GSUB - The Glyph Substitution Table

The Glyph Substitution table (GSUB) contains information for substituting glyphs to render the scripts and language systems supported in a font. Many language systems require glyph substitutes. For example, in the Arabic script, the glyph shape that depicts a particular character varies according to its position in a word or text string (see figure 1). In other language systems, glyph substitutes are aesthetic options for the user, such as the use of ligature glyphs in the English language (see Figure 2).

Figure 1. Isolated, initial, medial, and final forms of the Arabic character HAH
fig3a.gif

Figure 2. Two Latin glyphs and their associated ligature
fig3b.gif

Overview

Many fonts use limited character encoding standards that map glyphs to characters one-to-one, assigning a glyph to each character code value in a font. Multiple character codes cannot be mapped to a single glyph, as needed for ligature glyphs, and multiple glyphs cannot be mapped to a single character code, as needed to decompose a ligature into its component glyphs.

To supply glyph substitutes, font developers must assign different character codes to the glyphs, or they must create additional fonts or character sets. To access these glyphs, users must bear the burden of switching between character codes, character sets, or fonts.

Substituting Glyphs with OpenType™

The OpenType GSUB table fully supports glyph substitution. To access glyph substitutes, GSUB maps from the glyph index or indices defined in a cmap table to the glyph index or indices of the glyph substitutes. For example, if a font has three alternative forms of an ampersand glyph, the cmap table associates the ampersand's character code with only one of these glyphs. In GSUB, the indices of the other ampersand glyphs are then referenced by this one index.

The text-processing client uses the GSUB data to manage glyph substitution actions. GSUB identifies the glyphs that are input to and output from each glyph substitution action, specifies how and where the client uses glyph substitutes, and regulates the order of glyph substitution operations. Any number of substitutions can be defined for each script or language system represented in a font.

The GSUB table supports six types of glyph substitutions that are widely used in international typography:


Table Organization

The GSUB table begins with a header that defines offsets to a ScriptList, a FeatureList, and a LookupList (see Figure 3g):

For a detailed discussion of ScriptLists, FeatureLists, and LookupLists, see the chapter OpenType Common Table Formats.

 

Figure 7. High-level organization of GSUB table
fig3g.gif

This organization helps text-processing clients to easily locate the features and lookups that apply to a particular script or language system. To access GSUB information, clients should use the following procedure:

  1. Locate the current script in the GSUB ScriptList table.
  2. If the language system is known, search the script for the correct LangSys table; otherwise, use the script's default language system (DefaultLangSys table).
  3. The LangSys table provides index numbers into the GSUB FeatureList table to access a required feature and a number of additional features.
  4. Inspect the FeatureTag of each feature, and select the features to apply to an input glyph string. Each feature provides an array of index numbers into the GSUB LookupList table.
  5. Assemble all lookups from the set of chosen features, and apply the lookups in the order given in the LookupList table.

Lookup data is defined in one or more subtables that define the specific conditions, type, and results of a substitution action used to implement a feature. All subtables in a lookup must be of the same LookupType, as listed in the LookupType Enumeration table:


LookupType Enumeration table for glyph substitution

Value Type Description
1 Single (format 1.1 1.2) Replace one glyph with one glyph
2 Multiple (format 2.1) Replace one glyph with more than one glyph
3 Alternate (format 3.1) Replace one glyph with one of many glyphs
4 Ligature (format 4.1) Replace multiple glyphs with one glyph
5 Context (format 5.1 5.2 5.3) Replace one or more glyphs in context
6 Chaining Context (format 6.1 6.2 6.3) Replace one or more glyphs in chained context
7 Extension Substitution (format 7.1) Extension mechanism for other substitutions (i.e. this excludes the Extension type substitution itself)
8 Reverse chaining context single (format 8.1) Applied in reverse order, replace single glyph in chaining context
9+ Reserved For future use (set to zero)

Each LookupType subtable has one or more formats. The “best” format depends on the type of substitution and the resulting storage efficiency. When glyph information is best presented in more than one format, a single lookup may define more than one subtable, as long as all the subtables are for the same LookupType. For example, within a given lookup, a glyph index array format may best represent one set of target glyphs, whereas a glyph index range format may be better for another set.

A series of substitution operations on the same glyph or string requires multiple lookups, one for each separate action. Each lookup is given a different array number in the LookupList table and is applied in the LookupList order.

During text processing, a client applies a lookup to each glyph in the string before moving to the next lookup. A lookup is finished for a glyph after the client locates the target glyph or glyph context and performs a substitution, if specified. To move to the “next” glyph, the client will typically skip all the glyphs that participated in the lookup operation: glyphs that were substituted as well as any other glyphs that formed a context for the operation.

In the case of chained contextual lookups, glyphs comprising backtrack and lookahead sequences may participate in more than one context.

The rest of this chapter describes the GSUB header and the subtables defined for each GSUB LookupType. Examples at the end of this page illustrate each of the five LookupTypes, including the three formats available for contextual substitutions.

GSUB Header

The GSUB table begins with a header that contains a version number for the table (Version) and offsets to a three tables: ScriptList, FeatureList, and LookupList. For descriptions of each of these tables, see the chapter, OpenType Common Table Formats. Example 1 at the end of this chapter shows a GSUB Header table definition.

GSUB Header

Type Name Description
Fixed Version Version of the GSUB table-initially set to 0x00010000
Offset ScriptList Offset to ScriptList table-from beginning of GSUB table
Offset FeatureList Offset to FeatureList table-from beginning of GSUB table
Offset LookupList Offset to LookupList table-from beginning of GSUB table

LookupType 1: Single Substitution Subtable

Single substitution (SingleSubst) subtables tell a client to replace a single glyph with another glyph. The subtables can be either of two formats. Both formats require two distinct sets of glyph indices: one that defines input glyphs (specified in the Coverage table), and one that defines the output glyphs. Format 1 requires less space than Format 2, but it is less flexible.

1.1 Single Substitution Format 1

Format 1 calculates the indices of the output glyphs, which are not explicitly defined in the subtable. To calculate an output glyph index, Format 1 adds a constant delta value to the input glyph index. For the substitutions to occur properly, the glyph indices in the input and output ranges must be in the same order. This format does not use the Coverage Index that is returned from the Coverage table.

The SingleSubstFormat1 subtable begins with a format identifier (SubstFormat) of 1. An offset references a Coverage table that specifies the indices of the input glyphs. DeltaGlyphID is the constant value added to each input glyph index to calculate the index of the corresponding output glyph.

Example 2 at the end of this chapter uses Format 1 to replace standard numerals with lining numerals.


SingleSubstFormat1 subtable: Calculated output glyph indices

Type Name Description
uint16 SubstFormat Format identifier-format = 1
Offset Coverage Offset to Coverage table-from beginning of Substitution table
int16 DeltaGlyphID Add to original GlyphID to get substitute GlyphID

1.2 Single Substitution Format 2

Format 2 is more flexible than Format 1, but requires more space. It provides an array of output glyph indices (Substitute) explicitly matched to the input glyph indices specified in the Coverage table.

The SingleSubstFormat2 subtable specifies a format identifier (SubstFormat), an offset to a Coverage table that defines the input glyph indices, a count of output glyph indices in the Substitute array (GlyphCount), and a list of the output glyph indices in the Substitute array (Substitute).

The Substitute array must contain the same number of glyph indices as the Coverage table. To locate the corresponding output glyph index in the Substitute array, this format uses the Coverage Index returned from the Coverage table.

Example 3 at the end of this chapter uses Format 2 to substitute vertically oriented glyphs for horizontally oriented glyphs.


SingleSubstFormat2 subtable: Specified output glyph indices

Type Name Description
uint16 SubstFormat Format identifier-format = 2
Offset Coverage Offset to Coverage table-from beginning of Substitution table
uint16 GlyphCount Number of GlyphIDs in the Substitute array
GlyphID Substitute
[GlyphCount]
Array of substitute GlyphIDs-ordered by Coverage Index

LookupType 2: Multiple Substitution Subtable

A Multiple Substitution (MultipleSubst) subtable replaces a single glyph with more than one glyph, as when multiple glyphs replace a single ligature. The subtable has a single format: MultipleSubstFormat1. The subtable specifies a format identifier (SubstFormat), an offset to a Coverage table that defines the input glyph indices, a count of offsets in the Sequence array (SequenceCount), and an array of offsets to Sequence tables that define the output glyph indices (Sequence). The Sequence table offsets are ordered by the Coverage Index of the input glyphs.

