – added
|ULONG:28        |reserved  |                                   |5.0 – added
|ULONG:1         |octaboxes |Octaboxes are present flag         |5.0 – added
|Glyph_attrs[]   |entries   |Glyph attribute entries            |
|===============================================================================
For the compressed layout see <<comp_table>>.

.Glyph_attrs
[cols="3,3,8,3",options="unbreakable,header,compact",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type            |Name      |Description                         |Version notes

|Octabox_metrics |octabox    |Octabox metrics, only present if
                              the Glat header indicates          |5.0 – added
|Glat_entry[]    |entries    |Glyph attribute entries            |
|========================================================================

If the octaboxes flag is set in the Glat header then for each per glyph block
of data specified by the Gloc table, first set of data includes metrics that
approximate the glyph’s curves. The approximation uses “octoboxes”—rectangles
with corners that may be cut out at an angle of 45 degrees. Each octobox
requires 8 values to define. There are metrics for the entire glyph and for a
4x4 approximation grid, resulting in up to 16 sub-boxes. For some glyphs, no
sub-box data will be present, in which case the bitmap will be zero. Note that
the rectangle for the full glyph is not included here, as the bounding box
rectangle is stored elsewhere in the font.

.Octabox_metrics
[cols="3,3,8,3",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type           |Name          |Description                      |Version notes

|USHORT         |subbox_bitmap |Which subboxes exist on 4x4 grid; +
                                bit-index = (y-index*4) + x-index   |
|BYTE           |diag_neg_min  |Defines min negatively-sloped diagonal (di) |
|BYTE           |diag_neg_max  |Defines max negatively-sloped diagonal (da) |
|BYTE           |diag_pos_min  |Defines min positively-sloped diagonal (si) |
|BYTE           |diag_pos_max  |Defines max positively-sloped diagonal (sa) |
|Subbox_entry[] |subboxes      |One entry per bit in subbox_bitmap  |
|===============================================================================

Note that in the subbox bitmap, bit 3 indicates the presence of the lower right
cell, and bit 12 the upper left cell as per this diagram.

.subbox_bitmap
[cols=",,,",width="20%",grid="all"]
|==============
|12 |13 |14 |15
| 8 | 9 |10 |11
| 4 | 5 | 6 | 7
| 0 | 1 | 2 | 3
|==============

.Subbox_entry
[cols="3,3,8,3",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type |Name         |Description                                  |Version notes

|BYTE |left         |Left of subbox                               |
|BYTE |right        |Right of subbox                              |
|BYTE |bottom       |Bottom of subbox                             |
|BYTE |top          |Top of subbox                                |
|BYTE |diag_neg_min |Defines min negatively-sloped diagonal (si)  |
|BYTE |diag_neg_max |Defines max negatively-sloped diagonal (sa)  |
|BYTE |diag_pos_min |Defines min positively-sloped diagonal (di)  |
|BYTE |diag_pos_max |Defines max positively-sloped diagonal (da)  |
|===============================================================================

Following the glyph curve approximation data, the glyph attributes appear. The
glyph attributes associated with a particular glyph are identified by number and
value. To conserve space, this storage is run-length encoded. Thus a glyph will
have a series of Glat_entrys corresponding to each non-contiguous set of
attributes. The structure of a Glat_entry is:

.Glat_entry, version 2 & 3
[cols="2,2,8,5",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type   |Name         |Description                         |Version notes

|USHORT |attNum       |Attribute number of first attribute |4.0 – BYTE to SHORT
|USHORT |num          |Number of attributes in this run    |4.0 – BYTE to SHORT
|SHORT  |attributes[] |Array of num attributes             |
|===============================================================================

Notice that all glyph attributes are 16-bit signed values. If a 32-bit value is
required, then two attributes should be assigned and joined together by
the application.

Attribute numbers are application specific.

Note that if the font does not require more than 256 glyph attributes, version 1
of the Glat table will be generated, which is defined as follows.

.Glat version 1
[cols="3,3,8,3",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type         |Name    |Description                              |Version notes

|FIXED        |version |Table version: 00010000                  |
|Glat_entry[] |entries |Glyph attribute entries                  |
|===============================================================================

.Glat_entry, version 1
[cols="3,3,8,3",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type  |Name         |Description                                |Version notes

|BYTE  |attNum       |Attribute number of first attribute        |
|BYTE  |num          |Number of attributes in this run           |
|SHORT |attributes[] |Array of num attributes                    |
|===============================================================================

=== Gloc ===

The Gloc table is used to index the Glat table. It is structured identically to
the loca table type, except that it has a header.