For each input glyph listed in the Coverage table, a Sequence table defines the output glyphs. Each Sequence table contains a count of the glyphs in the output glyph sequence (GlyphCount) and an array of output glyph indices (Substitute).

Note: The order of the output glyph indices depends on the writing direction of the text. For text written left to right, the left-most glyph will be first glyph in the sequence. Conversely, for text written right to left, the right-most glyph will be first.

The use of multiple substitution for deletion of an input glyph is prohibited. GlyphCount should always be greater than 0.

Example 4 at the end of this chapter shows how to replace a single ligature with three glyphs.


MultipleSubstFormat1 subtable: Multiple output glyphs

Type Name Description
uint16 SubstFormat Format identifier-format = 1
Offset Coverage Offset to Coverage table-from beginning of Substitution table
uint16 SequenceCount Number of Sequence table offsets in the Sequence array
Offset Sequence
[SequenceCount]
Array of offsets to Sequence tables-from beginning of Substitution table-ordered by Coverage Index

Sequence table

Type Name Description
uint16 GlyphCount Number of GlyphIDs in the Substitute array. This should always be greater than 0.
GlyphID Substitute
[GlyphCount]
String of GlyphIDs to substitute

LookupType 3: Alternate Substitution Subtable

An Alternate Substitution (AlternateSubst) subtable identifies any number of aesthetic alternatives from which a user can choose a glyph variant to replace the input glyph. For example, if a font contains four variants of the ampersand symbol, the cmap table will specify the index of one of the four glyphs as the default glyph index, and an AlternateSubst subtable will list the indices of the other three glyphs as alternatives. A text-processing client would then have the option of replacing the default glyph with any of the three alternatives.

The subtable has one format: AlternateSubstFormat1. The subtable contains a format identifier (SubstFormat), an offset to a Coverage table containing the indices of glyphs with alternative forms (Coverage), a count of offsets to AlternateSet tables (AlternateSetCount), and an array of offsets to AlternateSet tables (AlternateSet).

For each glyph, an AlternateSet subtable contains a count of the alternative glyphs (GlyphCount) and an array of their glyph indices (Alternate). Because all the glyphs are functionally equivalent, they can be in any order in the array.

Example 5 at the end of this chapter shows how to replace the default ampersand glyph with alternative glyphs.


AlternateSubstFormat1 subtable: Alternative output glyphs

Type Name Description
uint16 SubstFormat Format identifier-format = 1
Offset Coverage Offset to Coverage table-from beginning of Substitution table
uint16 AlternateSetCount Number of AlternateSet tables
Offset AlternateSet
[AlternateSetCount]
Array of offsets to AlternateSet tables-from beginning of Substitution table-ordered by Coverage Index

AlternateSet table

Type Name Description
uint16 GlyphCount Number of GlyphIDs in the Alternate array
GlyphID Alternate[GlyphCount] Array of alternate GlyphIDs-in arbitrary order

LookupType 4: Ligature Substitution Subtable

A Ligature Substitution (LigatureSubst) subtable identifies ligature substitutions where a single glyph replaces multiple glyphs. One LigatureSubst subtable can specify any number of ligature substitutions.

The subtable uses a single format: LigatureSubstFormat1. It contains a format identifier (SubstFormat), a Coverage table offset (Coverage), a count of the ligature sets defined in this table (LigSetCount), and an array of offsets to LigatureSet tables (LigatureSet). The Coverage table specifies only the index of the first glyph component of each ligature set.


LigatureSubstFormat1 subtable: All ligature substitutions in a script

Type Name Description
uint16 SubstFormat Format identifier-format = 1
Offset Coverage Offset to Coverage table-from beginning of Substitution table
uint16 LigSetCount Number of LigatureSet tables
Offset LigatureSet
[LigSetCount]
Array of offsets to LigatureSet tables-from beginning of Substitution table-ordered by Coverage Index

A LigatureSet table, one for each covered glyph, specifies all the ligature strings that begin with the covered glyph. For example, if the Coverage table lists the glyph index for a lowercase “f,” then a LigatureSet table will define the “ffl,” “fl,” “ffi,” “fi,” and “ff” ligatures. If the Coverage table also lists the glyph index for a lowercase “e,” then a different LigatureSet table will define the “etc” ligature.

A LigatureSet table consists of a count of the ligatures that begin with the covered glyph (LigatureCount) and an array of offsets to Ligature tables, which define the glyphs in each ligature (Ligature). The order in the Ligature offset array defines the preference for using the ligatures. For example, if the “ffl” ligature is preferable to the “ff” ligature, then the Ligature array would list the offset to the “ffl” Ligature table before the offset to the “ff” Ligature table.


LigatureSet table: All ligatures beginning with the same glyph

Type Name Description
uint16 LigatureCount Number of Ligature tables
Offset Ligature
[LigatureCount]
Array of offsets to Ligature tables-from beginning of LigatureSet table-ordered by preference

For each ligature in the set, a Ligature table specifies the GlyphID of the output ligature glyph (LigGlyph); a count of the total number of component glyphs in the ligature, including the first component (CompCount); and an array of GlyphIDs for the components (Component). The array starts with the second component glyph (array index = 1) in the ligature because the first component glyph is specified in the Coverage table.

Note: The Component array lists GlyphIDs according to the writing direction of the text. For text written right to left, the right-most glyph will be first. Conversely, for text written left to right, the left-most glyph will be first.

Example 6 at the end of this chapter shows how to replace a string of glyphs with a single ligature.

Ligature table: Glyph components for one ligature

Type Name Description
GlyphID LigGlyph GlyphID of ligature to substitute
uint16 CompCount Number of components in the ligature
GlyphID Component
[CompCount - 1]
Array of component GlyphIDs-start with the second component-ordered in writing direction

LookupType 5: Contextual Substitution Subtable

A Contextual Substitution (ContextSubst) subtable defines the most powerful type of glyph substitution lookup: it describes glyph substitutions in context that replace one or more glyphs within a certain pattern of glyphs.

ContextSubst subtables can be any of three formats that define a context in terms of a specific sequence of glyphs, glyph classes, or glyph sets. Each format can describe one or more input glyph sequences and one or more substitutions for each sequence.

All three formats of ContextSubst subtables specify substitution data in a SubstLookupRecord. A description of that record follows.

SubstLookupRecord

Type Name Description
uint16 SequenceIndex Index into current glyph sequence-first glyph = 0
uint16 LookupListIndex Lookup to apply to that position-zero-based

The SequenceIndex in a SubstLookupRecord must take into consideration the order in which lookups are applied to the entire glyph sequence. Because multiple substitutions may occur per context, the SequenceIndex and LookupListIndex refer to the glyph sequence after the text-processing client has applied any previous lookups. In other words, the SequenceIndex identifies the location for the substitution at the time that the lookup is to be applied. For example, consider an input glyph sequence of four glyphs. The first glyph does not have a substitute, but the middle two glyphs will be replaced with a ligature, and a single glyph will replace the fourth glyph:

Note: This example assumes that the LookupList specifies the ligature substitution lookup before the single substitution lookup.

5.1 Context Substitution Format 1

Format 1 defines the context for a glyph substitution as a particular sequence of glyphs. For example, a context could be <xyz>, <holiday>, <!?*#@>, or any other glyph sequence.

Within a context sequence, Format 1 identifies particular glyph positions (not glyph indices) as the targets for specific substitutions. When a text-processing client locates a context in a string of text, it finds the lookup data for a targeted position and makes a substitution by applying the lookup data at that location.

For example, if a client is to replace the glyph string <abc> with its reverse glyph string <cba>, the input context is defined as the glyph sequence, <abc>, and the lookups defined for the context are (1) “a” to “c” and (2) “c” to “a”. When a client encounters the context <abc>, the lookups are performed in the order stored. First, “c” is substituted for “a” resulting in <cbc>. Second, “a” is substituted for the “c” that has not yet been touched, resulting in <cba>.

To specify a context, a Coverage table lists the first glyph in the sequence, and a SubRule table identifies the remaining glyphs. To describe the >abc< context used in the previous example, the Coverage table lists the glyph index of the first component of the sequence-the “a” glyph. A SubRule table defines indices for the “b” and “c” glyphs.