TODO: add a field indicating the number of glyphs in the table (the current
dependence on the Silf table is not architecturally clean).

.Gloc
[cols="3,3,8,3",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type         |Name        |Description                          |Version notes

|FIXED        |version     |Table version: 00010000              |
|USHORT       |flags       |bit 0 = 1 for Long format,
                                    0 for short format; +
                            bit 1 = 1 for attribute names,
                                    0 for stripped               |
|USHORT       |numAttribs  |Number of attributes                 |
|USHORT/ULONG |locations[] |Offsets into Glat for each glyph; +
                            (number of  glyph IDs + 1) of these  |

|USHORT       |attribIds[] |Debug id for each attribute          |
|===============================================================================

A version 1.1 (0x00010001) table indicates that the Glat table has octabox
information. But such a version is not required in that case.

The flags entry contains a bit to indicate whether the locations array is of
type USHORT or ULONG. The locations array is identically structured to that of
the loca table. There is one entry per glyph and an extra entry to identify the
length of the final glyph’s attribute entries. Offsets are given to a Glat_entry
in the Glat table. The second bit indicates whether there is an attribIds array
at the end of this table. If there is, then it contains name IDs for each
attribute. If this bit is not set, then there is no array and the table ends
after the locations array.

As of version 2 of the Silf table, the values of the breakweight attribute are
interpreted as follows:

....
BREAK_WHITESPACE = 10
BREAK_WORD = 15
BREAK_INTRA = 20
BREAK_LETTER = 30
BREAK_CLIP = 40
....

=== Feat ===

Graphite stores features in a table whose format is very similar to the GX feat
table. This makes reference to the name table which is use for storing feature
names and feature value names.

.Feat
[cols="3,3,8,5",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type               |Name |Description                           |Version notes

|FIXED              |version        |Table version: 00020001     |
|USHORT             |numFeat        |Number of features          |
|USHORT             |reserved       |                            |
|ULONG              |reserved       |                            |
|FeatureDefn        |features[]     |Array of numFeat features   |
|Feature­SettingDefn |featSettings[] |Array of feature setting
                                     values, indexed by offset   |
|===============================================================================

.FeatureDefn
[cols="3,3,8,3",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type   |Name        |Description                       |Version notes

|ULONG  |id          |Feature ID number                 |2.0 – added
|USHORT |numSettings |Number of settings                |
|USHORT |reserved    |                                  |2.0 – inserted
|ULONG  |offset      |Offset into featSettings list     |
|USHORT |flags       |                                  |2.1 - reserved to flags
|USHORT |label       |Index in name table for UI label  |
|===============================================================================

The flags value has its own bit structure:

.FeatureFlags
[cols="3,3,8,3",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Bits |Mask   |Description                                       |Version notes

|0    |0x0001 |This feature is an alias of the previous one.     |2.1 - added
|1-15 |0xFFFE |Reserved                                          |
|===============================================================================

.FeatureSettingDefn
[cols="3,3,8,3",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type   |Name  |Description                                      |Version notes

|SHORT  |value |Feature setting value                            |
|USHORT |label |Index into name table for UI label               |
|===============================================================================

=== Silf ===

The “Silf” table will be used for storing rules and actions for the various
types of tables in a rendering description. The structure of the Silf table is:

.Silf
[cols="2,3,8,5",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type     |Name            |Description               |Version notes

|FIXED    |version         |Table version: 00050000   |2.0 – changed to 00020000
|         |                |                          |3.0 – changed to 00030000
|         |                |                          |5.0 – changed to 00050000
|ULONG:5  |scheme          |scheme must be 0          |5.0 – added
|FIXED:27 |compilerVersion |Compiler version that
                            generated this font       |3.0 – added +
                                                       5.0 – changed to 27 bits
|USHORT   |numSub          |Number of SIL subtables   |
|USHORT   |reserved        |                          |
|ULONG    |offset[]        |Array of numSub offsets to
                            the subtables relative to
                            the start of this table   |
|SIL_Sub  |tables[]        |Array of independent
                            rendering description
                            subtables                 |
|===============================================================================
For the compressed layout see <<comp_table>>. Since one TrueType file may hold
multiple independent rendering descriptions, each rendering description is
described in a subtable. The subtable contains all that is necessary to describe
the rendering of one set of writing systems.