A single ContextSubstFormat1 subtable may define more than one context glyph sequence. If different context sequences begin with the same glyph, then the Coverage table should list the glyph only once because all glyphs in the table must be unique. For example, if three contexts each start with an “s” and two start with a “t,” then the Coverage table will list one “s” and one “t.”

For each context, a SubRule table lists all the glyphs that follow the first glyph. The table also contains an array of SubstLookupRecords that specify the substitution lookup data for each glyph position (including the first glyph position) in the context.

All of the SubRule tables defining contexts that begin with the same first glyph are grouped together and defined in a SubRuleSet table. For example, the SubRule tables that define the three contexts that begin with an “s” are grouped in one SubRuleSet table, and the SubRule tables that define the two contexts that begin with a “t” are grouped in a second SubRuleSet table. Each glyph listed in the Coverage table must have a SubRuleSet table defining all the SubRule tables that apply to a covered glyph.

To locate a context glyph sequence, the text-processing client searches the Coverage table each time it encounters a new text glyph. If the glyph is covered, the client reads the corresponding SubRuleSet table and examines each SubRule table in the set to determine whether the rest of the context matches the subsequent glyphs in the text. If the context and text string match, the client finds the target glyph positions, applies the lookups for those positions, and completes the substitutions.

A ContextSubstFormat1 subtable contains a format identifier (SubstFormat), an offset to a Coverage table (Coverage), a count of defined SubRuleSets (SubRuleSetCount), and an array of offsets to the SubRuleSet tables (SubRuleSet). As mentioned, one SubRuleSet table must be defined for each glyph listed in the Coverage table.

In the SubRuleSet array, the SubRuleSet table offsets are ordered in the Coverage Index order. The first SubRuleSet in the array applies to the first GlyphID listed in the Coverage table, the second SubRuleSet in the array applies to the second GlyphID listed in the Coverage table, and so on.


ContextSubstFormat1 subtable: Simple context glyph substitution

Type Name Description
uint16 SubstFormat Format identifier-format = 1
Offset Coverage Offset to Coverage table-from beginning of Substitution table
uint16 SubRuleSetCount Number of SubRuleSet tables-must equal GlyphCount in Coverage table
Offset SubRuleSet
[SubRuleSetCount]
Array of offsets to SubRuleSet tables-from beginning of Substitution table-ordered by Coverage Index

A SubRuleSet table consists of an array of offsets to SubRule tables (SubRule), ordered by preference, and a count of the SubRule tables defined in the set (SubRuleCount). The order in the SubRule array can be critical. Consider two contexts, <abc> and <abcd>. If <abc> is first in the SubRule array, all instances of <abc> in the text-including all instances of <abcd>-will be changed. If <abcd> comes first in the array, however, only <abcd> sequences will be changed, without affecting any instances of <abc>.


SubRuleSet table: All contexts beginning with the same glyph

Type Name Description
uint16 SubRuleCount Number of SubRule tables
Offset SubRule
[SubRuleCount]
Array of offsets to SubRule tables-from beginning of SubRuleSet table-ordered by preference

A SubRule table consists of a count of the glyphs to be matched in the input context sequence (GlyphCount), including the first glyph in the sequence, and an array of glyph indices that describe the context (Input). The Coverage table specifies the index of the first glyph in the context, and the Input array begins with the second glyph (array index = 1) in the context sequence.

Note: The Input array lists the indices in the order the corresponding glyphs appear in the text. For text written from right to left, the right-most glyph will be first; conversely, for text written from left to right, the left-most glyph will be first.

A SubRule table also contains a count of the substitutions to be performed on the input glyph sequence (SubstCount) and an array of SubstitutionLookupRecords (SubstLookupRecord). Each record specifies a position in the input glyph sequence and a LookupListIndex to the substitution lookup that is applied at that position. The array should list records in design order, or the order the lookups should be applied to the entire glyph sequence.

SubRule table: One simple context definition

Type Name Description
uint16 GlyphCount Total number of glyphs in input glyph sequence-includes the first glyph
uint16 SubstCount Number of SubstLookupRecords
GlyphID Input
[GlyphCount - 1]
Array of input GlyphIDs-start with second glyph
struct SubstLookupRecord
[SubstCount]
Array of SubstLookupRecords-in design order

Example 7 at the end of the chapter shows how to use the ContextSubstFormat1 subtable to replace a sequence of three glyphs with a sequence preferred for the French language system.

5.2 Context Substitution Format 2

Format 2, a more flexible format than Format 1, describes class-based context substitution. For this format, a specific integer, called a class value, must be assigned to each glyph component in all context glyph sequences. Contexts are then defined as sequences of glyph class values. More than one context may be defined at a time.

For example, suppose that a swash capital glyph should replace each uppercase letter glyph that is preceded by a space glyph and followed by a lowercase letter glyph (a glyph sequence of space - uppercase - lowercase). The set of uppercase glyphs would constitute one glyph class (Class 1), the set of lowercase glyphs would constitute a second class (Class 2), and the space glyph would constitute a third class (Class 3). The input context might be specified with a context rule (called a SubClassRule) that describes “the set of glyph strings that form a sequence of three glyph classes, one glyph from Class 3, followed by one glyph from Class 1, followed by one glyph from Class 2.”

Each ContextSubstFormat2 subtable contains an offset to a class definition table (ClassDef), which defines the glyph class values of all input contexts. Generally, a unique ClassDef table will be declared in each instance of the ContextSubstFormat2 table that is included in a font, even though several Format 2 tables could share ClassDef tables. Class assignments are fixed (the same for each position in the context), and classes are exclusive (a glyph cannot be in more than one class at a time). The output glyphs that replace the glyphs in the context sequences do not need class values because they are specified elsewhere by GlyphID.

The ContextSubstFormat2 subtable also contains a format identifier (SubstFormat) and defines an offset to a Coverage table (Coverage). For this format, the Coverage table lists indices for the complete set of unique glyphs (not glyph classes) that may appear as the first glyph of any class-based context. In other words, the Coverage table contains the list of glyph indices for all the glyphs in all classes that may be first in any of the context class sequences. For example, if the contexts begin with a Class 1 or Class 2 glyph, then the Coverage table will list the indices of all Class 1 and Class 2 glyphs.

A ContextSubstFormat2 subtable also defines an array of offsets to the SubClassSet tables (SubClassSet) and a count of the SubClassSet tables (SubClassSetCnt). The array contains one offset for each class (including Class 0) in the ClassDef table. In the array, the class value defines an offset's index position, and the SubClassSet offsets are ordered by ascending class value (from 0 to SubClassSetCnt - 1).

For example, the first SubClassSet listed in the array contains all contexts beginning with Class 0 glyphs, the second SubClassSet contains all contexts beginning with Class 1 glyphs, and so on. If no contexts begin with a particular class (that is, if a SubClassSet contains no SubClassRule tables), then the offset to that particular SubClassSet in the SubClassSet array will be set to NULL.


ContextSubstFormat2 subtable: Class-based context glyph substitution

Type Name Description
uint16 SubstFormat Format identifier-format = 2
Offset Coverage Offset to Coverage table-from beginning of Substitution table
Offset ClassDef Offset to glyph ClassDef table-from beginning of Substitution table
uint16 SubClassSetCnt Number of SubClassSet tables
Offset SubClassSet
[SubClassSetCnt]
Array of offsets to SubClassSet tables-from beginning of Substitution table-ordered by class-may be NULL

Each context is defined in a SubClassRule table, and all SubClassRules that specify contexts beginning with the same class value are grouped in a SubClassSet table. Consequently, the SubClassSet containing a context identifies a context's first class component.

Each SubClassSet table consists of a count of the SubClassRule tables defined in the SubClassSet (SubClassRuleCnt) and an array of offsets to SubClassRule tables (SubClassRule). The SubClassRule tables are ordered by preference in the SubClassRule array of the SubClassSet.

SubClassSet subtable

Type Name Description
uint16 SubClassRuleCnt Number of SubClassRule tables
Offset SubClassRule
[SubClassRuleCount]
Array of offsets to SubClassRule tables-from beginning of SubClassSet-ordered by preference

For each context, a SubClassRule table contains a count of the glyph classes in the context sequence (GlyphCount), including the first class. A Class array lists the classes, beginning with the second class (array index = 1), that follow the first class in the context.

Note: Text order depends on the writing direction of the text. For text written from right to left, the right-most class will be first. Conversely, for text written from left to right, the left-most class will be first.

The values specified in the Class array are the values defined in the ClassDef table. For example, a context consisting of the sequence “Class 2, Class 7, Class 5, Class 0” will produce a Class array of 7,5,0. The first class in the sequence, Class 2, is identified in the ContextSubstFormat2 table by the SubClassSet array index of the corresponding SubClassSet.

A SubClassRule also contains a count of the substitutions to be performed on the context (SubstCount) and an array of SubstLookupRecords (SubstLookupRecord) that supply the substitution data. For each position in the context that requires a substitution, a SubstLookupRecord specifies a LookupList index and a position in the input glyph sequence where the lookup is applied. The SubstLookupRecord array lists SubstLookupRecords in design order-that is, the order in which lookups should be applied to the entire glyph sequence.


SubClassRule table: Context definition for one class

Type Name Description
uint16 GlyphCount Total number of classes specified for the context in the rule-includes the first class
uint16 SubstCount Number of SubstLookupRecords
uint16 Class
[GlyphCount - 1]
Array of classes-beginning with the second class-to be matched to the input glyph class sequence
struct SubstLookupRecord
[SubstCount]
Array of Substitution lookups-in design order

Example 8 at the end of this chapter uses Format 2 to substitute Arabic mark glyphs for base glyphs of different heights.

5.3 Context Substitution Format 3

Format 3, coverage-based context substitution, defines a context rule as a sequence of coverage tables. Each position in the sequence may define a different Coverage table for the set of glyphs that matches the context pattern. With Format 3, the glyph sets defined in the different Coverage tables may intersect, unlike Format 2 which specifies fixed class assignments (identical for each position in the context sequence) and exclusive classes (a glyph cannot be in more than one class at a time).

For example, consider an input context that contains a lowercase glyph (position 0), followed by an uppercase glyph (position 1), either a lowercase or numeral glyph (position 2), and then either a lowercase or uppercase vowel (position 3). This context requires four Coverage tables, one for each position:

Unlike Formats 1 and 2, this format defines only one context rule at a time. It consists of a format identifier (SubstFormat), a count of the glyphs in the sequence to be matched (GlyphCount), and an array of Coverage offsets that describe the input context sequence (Coverage).

Note: The order of the Coverage tables listed in the Coverage array must follow the writing direction. For text written from right to left, then the right-most glyph will be first. Conversely, for text written from left to right, the left-most glyph will be first.

The subtable also contains a count of the substitutions to be performed on the input Coverage sequence (SubstCount) and an array of SubstLookupRecords (SubstLookupRecord) in design order-that is, the order in which lookups should be applied to the entire glyph sequence.

Example 9 at the end of this chapter substitutes swash glyphs for two out of three glyphs in a sequence.


ContextSubstFormat3 subtable: Coverage-based context glyph substitution

Type Name Description
uint16 SubstFormat Format identifier-format = 3
uint16 GlyphCount Number of glyphs in the input glyph sequence
uint16 SubstCount Number of SubstLookupRecords
Offset Coverage[GlyphCount] Array of offsets to Coverage table-from beginning of Substitution table-in glyph sequence order
struct SubstLookupRecord
[SubstCount]
Array of SubstLookupRecords-in design order

LookupType 6: Chaining Contextual Substitution Subtable

A Chaining Contextual Substitution subtable (ChainContextSubst) describes glyph substitutions in context with an ability to look back and/or look ahead in the sequence of glyphs. The design of the Chaining Contextual Substitution subtable is parallel to that of the Contextual Substitution subtable, including the availability of three formats for handling sequences of glyphs, glyph classes, or glyph sets. Each format can describe one or more backtrack, input, and lookahead sequences and one or more substitutions for each sequence.

6.1 Chaining Context Substitution Format 1: Simple Chaining Context Glyph Substitution

Format 1 defines the context for a glyph substitution as a particular sequence of glyphs. For example, a context could be <xyz>, <holiday>, <!?*#@>, or any other glyph sequence.

Within a context sequence, Format 1 identifies particular glyph positions (not glyph indices) as the targets for specific substitutions. When a text-processing client locates a context in a string of text, it finds the lookup data for a targeted position and makes a substitution by applying the lookup data at that location.

To specify the context, the coverage table lists the first glyph in the input sequence, and the ChainSubRule subtable defines the rest. Once a covered glyph is found at position i, the client reads the corresponding ChainSubRuleSet table and examines each table to determine if it matches the surrounding glyphs in the text. In the simplest of cases, there is a match if the string <backtrack sequence>+<input sequence>+<lookahead sequence> matches with the glyphs at position i - BacktrackGlyphCount in the text. LookupFlag values affect backtrack/lookahead sequences.

To clarify the ordering of glyph arrays for input, backtrack and lookahead sequences, the following illustration is provided. Input sequence match begins at i where the input sequence match begins. The backtrack sequence is ordered beginning at i - 1 and increases in offset value as one moves away from i. The lookahead sequence begins after the input sequence and increases in logical order.

Logical order -
                   
Input sequence -                
Backtrack sequence -            
Lookahead sequence -            

If there is a match, then the client finds the target glyph positions for substitutions and completes the substitutions. Please note that (just like in the ContextSubstFormat1 subtable) these lookups are required to operate within the range of text from the covered glyph to the end of the input sequence. No substitutions can be defined for the backtracking sequence or the lookahead sequence.

Once the substitutions are complete, the client should move to the glyph position immediately following the matched input sequence and resume the lookup process from there.

A single ChainContextSubstFormat1 subtable may define more than one context glyph sequence. If different context sequences begin with the same glyph, then the Coverage table should list the glyph only once because all glyphs in the table must be unique. For example, if three contexts each start with an “s” and two start with a “t,” then the Coverage table will list one “s” and one “t.”

All of the ChainSubRule tables defining contexts that begin with the same first glyph are grouped together and defined in a ChainSubRuleSet table. For example, the ChainSubRule tables that define the three contexts that begin with an “s” are grouped in one ChainSubRuleSet table, and the ChainSubRule tables that define the two contexts that begin with a “t” are grouped in a second ChainSubRuleSet table. Each glyph listed in the Coverage table must have a ChainSubRuleSet table defining all the ChainSubRule tables that apply to a covered glyph.

A ChainContextSubstFormat1 subtable contains a format identifier (SubstFormat), an offset to a Coverage table (Coverage), a count of defined ChainSubRuleSets (ChainSubRuleSetCount), and an array of offsets to the ChainSubRuleSet tables (ChainSubRuleSet). As mentioned, one ChainSubRuleSet table must be defined for each glyph listed in the Coverage table.

In the ChainSubRuleSet array, the ChainSubRuleSet table offsets are ordered in the Coverage Index order. The first ChainSubRuleSet in the array applies to the first GlyphID listed in the Coverage table, the second ChainSubRuleSet in the array applies to the second GlyphID listed in the Coverage table, and so on.


ChainContextSubstFormat1 subtable: Simple context glyph substitution

Type Name Description
uint16 SubstFormat Format identifier-format = 1
Offset Coverage Offset to Coverage table-from beginning of Substitution table
uint16 ChainSubRuleSetCount Number of ChainSubRuleSet tables-must equal GlyphCount in Coverage table
Offset ChainSubRuleSet
[ChainSubRuleSetCount]
Array of offsets to ChainSubRuleSet tables-from beginning of Substitution table-ordered by Coverage Index

A ChainSubRuleSet table consists of an array of offsets to ChainSubRule tables (ChainSubRule), ordered by preference, and a count of the ChainSubRule tables defined in the set (ChainSubRuleCount).

The order in the ChainSubRule array can be critical. Consider two contexts, <abc> and <abcd>. If <abc> is first in the ChainSubRule array, all instances of <abc> in the text-including all instances of <abcd>-will be changed. If <abcd> comes first in the array, however, only <abcd> sequences will be changed, without affecting any instances of <abc>.


ChainSubRuleSet table: All contexts beginning with the same glyph

Type Name Description
uint16 ChainSubRuleCount Number of ChainSubRule tables
Offset ChainSubRule
[ChainSubRuleCount]
Array of offsets to ChainSubRule tables-from beginning of ChainSubRuleSet table-ordered by preference

A ChainSubRule table consists of a count of the glyphs to be matched in the backtrack, input, and lookahead context sequences, including the first glyph in each sequence, and an array of glyph indices that describe each portion of the contexts. The Coverage table specifies the index of the first glyph in each context, and each array begins with the second glyph (array index = 1) in the context sequence.