.SIL_Sub
[cols="4,6,28,7",options="header",frame="topbot",grid="none"]
|===============================================================================
|Type      |Name               |Description                     |Version notes

|FIXED     |ruleVersion        |Stack-machine language version  |3.0 – added
|USHORT    |passOffset         |offset of oPasses[0] relative to
                                start of sub-table              |3.0 – added
|USHORT    |pseudosOffset      |offset of pMaps[0] relative to
                                start of sub-table              |3.0 – added
|USHORT    |maxGlyphID         |Maximum valid glyph ID (including
                                line-break & pseudo-glyphs)     |
|SHORT     |extraAscent        |Em-units to be added to the
                                font’s ascent                   |
|SHORT     |extraDescent       |Em-units to be added to the
                                font’s descent                  |
|BYTE      |numPasses          |Number of rendering description
                                passes                          |
|BYTE      |iSubst             |Index of 1st substitution pass  |
|BYTE      |iPos               |Index of 1st Positioning pass   |
|BYTE      |iJust              |Index of 1st Justification pass |
|BYTE      |iBidi              |Index of 1st pass after the
                                bidi pass(must be <= iPos); +
                                0xFF implies no bidi pass       |
|BYTE      |flags              |0   - has line end contextuals, +
                                1   - contextuals, +
                                2-4 - space contextuals, +
                                5 - has collision pass          |4.0 – add Bit 1
|BYTE      |maxPreContext      |Max range for preceding
                                cross-line-boundary contextualization |
|BYTE      |maxPostContext     |Max range for following
                                cross-line-boundary contextualization |
|BYTE      |attrPsuedo         |Glyph attribute number that is
                                used for actual glyph ID for a
                                pseudo-glyph                    |
|BYTE      |attrBreakWeight    |Glyph attribute number of
                                breakweight attribute           |
|BYTE      |attrDirectionality |Glyph attribute number for
                                directionality attribute        |
|BYTE      |attrMirroring      |Glyph attribute number for
                                mirror.glyph (mirror.isEncoded
                                directly after)                 |2.0 – added; +
                                                                 4.0 – used
|BYTE      |attrSkipPasses     |Glyph attribute of bitmap
                                indicating key glyphs for pass
                                optimization                    |2.0 – added; +
                                                                 4.0 – used
|BYTE      |numJLevels         |Number of justification levels;+
                                0 if no justification           |2.0 – added
|JestLevel |jLevels[]          |Justification information for
                                each level.                     |2.0 – added
|USHORT    |numLigComp         |Number of initial glyph
                                attributes that represent
                                ligature components             |
|BYTE      |numUserDefn        |Number of user-defined slot
                                attributes                      |
|BYTE      |maxCompPerLig      |Maximum number of components per
                                ligature                        |
|BYTE      |direction          |Supported direction(s)          |
|BYTE      |attCollisions      |First of a set of attributes
                                that hold collision flags and
                                constraint box                  | 5.0 - used
|BYTE      |reserved           |                                |
|BYTE      |reserved           |                                |
|BYTE      |reserved           |                                |2.0 – added
|BYTE      |numCritFeatures    |Number of critical features     |2.0 – added
|USHORT    |critFeatures[]     |Array of critical features      |2.0 – added
|BYTE      |reserved           |                                |2.0 – added
|BYTE      |numScriptTag       |Number of scripts in scriptTag  |
|ULONG     |scriptTag[]        |Array of script tags            |
|USHORT    |lbGID              |Glyph ID for line-break
                                psuedo-glyph                    |
|ULONG     |oPasses[]          |Offets to passes relative to the
                                start of this subtable; +
                                numPasses + 1 of these          |
|USHORT    |numPseudo          |Number of Unicode ->
                                pseudo-glyph mappings           |
|USHORT    |searchPseudo       |(max power of 2 <= numPseudo)
                                * sizeof(PseudoMap) [Deprecated]|
|USHORT    |pseudoSelector     |log2(max power of 2<= numPseudo)
                                [Deprecated]                    |
|USHORT    |pseudoShift        |numPseudo - searchPseudo
                                [Deprecated]                    |
|PseudoMap |pMaps[]            |Unicode ->pseudo-glyph mappings
                                in Unicode order                |
|ClassMap  |classes            |Classes map storing replacement
                                classes used in actions         |
|SIL_Pass  |passes[]]          |Array of passes                 |
|===============================================================================

Deprecated values will not be removed from the structure, but their meaning is
lost, they become reserved values that will not be reassigned. Each
justification level has several glyph attributes associated with it.

This structure was new as of version 2.0.

.JustificationLevel
[cols="3,3,8,3",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type |Name        |Description                                  |Version notes

|BYTE |attrStretch |Glyph attribute number for justify.X.stretch |
|BYTE |attrShrink  |Glyph attribute number for justify.X.shrink  |
|BYTE |attrStep    |Glyph attribute number for justify.X.step    |
|BYTE |attrWeight  |Glyph attribute number for justify.X.weight  |
|BYTE |runto       |Which level starts the next stage            |
|BYTE |reserved    |                                             |
|BYTE |reserved    |                                             |
|BYTE |reserved    |                                             |
|===============================================================================

A pseudo-glyph is a glyph which contains no font metrics (it has a GID greater
than the numGlyphs entry in the maxp table) but is used in the rendering
process. Each pseudo-glyph has an attribute which is the glyph ID of a real
glyph which will be used to actually render the glyph. The pseudo-glyph map
contains a mapping between Unicode and pseudo-glyph number:

.PseudoMap
[cols="3,3,8,3",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type   |Name    |Description               |Version notes

|ULONG  |unicode |Unicode codepoint         |2.0 – changed from USHORT to ULONG
|USHORT |nPseudo |Glyph ID of pseudo-glyph  |
|===============================================================================

The ClassMap stores the replacement class information for the passes in this
description. Replacement classes are used during substitution where a glyph id
is looked up in one class and the glyph ID at the corresponding index in another
class is substituted. The difficulty with the storage of such classes is in
looking up a glyph ID in an arbitrarily ordered list. One approach is to use a
linear search; this is very slow, but is stored very simply. Another approach is
to order the glyphs in the class and to store the index against the glyph. Both
approaches are supported in the ClassMap table structure:

.ClassMap
[cols="3,3,8,3",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type        |Name      |Description                   |Version notes

|USHORT      |numClass  |Number of replacement classes |
|USHORT      |numLinear |Number of linearly stored
                         replacement classes           |
|ULONG       |oClass[]  |Array of numClass + 1 offsets
                        to class arrays from the
                        beginning of the class map     | 4.0 changed from USHORT
|USHORT      |glyphs[]  |Glyphs for linear classes     |
|LookupClass |lookups[] |An array of numClass –
                        numLinear lookups              |
|===============================================================================

The LookupClass stores a fast lookup association between glyph ID and index.
Each lookup consists of an ordered list of glyph IDs with the corresponding
index for that glyph. The number of elements in the lookup is specified by
numIds along with a search Range and shift to initialize a fast binary search
engine:

.LookupClass
[cols="3,3,8,3",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type       |Name          |Description                          |Version notes

|USHORT     |numIDs        |Number of elements in the lookup     |
|USHORT     |searchRange   |(max power of 2<= numIDs)            |
|USHORT     |entrySelector |log2(max power of 2<= numIDs)        |
|USHORT     |rangeShift    |numIds – searchRange                 |
|LookupPair |lookups[]     |lookups; there are numIDs of these   |
|===============================================================================

Each element in the lookup consists of a glyphId and the corresponding index in
the original ordered list.

.LookupPair
[cols="3,3,8,3",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type   |Name    |Description                                    |Version notes

|USHORT |glyphId |glyph id to be compared                        |
|USHORT |index   |index corresponding to glyphId in ordered list |
|===============================================================================

=== Pass ===

Each processing pass consists of a finite state machine description for rule
finding, and the actions that are executed when a rule is matched.