Note: All arrays list the indices in the order the corresponding glyphs appear in the text. For text written from right to left, the right-most glyph will be first; conversely, for text written from left to right, the left-most glyph will be first.

A ChainSubRule table also contains a count of the substitutions to be performed on the input glyph sequence (SubstCount) and an array of SubstitutionLookupRecords (SubstLookupRecord). Each record specifies a position in the input glyph sequence and a LookupListIndex to the substitution lookup that is applied at that position. The array should list records in design order, or the order the lookups should be applied to the entire glyph sequence.

ChainSubRule subtable

Type Name Description
uint16 BacktrackGlyphCount Total number of glyphs in the backtrack sequence (number of glyphs to be matched before the first glyph)
GlyphID Backtrack
[BacktrackGlyphCount]
Array of backtracking GlyphID's (to be matched before the input sequence)
uint16 InputGlyphCount Total number of glyphs in the input sequence (includes the first glyph)
GlyphID Input
[InputGlyphCount - 1]
Array of input GlyphIDs (start with second glyph)
uint16 LookaheadGlyphCount Total number of glyphs in the look ahead sequence (number of glyphs to be matched after the input sequence)
GlyphID LookAhead
[LookAheadGlyphCount]
Array of lookahead GlyphID's (to be matched after the input sequence)
uint16 SubstCount Number of SubstLookupRecords
struct SubstLookupRecord
[SubstCount]
Array of SubstLookupRecords (in design order)

6.2 Chaining Context Substitution Format 2: Class-based Chaining Context Glyph Substitution

Format 2 describes class-based chaining context substitution. For this format, a specific integer, called a class value, must be assigned to each glyph component in all context glyph sequences. Contexts are then defined as sequences of glyph class values. More than one context may be defined at a time.

To chain contexts, three classes are used in the glyph ClassDef table: Backtrack ClassDef, Input ClassDef, and Lookahead ClassDef.

The ChainContextSubstFormat2 subtable also contains a format identifier (SubstFormat) and defines an offset to a Coverage table (Coverage). For this format, the Coverage table lists indices for the complete set of unique glyphs (not glyph classes) that may appear as the first glyph of any class-based context. In other words, the Coverage table contains the list of glyph indices for all the glyphs in all classes that may be first in any of the context class sequences. For example, if the contexts begin with a Class 1 or Class 2 glyph, then the Coverage table will list the indices of all Class 1 and Class 2 glyphs.

A ChainContextSubstFormat2 subtable also defines an array of offsets to the ChainSubClassSet tables (ChainSubClassSet) and a count of the ChainSubClassSet tables (ChainSubClassSetCnt). The array contains one offset for each class (including Class 0) in the ClassDef table. In the array, the class value defines an offset's index position, and the ChainSubClassSet offsets are ordered by ascending class value (from 0 to ChainSubClassSetCnt - 1).

If no contexts begin with a particular class (that is, if a ChainSubClassSet contains no ChainSubClassRule tables), then the offset to that particular ChainSubClassSet in the ChainSubClassSet array will be set to NULL.


ChainContextSubstFormat2 subtable: Class-based chaining context glyph substitution

Type Name Description
uint16 SubstFormat Format identifier-format = 2
Offset Coverage Offset to Coverage table-from beginning of Substitution table
Offset BacktrackClassDef Offset to glyph ClassDef table containing backtrack sequence data-from beginning of Substitution table
Offset InputClassDef Offset to glyph ClassDef table containing input sequence data-from beginning of Substitution table
Offset LookaheadClassDef Offset to glyph ClassDef table containing lookahead sequence data-from beginning of Substitution table
uint16 ChainSubClassSetCnt Number of ChainSubClassSet tables
Offset ChainSubClassSet
[ChainSubClassSetCnt]
Array of offsets to ChainSubClassSet tables-from beginning of Substitution table-ordered by input class-may be NULL

Each context is defined in a ChainSubClassRule table, and all ChainSubClassRules that specify contexts beginning with the same class value are grouped in a ChainSubClassSet table. Consequently, the ChainSubClassSet containing a context identifies a context's first class component.

Each ChainSubClassSet table consists of a count of the ChainSubClassRule tables defined in the ChainSubClassSet (ChainSubClassRuleCnt) and an array of offsets to ChainSubClassRule tables (ChainSubClassRule). The ChainSubClassRule tables are ordered by preference in the ChainSubClassRule array of the ChainSubClassSet.

ChainSubClassSet subtable

Type Name Description
uint16 ChainSubClassRuleCnt Number of ChainSubClassRule tables
Offset ChainSubClassRule
[ChainSubClassRuleCount]
Array of offsets to ChainSubClassRule tables-from beginning of ChainSubClassSet-ordered by preference

For each context, a ChainSubClassRule table contains a count of the glyph classes in the context sequence (GlyphCount), including the first class. A Class array lists the classes, beginning with the second class (array index = 1), that follow the first class in the context.

Note: Text order depends on the writing direction of the text. For text written from right to left, the right-most class will be first. Conversely, for text written from left to right, the left-most class will be first.

The values specified in the Class array are the values defined in the ClassDef table. The first class in the sequence, Class 2, is identified in the ChainContextSubstFormat2 table by the ChainSubClassSet array index of the corresponding ChainSubClassSet.

A ChainSubClassRule also contains a count of the substitutions to be performed on the context (SubstCount) and an array of SubstLookupRecords (SubstLookupRecord) that supply the substitution data. For each position in the context that requires a substitution, a SubstLookupRecord specifies a LookupList index and a position in the input glyph sequence where the lookup is applied. The SubstLookupRecord array lists SubstLookupRecords in design order-that is, the order in which lookups should be applied to the entire glyph sequence.


ChainSubClassRule table: Chaining context definition for one class

Type Name Description
uint16 BacktrackGlyphCount Total number of glyphs in the backtrack sequence (number of glyphs to be matched before the first glyph)
uint16 Backtrack
[BacktrackGlyphCount]
Array of backtracking classes(to be matched before the input sequence)
uint16 InputGlyphCount Total number of classes in the input sequence (includes the first class)
uint16 Input
[InputGlyphCount - 1]
Array of input classes(start with second class; to be matched with the input glyph sequence)
uint16 LookaheadGlyphCount Total number of classes in the look ahead sequence (number of classes to be matched after the input sequence)
uint16 LookAhead
[LookAheadGlyphCount]
Array of lookahead classes(to be matched after the input sequence)
uint16 SubstCount Number of SubstLookupRecords
struct SubstLookupRecord
[SubstCount]
Array of SubstLookupRecords (in design order)

6.3 Chaining Context Substitution Format 3: Coverage-based Chaining Context Glyph Substitution

Format 3 defines a chaining context rule as a sequence of Coverage tables. Each position in the sequence may define a different Coverage table for the set of glyphs that matches the context pattern. With Format 3, the glyph sets defined in the different Coverage tables may intersect, unlike Format 2 which specifies fixed class assignments (identical for each position in the backtrack, input, or lookahead sequence) and exclusive classes (a glyph cannot be in more than one class at a time).

Note: The order of the Coverage tables listed in the Coverage array must follow the writing direction. For text written from right to left, then the right-most glyph will be first. Conversely, for text written from left to right, the left-most glyph will be first.

The subtable also contains a count of the substitutions to be performed on the input Coverage sequence (SubstCount) and an array of SubstLookupRecords (SubstLookupRecord) in design order: that is, the order in which lookups should be applied to the entire glyph sequence. (SubstLookupRecords are described next.)


ChainContextSubstFormat3 subtable: Coverage-based chaining context glyph substitution

Type Name Description
uint16 SubstFormat Format identifier-format = 3
uint16 BacktrackGlyphCount Number of glyphs in the backtracking sequence
Offset Coverage[BacktrackGlyphCount] Array of offsets to coverage tables in backtracking sequence, in glyph sequence order
uint16 InputGlyphCount Number of glyphs in input sequence
Offset Coverage[InputGlyphCount] Array of offsets to coverage tables in input sequence, in glyph sequence order
uint16 LookaheadGlyphCount Number of glyphs in lookahead sequence
Offset Coverage[LookaheadGlyphCount] Array of offsets to coverage tables in lookahead sequence, in glyph sequence order
uint16 SubstCount Number of SubstLookupRecords
struct SubstLookupRecord
[SubstCount]
Array of SubstLookupRecords, in design order

LookupType 7: Extension Substitution

This lookup provides a mechanism whereby any other lookup type's subtables are stored at a 32-bit offset location in the 'GSUB' table. This is needed if the total size of the subtables exceeds the 16-bit limits of the various other offsets in the 'GSUB' table. In this specification, the subtable stored at the 32-bit offset location is termed the “extension” subtable.