.SIL_Pass
[cols="5,9,34,8",options="header",frame="topbot",grid="none"]
|===============================================================================
|Type |Name |Description                                     |Version notes

|BYTE       |flags             |0-2 - number of collision runs; +
                                3-4 - kerning collisions; +
                                5 - reverse direction pass   |5.0 - add bits 0-5
|BYTE       |maxRuleLoop       |MaxRuleLoop for this pass    |
|BYTE       |maxRuleContext    |Number of slots of input
                                needed to run this pass      |
|BYTE       |maxBackup         |Maximum number of slots this
                                pass is allowed to back up)  |
|USHORT     |numRules          |Number of action code blocks |
|USHORT     |fsmOffset         |offset to numRows relative to
                                the beginning of the SIL_Pass
                                block                        |2.0 - added; +
                                                              3.0 - used
|ULONG      |pcCode            |Offset to pass constraint
                                code from start of subtable
                                (*passConstraints[0]*)       |2.0 - added
|ULONG      |rcCode            |Offset to rule constraint
                                code from start of subtable
                                (*ruleConstraints[0]*)       |
|ULONG      |aCode             |Offset to action code from
                                start of subtable
                                (*actions[0]*)               |
|ULONG      |oDebug            |Offset to debug arrays
                                (*dActions[0]*); +
                                0 if debug stripped          |
|USHORT     |numRows           |Number of FSM states         |
|USHORT     |numTransitional   |Number of transitional states
                                in the FSM (length of
                                *states* matrix)             |
|USHORT     |numSuccess        |Number of success states in
                                the FSM (size of *oRuleMap*
                                array)                       |
|USHORT     |numColumns        |Number of FSM columns        |
|USHORT     |numRange          |Number of contiguous glyph ID
                                ranges which map to columns  |
|USHORT     |searchRange       |(maxi power of 2 <= numRange)
                                *sizeof(Pass_Range)[Deprecated]|
|USHORT     |entrySelector     |log2(maximum power of 2
                                <= numRange) [Deprecated]    |
|USHORT     |rangeShift        |numRange*sizeof(Pass_Range)
                                - searchRange [Deprecated]   |
|Pass_Range |ranges[]          |Ranges of glyph IDs in this FSM; +
                                *numRange* of these |
|USHORT     |oRuleMap[]        |Maps from success state to offset into ruleMap
                                array from start of array. 1st item corresponds
                                to state # (numRows – numSuccess);
                                ie, non-success states are omitted.
                                [0xFFFF implies rule number is equal to state
                                number (no entry in ruleMap) – NOT IMPLEMENTED].
                                There are (numSuccess + 1) entries. |
|USHORT     |ruleMap[]         |Linear arrays of rule numbers
                                maping to a success state number |
|BYTE       |minRulePreContext |Min number of items in any rule
                                context before 1st modified rule item |
|BYTE       |maxRulePreContext |Max number of items in any rule
                                context before 1st modified rule item |
|USHORT     |startStates[]     |Array of size (maxRulePreContext
                                - minRulePreContext + 1), indicating the start
                                state in the state machine based on how many
                                pre-context items a rule has |
|USHORT     |ruleSortKeys[]    |Array of *numRules* sort keys,
                                indicating precedence of rules |
|BYTE       |rulePreContext[]  |Array of *numRules* items indicating
                                the number of items in the context
                                before the 1st modified item,
                                one for each rule            |
|BYTE       |collisionThreshold|                             |2.0 - inserted, +
                                                              5.0 – used
|USHORT     |pConstraint       |passConstraint block length  |2.0 – added
|USHORT     |oConstraints[]    |numRules + 1 offsets to constraint code blocks
                                from start of ruleConstraints. |
|USHORT     |oActions[]        |numRules + 1 offsets to action code blocks
                                from start of actions.       |
|USHORT     |stateTrans[][]    |Array of *numTransitional* rows of *numColumns*
                                state transitions.           |
|BYTE       |reserved          |                             |2.0 – inserted
|BYTE       |passConstraints[] |Sequences of constraint code
                                for pass-level constraints   |2.0 – added
|BYTE       |ruleConstraints[] |Sequences of constraint code
                                for rules                    |
|BYTE       |actions[]         |Sequences of action code     |
|footnote:[ Should debug tables go at the end, and be
            marked via a flag as per Gloc?
]USHORT     |dActions[]        |Name index for each action for
                                documentation purposes. +
                                0 = stripped. length numRules |
|USHORT     |dStates[]         |Name index for each intermediate
                                FSM row/state for debugging. +
                                0 = stripped.
                                Corresponds to the
                                last numRows – numRules      |
|USHORT     |dCols[]           |Name index for each state
                                (length numRows)             |
|===============================================================================

Deprecated values will not be removed from the structure, but their meaning is
lost, they become reserved values that will not be reassigned.