ExtensionSubstFormat1 subtable

Type Name Description
USHORT SubstFormat Format identifier. Set to 1.
USHORT ExtensionLookupType Lookup type of subtable referenced by ExtensionOffset (i.e. the extension subtable).
ULONG ExtensionOffset Offset to the extension subtable, of lookup type ExtensionLookupType, relative to the start of the ExtensionSubstFormat1 subtable.

ExtensionLookupType must be set to any lookup type other than 7. All subtables in a LookupType 7 lookup must have the same ExtensionLookupType. All offsets in the extension subtables are set in the usual way, i.e. relative to the extension subtables themselves.

When an OpenType layout engine encounters a LookupType 7 Lookup table, it shall:

Substitution Lookup Record

All contextual substitution subtables specify the substitution data in a Substitution Lookup Record (SubstLookupRecord). Each record contains a SequenceIndex, which indicates the position where the substitution will occur in the glyph sequence. In addition, a LookupListIndex identifies the lookup to be applied at the glyph position specified by the SequenceIndex.

The contextual substitution subtables defined in Examples 7, 8, and 9 at the end of this chapter show SubstLookupRecords.

LookupType 8: Reverse Chaining Contextual Single Substitution Subtable

Reverse Chaining Contextual Single Substitution  subtable (ReverseChainSingleSubst) describes single glyph substitutions in context with an ability to look back and/or look ahead in the sequence of glyphs. The major difference between this and other lookup types is that processing of input glyph sequence goes from end to start. Comparing to Chaining Contextual Sustitution this format is restricted to only coverage based subtable format, input sequence could contain only single glyph and only single substitution allowed on this glyph. This substitution rule is integrated into subtable format.

This lookup type is designed specifically for the Arabic script writing styles, like nastaliq, where the shape of the glyph is determined by the following glyph, beginning at the last glyph of the “joor”, or set of connected glyphs. An example of this lookup type is defined in Example 10 at the end of this chapter.

8.1 Reverse Chaining Contextual Single Substitution Format 1: Coverage Based Reverse Chaining Contextual Single Glyph Substitution.

Format 1 defines a chaining context rule as a sequence of Coverage tables. Each position in the sequence may define a different Coverage table for the set of glyphs that matches the context pattern. With Format 1, the glyph sets defined in the different Coverage tables may intersect.

Note: Despite reverse order processing, the order of the Coverage tables listed in the Coverage array must be in logical order (follow the writing direction). The backtrack sequence is as illustrated in the LookupType 6: Chaining Contextual Substitution subtable. The input sequence is one glyph located at i in the logical string. The backtrack begins at i - 1 and increases in offset value as one moves toward the logical beginning of the string. The lookahead sequence begins at i + 1 and increases in offset value as one moves toward the logical end of the string. In the reverse chaining process i began at the logical end of the string and moves to the beginning.

The subtable contains Coverage table for input glyph and Coverage table arrays for lookahead and backtrack sequences, also count of output glyph indices in the Substitute array (GlyphCount), and a list of the output glyph indices (Substitute array). The Substitute array must contain the same number of glyph indices as the Coverage table. To locate the corresponding output glyph index in the Substitute array, this format uses the Coverage Index returned from the Coverage table.

ReverseChainSingleSubstFormat1 subtable: Coverage-based Reverse Chaining Contextual Single Glyph substitution .

Type Name Description
uint16 SubstFormat Format identifier-format = 1
Offset Coverage Offset to Coverage table - from beginning of Substitution table
uint16 BacktrackGlyphCount Number of glyphs in the backtracking sequence
Offset Coverage[BacktrackGlyphCount] Array of offsets to coverage tables in backtracking sequence, in glyph sequence order
uint16 LookaheadGlyphCount Number of glyphs in lookahead sequence
Offset Coverage[LookaheadGlyphCount] Array of offsets to coverage tables in lookahead sequence, in glyph sequence order
uint16 GlyphCount Number of GlyphIDs in the Substitute array
GlyphID Substitute[GlyphCount] Array of substitute GlyphIDs-ordered by Coverage Index


GSUB Subtable Examples

The rest of this chapter describes and illustrates examples of all the GSUB subtables, including each of the three formats available for contextual substitutions. All the examples reflect unique parameters described below, but the samples provide a useful reference for building subtables specific to other situations.

All the examples have three columns showing hex data, source, and comments.

Example 1: GSUB Header Table

Example 1 shows a typical GSUB Header table definition.

Example 1

Hex Data Source Comments
  GSUBHeader
TheGSUBHeader
GSUBHeader table definition
00010000 0x00010000 version
000A TheScriptList offset to ScriptList table
001E TheFeatureList offset to FeatureList table
002C TheLookupList offset to LookupList table

Example 2: SingleSubstFormat1 Subtable

Example 2 illustrates the SingleSubstFormat1 subtable , which uses ranges to replace single input glyphs with their corresponding output glyphs. The indices of the output glyphs are calculated by adding a constant delta value to the indices of the input glyphs. In this example, the Coverage table has a format identifier of 1 to indicate the range format, which is used because the input glyph indices are in consecutive order in the font. The Coverage table specifies one range that contains a StartGlyphID for the “0” (zero) glyph and an EndGlyphID for the “9” glyph.

Example 2

Hex Data Source Comments
  SingleSubstFormat1
LiningNumeralSubtable
SingleSubst subtable definition
0001 1 SubstFormat, ranges
0006 LiningNumeralCoverage offset to Coverage table for input glyphs
00C0 192 DeltaGlyphID = 192, add to each input glyph index to produce output glyph index
  CoverageFormat2
LiningNumeralCoverage
Coverage table definition
0002 2 CoverageFormat, ranges
  1 RangeCount
RangeRecord[0]
004E 78 Start GlyphID for numeral zero glyph
0058 87 End GlyphID for numeral nine glyph
0000 0 StartCoverageIndex first CoverageIndex = 0

Example 3: SingleSubstFormat2 Subtable

Example 3 uses the SingleSubstFormat2 subtable for lists to substitute punctuation glyphs in Japanese text that is written vertically. Horizontally oriented parentheses and square brackets (the input glyphs) are replaced with vertically oriented parentheses and square brackets (the output glyphs).

The Coverage table, Format 1, identifies each input glyph index. The number of input glyph indices listed in the Coverage table matches the number of output glyph indices listed in the subtable. For correct substitution, the order of the glyph indices in the Coverage table (input glyphs) must match the order in the Substitute array (output glyphs).

Example 3

Hex Data Source Comments
  SingleSubstFormat2
VerticalPunctuationSubtable
SingleSubst subtable definition
0002 2 SubstFormat lists
000E VerticalPunctuationCoverage offset to Coverage table
0004 4 GlyphCount, equals GlyphCount in Coverage table
0131 VerticalOpenBracketGlyph Substitute[0], ordered by Coverage Index
0135 VerticalClosedBracketGlyph Substitute[1]
013E VerticalOpenParenthesisGlyph Substitute[2]
0143 VerticalClosedParenthesisGlyph Substitute[3]
  CoverageFormat1
VerticalPunctuationCoverage
Coverage table definition
0001 1 CoverageFormat lists
0004 4 GlyphCount
003C HorizontalOpenBracketGlyph GlyphArray[0], ordered by GlyphID
0040 HorizontalClosedBracketGlyph GlyphArray[1]
004B HorizontalOpenParenthesisGlyph GlyphArray[2]
004F HorizontalClosedParenthesisGlyph GlyphArray[3]

Example 4: MultipleSubstFormat1 Subtable

Example 4 uses a MultipleSubstFormat1 subtable to replace a single “ffi” ligature with three individual glyphs that form the string <ffi>. The subtable defines a format identifier of 1, an offset to a Coverage table that specifies the glyph index of the “ffi” ligature (the input glyph), an offset to a Sequence table that specifies the sequence of glyph indices for the <ffi> string in its substitute array (the output glyph sequence), and a count of Sequence table offsets.