Notice that the ranges array has fast lookup information on the front to allow
for the quick identification of which range a particular glyph id is in. Each
range consists of the first and last glyph id in the range.

.Pass_Range
[cols="3,3,8",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type   |Name    |Description

|USHORT |firstId |First Glyph id in the range
|USHORT |lastId  |Last Glyph id in the range
|USHORT |colId   |Column index for this range
|===============================================================================

==== Pass Contents ====

A pass contains a Finite State Machine (FSM) which is used to match input
strings to rules. It also contains constraints for further testing whether a
matched string should fire, and it contains the action code to execute against
the matched string.

The FSM consists of a set of states. A state consists of a row of transitions
between that state and another state dependent upon the next glyph in the input
stream. Each state may be an acceptance state, in which case it corresponds to a
rule match, or a transition state, in which case the state is on the way to
matching a rule, or both. A null state transition is one in which the occurrence
of this particular class of the following glyph, will result in no extension of
a rule match anywhere, just fail on all further searching. A final state is one
in which all its transitions are null transitions.

Note that the stateTrans array only needs to represent transitional states, not
final states. Similarly, the oRuleMap array only needs entries for acceptance
states (whether final or transitional). For this reason the FSM is set up
(conceptually) in the following order: transitional non-accepting states first,
followed by transitional accepting states, followed by final (accepting) states.

Note also that because there may be more than one matched rule for a given state,
oRuleMap indicates a list of rule indices in the ruleMap array;
oRuleMap[i+1] - oRuleMap[i] indicates how many there are for state i.

Normally the start state for an FSM is zero. But for each pass there is the idea
of a “pre-context,” that is, there are slots that need to be taken into
consideration in the rule-matching process that are before the current position
of the input stream. If we are very near the beginning of the input, we may need
to adjust by skipping some states, which corresponds to skipping the
“pre-context” slots that not present due to being prior to the beginning of the
input.  This is what the maxRulePreContext, minRulePreContext, and startStates
items are used for. Specifically, we need to skip the number of transitions
equal to the difference between the maxRulePreContext and the current stream
position, if greater than zero. The startStates array indicates what the
adjusted start state should be. If the current input position is less than
minRulePreContext, no rule will match at all.

Rules are matched in order of length, so that longest rules are given precedence
over shorter rules. However, the length of some rules may have been adjusted to
allow for a consistent “pre-context” for all rules, so the number of matched
states in the FSM may not correspond to the actual number of matched items in
the rule. For this reason, it is not adequate to simply order rules based on the
number of traversed states in the FSM. Rather, rules are given sort keys
indicating their precedence, which is based primarily on the length of the rule
and secondarily on its original position within the source code.

The FSM engine keeps track of all the acceptance states it passes through on its
path to a final state. This results in a list of rules matched by the string
sorted by precedence. The engine takes the first rule index off the list and
looks up the offset to some constraint code. This code is executed and if the
constraint passes, then the action code associated with that offset is executed
and the FSM restarts at the returned slot position. If the constraint fails,
then the FSM considers the next-preferred rule, tests that constraint, and so
forth. If no accepting state is found or all rules fail their constraints, then
no rule applies, in which case a single glyph is put into the output stream and
the current position advances by one slot.

The action strings are simply byte strings of actions, much like hinting code,
but using a completely different language. (See “Stack Machine Commands.doc”.)

=== Sile ===

This table is used in Graphite table files that rely on an external font for
rendering of the glyphs. When this table is present, the Graphite file is in
effect a minimal font that contains information about the actual font to use in
rendering. This information is stored in the Sile table.

This table was added as of version 2. It is not currently being used.

.Sile
[cols="3,3,8",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type   |Name           |Description

|FIXED  |version        |Table version: 00010000
|ULONG  |checksum       |master checksum (checkSumAdjustment) from the head
                         table of the base font
|ULONG  |createTime[2]  |Create time of the base font (64-bits)
                         from the head table
|ULONG  |modifyTime[2]  |Modify time of the base font (64-bits)
                         from the headtable
|USHORT |fontNameLength |Number of characters in fontName
|USHORT |fontName[]     |Family name of base font
|USHORT |fontFileLength |Number of characters in baseFile
|USHORT |baseFile[]     |Original path and name of base font file
|===============================================================================

There are four possible situations with regard to the Sile table. The first two
are considered normal and the second two pathological.

No Sile table is present. In this case, it is assumed that the Graphite table
file is a normal font containing not only the Graphite tables but also the
glyphs and metrics needed for rendering.

The base font named in the Sile table is present on the system, and its master
checksum and dates match those in the Sile table. In this case, the Graphite
tables are read from the Graphite table file, but the glyphs, metrics, and cmap
from the base font are what are used for rendering (with the modification
performed by the Graphite tables).

The base font named in the Sile table is present, but its master checksum and/or
dates do not match those in the Sile table. In this case the base font is used
to perform the rendering, but with no Graphite behaviors.

The base font named in the Sile table is not present on the system. In this case
the Graphite table file is used for the rendering, with no Graphite behaviors,
resulting in square boxes in place of the expected glyphs.

=== Sill ===

This table maps ISO-639-3 language codes onto feature values. Each language code
can be a maxmum of 4 ASCII characters (although 2 or 3 characters is what is
used by the ISO standard).

This table was added as of version 3.

.Sill
[cols="4,3,8,3",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type               |Name          |Description                  |Version notes

|FIXED              |version       |Table version: 00010000      |
|USHORT             |numLangs      |Number of languages supported|
|USHORT             |searchRange   |(max power of 2 <= numLangs)
                                    [Deprecated]                 |
|USHORT             |entrySelector |log2(max power of
                                    2 <= numLangs) [Deprecated]  |
|USHORT             |rangeShift    |numLangs-searchRange
                                    [Deprecated]                 |
|LanguageEntry      |entries[]     |Languages and pointers to
                                    feature settings; +
                                    numLang + 1 length           |
|LangFeatureSetting |settings[]    |Feature ID / value pairs     |
|===============================================================================

Each language entry contains a 4-character language code and an offset to the
list of features. There is one bogus entry at the end that facilitates finding
the size of the last entry. The offsets are relative to the beginning of the
Sill table.

The language code is left-aligned with any unused characters padded with NULLs.
For instance, the code “en” is represented by the four bytes [101, 110, 0, 0].

.LanguageEntry
[cols="3,3,8,3",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type   |Name        |Description                                |Version notes

|BYTE   |langcode[4] |4-char ISO-639-3 language code             |
|USHORT |numSettings |Number of feature settings of language     |
|USHORT |offset      |Offset to 1st feature setting of language  |
|===============================================================================

.LangFeatureSetting
[cols="3,3,8,3",options="unbreakable,header",width="90%",frame="topbot",grid="none"]
|===============================================================================
|Type   |Name      |Description                                  |Version notes

|ULONG  |featureId |Feature identifer number (matches ID in
                    Feat table)                                  |
|SHORT  |value     |Default feature value for this language      |
|USHORT |reserved  |Pad bytes                                    |
|===============================================================================

=== Sild ===

This table holds the debug strings for debugging purposes. Since the strings are
only used for debugging, they are held somewhat optimised for space over speed
and are not considered to be multilingual. Thus strings are considered to be
7-bit ASCII, with a possible extension to UTF-8 at a later stage. The table
consists of a sequence of strings each preceded by a length byte. The first
string is id 0 and so on to the end of the table.

NOTE: this table has not been implemented.

== Multiple Descriptions ==

In the case where multiple descriptions are to be stored in the same set of
tables, the following unifications need to occur:

The feature sets must be unified, thus limiting two features with the same name
to having the same settings and corresponding values.

The glyph attributes must be unified. This can be done by using different
attribute number ranges, or by examining for identical attribute mappings or for
non-intersecting attribute mappings.

The use of the name table must be unified to ensure that two features or feature
settings do not refer to the same entry in the name table.

Notice that the requirement that any tables declared in an external binary
description override the corresponding font table in the font, means that a name
table in an external binary description must be complete, including all the
strings from the original font.

include::OpCodes.txt[]