Example 4

Hex Data Source Comments
  MultipleSubstFormat1
FfiDecompSubtable
MultipleSubst subtable definition
0001 1 SubstFormat
0008 FfiDecompCoverage offset to Coverage table
0001 1 SequenceCount, equals GlyphCount in Coverage table
000E FfiDecompSequence offset to Sequence[0] table
  CoverageFormat1
FfiDecompCoverage
Coverage table definition
0001 1 CoverageFormat lists
0001 1 GlyphCount
00F1 ffiGlyphID ligature glyph
  Sequence
FfiDecompSequence
Sequence table definition
0003 3 GlyphCount
001A fGlyphID first glyph in sequence order
001A fGlyphID second glyph
001D iGlyphID third glyph

Example 5: AlternateSubstFormat 1 Subtable

Example 5 uses the AlternateSubstFormat1 subtable to replace the default ampersand glyph (input glyph) with one of two alternative ampersand glyphs (output glyph).

In this case, the Coverage table specifies the index of a single glyph, the default ampersand, because it is the only glyph covered by this lookup. The AlternateSet table for this covered glyph identifies the alternative glyphs: AltAmpersand1GlyphID and AltAmpersand2GlyphID.

In Example 5, the index position of the AlternateSet table offset in the AlternateSet array is zero (0), which correlates with the index position (also zero) of the default ampersand glyph in the Coverage table.

Example 5

Hex Data Source Comments
  AlternateSubstFormat1
AltAmpersandSubtable
AlternateSubstFormat1 subtable definition
0001 1 SubstFormat
0008 AltAmpersandCoverage offset to Coverage table
0001 1 AlternateSetCnt, equals GlyphCount in Coverage table
000E AltAmpersandSet offset to AlternateSet[0] table
  CoverageFormat1
AltAmpersandCoverage
Coverage table definition
0001 1 CoverageFormat
0001 1 GlyphCount
003A DefaultAmpersandGlyphID GlyphArray[0]
  AlternateSet
AltAmpersandSet
AlternateSet table definition
0002 2 GlyphCount
00C9 AltAmpersand1GlyphID offset to Alternate[0], in arbitrary order
00CA AltAmpersand2GlyphID offset to Alternate[1]

Example 6: LigatureSubstFormat1 Subtable

Example 6 shows a LigatureSubstFormat1 subtable that defines data to replace a string of glyphs with a single ligature glyph. Because a LigatureSubstFormat1 subtable can specify glyph substitutions for more than one ligature, this subtable defines three ligatures: “etc,” “ffi,” and “fi.”

The sample subtable contains a format identifier (4) and an offset to a Coverage table. The Coverage table, which lists an index for each first glyph in the ligatures, lists indices for the “e” and “f” glyphs. The Coverage table range format is used here because the “e” and “f” glyph indices are numbered consecutively.

In the LigatureSubst subtable, LigSetCount specifies two LigatureSet tables, one for each covered glyph, and the LigatureSet array stores offsets to them. In this array, the “e” LigatureSet precedes the “f” LigatureSet, matching the order of the corresponding first-glyph components in the Coverage table.

Each LigatureSet table identifies all ligatures that begin with a covered glyph. The sample LigatureSet table defined for the “e” glyph contains only one ligature, “etc.” A LigatureSet table defined for the “f” glyph contains two ligatures, “ffi” and “fi.”

The sample FLigaturesSet table has offsets to two Ligature tables, one for “ffi” and one for “fi.” The Ligature array lists the “ffi” Ligature table first to indicate that the “ffi” ligature is preferred to the “fi” ligature.

Example 6

Hex Data Source Comments
  LigatureSubstFormat1
LigaturesSubtable
LigatureSubstFormat1 subtable definition
0001 1 SubstFormat
000A LigaturesCoverage offset to Coverage table
0002 2 LigSetCount
0014 ELigaturesSet offset to LigatureSet[0] table in Coverage Index order
0020 FLigaturesSet offset to LigatureSet[1] table
  CoverageFormat2
LigaturesCoverage
Coverage table definition
0002 2 CoverageFormat, ranges
0001 1 RangeCount
RangeRecord[0]
0019 eGlyphID Start, first GlyphID
001A fGlyphID End, last GlyphID in range
0000 0 StartCoverageIndex, coverage index of start glyphID
  LigatureSet
ELigaturesSet
LigatureSet table definition
all ligatures that start with e
0001 1 LigatureCount
0004 etcLigature offset to Ligature[0] table
  Ligature
etcLigature
Ligature table definition
015B etcGlyphID LigGlyph, output GlyphID
0003 3 CompCount number of components
0028 tGlyphID Component[1], second component in ligature
0017 cGlyphID Component[2], third component in ligature
  LigatureSet
FLigaturesSet
LigatureSet table definition all ligatures start with f
0002 2 LigatureCount
0006 ffiLigature offset to Ligature[0] table, listed first because ffi ligature is preferred to fi ligature
000E fiLigature offset to Ligature[1] table
  Ligature
ffiLigature
Ligature table definition
00F1 ffiGlyphID LigGlyph, output GlyphID
0003 3 CompCount
001A fGlyphID Component[1], second component in ligature
001D iGlyphID Component[2], third component in ligature
  Ligature
fiLigature
Ligature table definition
00F0 fiGlyphID LigGlyph, output GlyphID
0002 2 CompCount
001D iGlyphID Component[1] second component in ligature

Example 7: ContextSubstFormat1 Subtable and SubstLookupRecord

Example 7 uses a ContextSubstFormat1 subtable for glyph sequences to replace a string of three glyphs with another string. For the French language system, the subtable defines a contextual substitution that replaces the input sequence, space-dash-space, with the output sequence, thin space-dash-thin space.

The contextual substitution, called Dash Lookup in this example, contains one ContextSubstFormat1 subtable called the DashSubtable. The subtable specifies two contexts: a SpaceGlyph followed by a DashGlyph, and a DashGlyph followed by a SpaceGlyph. In each sequence, a single substitution replaces the SpaceGlyph with a ThinSpaceGlyph.

The Coverage table, labeled DashCoverage, lists two GlyphIDs for the first glyphs in the SpaceGlyph and DashGlyph sequences. One SubRuleSet table is defined for each covered glyph.

SpaceAndDashSubRuleSet lists all the contexts that begin with a SpaceGlyph. It contains an offset to one SubRule table (SpaceAndDashSubRule), which specifies two glyphs in the context sequence, the second of which is a DashGlyph. The SubRule table contains an offset to a SubstLookupRecord that lists the position in the sequence where the glyph substitution should occur (position 0) and the index of the SpaceToThinSpaceLookup applied there to replace the SpaceGlyph with a ThinSpaceGlyph. DashAndSpaceSubRuleSet lists all the contexts that begin with a DashGlyph. An offset to a SubRule table (DashAndSpaceSubRule) specifies two glyphs in the context sequence, and the second one is a SpaceGlyph. The SubRule table contains an offset to a SubstLookupRecord, which lists the position in the sequence where the glyph substitution should occur, and an index to the same lookup used in the SpaceAndDashSubRule. The lookup replaces the SpaceGlyph with a ThinSpaceGlyph.

Example 7

Hex Data Source Comments
  ContextSubstFormat1
DashSubtable
ContextSubstFormat1 subtable definition for Lookup[0], DashLookup
0001 1 SubstFormat
000A DashCoverage offset to Coverage table
0002 2 SubRuleSetCount
0012 SpaceAndDashSubRuleSet offset to SubRuleSet[0], ordered by Coverage Index
0020 DashAndSpaceSubRuleSet offset to SubRuleSet[1]
  CoverageFormat1
DashCoverage
Coverage table definition
0001 1 CoverageFormat lists
0002 2 GlyphCount
0028 SpaceGlyph GlyphArray[0], in numeric order
005D DashGlyph GlyphArray[1], dash GlyphID
  SubRuleSet
SpaceAndDashSubRuleSet
SubRuleSet[0] table definition
0001 1 SubRuleCount
0004 SpaceAndDashSubRule offset to SubRule[0], ordered by preference
  SubRule
SpaceAndDashSubRule
SubRule[0] table definition
0002 2 GlyphCount number in input sequence
0001 1 SubstCount
005D DashGlyph Input[1], starting with second glyph SpaceGlyph in Coverage table is first glyph
SubstLookupRecord[0]
0000 0 SequenceIndex substitution at first glyph position (0)
0001 1 LookupListIndex index for SpaceToThinSpaceLookup in LookupList
  SubRuleSet
DashAndSpaceSubRuleSet
SubRuleSet[0] table definition
0001 1 SubRuleCount
0004 DashAndSpaceSubRule offset to SubRule[0], ordered by preference
  SubRule
DashAndSpaceSubRule
SubRule[0] table definition
0002 2 GlyphCount number in the input glyph sequence
0001 1 SubstCount
0028 SpaceGlyph Input[1], starting with second glyph SubstLookupRecord definition
0001 1 SequenceIndex substitution at second glyph position(1)
0001 1 LookupListIndex for SpaceToThinSpaceLookup

Example 8: ContextSubstFormat2 Subtable

Example 8 uses a ContextSubstFormat2 subtable with glyph classes to replace default mark glyphs with their alternative forms. Glyph alternatives are selected depending upon the height of the base glyph that they combine with-that is, the mark glyph used above a high base glyph differs from the mark glyph above a very high base glyph.

In the example, SetMarksHighSubtable contains a Class table that defines four glyph classes: medium-height glyphs (Class 0), all default mark glyphs (Class 1), high glyphs (Class 2), and very high glyphs (Class 3). The subtable also contains a Coverage table that lists each base glyph that functions as a first component in a context, ordered by glyph index.

Two SubClassSets are defined, one for substituting high marks and one for very high marks. No SubClassSets are specified for Class 0 and Class 1 glyphs because no contexts begin with glyphs from these classes. The SubClassSet array lists SubClassSets in numerical order, so SubClassSet 2 precedes SubClassSet 3.

Within each SubClassSet, a SubClassRule is defined. In SetMarksHighSubClassSet2, the SubClassRule table specifies two glyphs in the context, the first glyph in Class 2 (a high glyph) and the second in Class 1 (a mark glyph). The SubstLookupRecord specifies applying SubstituteHighMarkLookup at the second position in the sequence-that is, a high mark glyph will replace the default mark glyph.

In SetMarksVeryHighSubClassSet3, the SubClassRule specifies two glyphs in the context, the first in Class 3 (a very high glyph) and the second in Class 1 (a mark glyph). The SubstLookupRecord specifies applying SubstituteVeryHighMarkLookup at the second position in the sequence-that is, a very high mark glyph will replace the default mark glyph.

Example 8

Hex Data Source Comments
  ContextSubstFormat2
SetMarksHighSubtable
ContextSubstFormat2 subtable definition
0002 2 SubstFormat
0010 SetMarksHighCoverage offset to Coverage table
001C SetMarksHighClassDef offset to Class Def table
0004 4 SubClassSetCnt
0000 NULL offset to SubClassSet[0] table, no contexts that begin with Class 0 glyphs are defined
0000 NULL offset to SubClassSet[1] table no contexts that begin with Class 1 glyphs are defined
0032 SetMarksHighSubClassSet2 offset to SubClassSet[2] table for contexts that begin with Class 2 glyphs (high base glyphs)
0040 SetMarksVeryHighSubClassSet3 offset to SubClassSet[3] table for contexts that begin with Class 3 glyphs (very high base glyphs)
  CoverageFormat1
SetMarksHighCoverage
Coverage table definition
0001 1 CoverageFormat, lists
0004 4 GlyphCount
0030 tahGlyphID GlyphArray[0], high base glyph
0031 dhahGlyphID GlyphArray[1], high base glyph
0040 cafGlyphID GlyphArray[2], very high base glyph
0041 gafGlyphID GlyphArray[3], very high base glyph
  ClassDefFormat2
SetMarksHighClassDef
Class table definition
0002 2 Class Format, ranges
0003 3 ClassRangeCount ClassRange[0] ordered by StartGlyphID for Class 2, high base glyphs
0030 tahGlyphID Start, first Glyph ID in range
0031 dhahGlyphID End, last Glyph ID in range
0002 2 Class
ClassRange[1] for Class 3, very high base glyphs
0040 cafGlyphID Start, first Glyph ID in the range
0041 gafGlyphID End, last Glyph ID in the range
0003 3 Class
ClassRange[2] for Class 1, mark gyphs
00D2 fathatanDefaultGlyphID Start, first Glyph ID in range default fathatan mark
00D3 dammatanDefaultGlyphID End, last Glyph ID in the range default dammatan mark
0001 1 Class
  SubClassSet
SetMarksHighSubClassSet2
SubClassSet[2] table definition
all contexts that begin with Class 2 glyphs
0001 1 SubClassRuleCnt
0004 SetMarksHighSubClassRule2 offset to SubClassRule[0] table ordered by preference
  SubClassRule
SetMarksHighSubClassRule2
SubClassRule[0] table definition, Class 2 glyph (high base) glyph followed by a Class 1 glyph (mark)
0002 2 GlyphCount
0001 1 SubstCount
0001 1 offset to Class[1], beginning with the second Class in the context sequence (mark = Class 1) begin SubstLookupRecord array in design order SubstLookupRecord[0]
0001 1 SequenceIndex, apply substitution to position 2, a mark
0001 1 LookupListIndex
  SubClassSet
SetMarksVeryHighSubClassSet3
SubClassSet[3] table definition all contexts that begin with Class 3 glyphs
0001 1 SubClassRuleCnt
0004 SetMarksVeryHighSubClassRule3 offset to SubClassRule[0] table ordered by preference
  SubClassRule
SetMarksVeryHighSubClassRule3
SubClassRule[0] table definition Class 3 glyph (very high base glyph) followed by a Class 1 glyph (mark)
0002 2 GlyphCount
0001 1 SubstCount
0001 1 offset to Class[1], beginning with the second Class in the context sequence = marks, Class 1 begin SubstLookupRecord array in design order SubstLookupRecord[0]
0001 1 SequenceIndex, apply substitution to position 2, second glyph class (mark)
0002 2 LookupListIndex

Example 9: ContextualSubstFormat3 Subtable

Example 9 uses the ContextSubstFormat3 subtable with Coverage tables to describe a context sequence of three lowercase glyphs in the pattern: any ascender or descender glyph in position 0 (zero), any x-height glyph in position 1, and any descender glyph in position 2. The overlapping sets of covered glyphs for positions 0 and 2 make Format 3 better for this context than the class-based Format 2.

In positions 0 and 2, swash versions of the glyphs replace the default glyphs. The contextual-substitution lookup is SwashLookup (LookupList index = 0), and its subtable is SwashSubtable. The SwashSubtable defines three Coverage tables: AscenderDescenderCoverage, XheightCoverage, and DescenderCoverage-one for each glyph position in the context sequence, respectively.

The SwashSubtable also defines two SubstLookupRecords: one that applies to position 0, and one for position 2. (No substitutions are applied to position 1.) The record for position 0 uses a single substitution lookup called AscDescSwashLookup to replace the current ascender or descender glyph with a swash ascender or descender glyph. The record for position 2 uses a single substitution lookup called DescSwashLookup to replace the current descender glyph with a swash descender glyph.

Example 9

Hex Data Source Comments
  ContextSubstFormat3
SwashSubtable
ContextSubstFormat3 subtable definition
0003 3 SubstFormat
0003 3 GlyphCount in input glyph sequence
0002 2 SubstCount
0030 AscenderDescenderCoverage offset to Coverage[0] table in context sequence order
004C XheightCoverage offset to Coverage[1] table
006E DescenderCoverage offset to Coverage[2] table SubstLookupRecord[0] in glyph position order
0000 0 SequenceIndex
0001 1 LookupListIndex, single substitution to output ascender or descender swash SubstLookupRecord[1]
0002 2 SequenceIndex
0002 2 LookupListIndex single substitution to output descender swash
  CoverageFormat1
AscenderDescenderCoverage
Coverage table definition
0001 1 CoverageFormat, lists
000C 12 GlyphCount
0033 bGlyphID GlyphArray[0] in GlyphID order
0035 dGlyphID GlyphArray[1]
0037 fGlyphID GlyphArray[2]
0038 gGlyphID GlyphArray[3]
0039 hGlyphID GlyphArray[4]
003B jGlyphID GlyphArray[5]
003C kGlyphID GlyphArray[6]
003D lGlyphID GlyphArray[7]
0041 pGlyphID GlyphArray[8]
0042 qGlyphID GlyphArray[9]
0045 tGlyphID GlyphArray[10]
004A yGlyphID GlyphArray[11]
  CoverageFormat1
XheightCoverage
Coverage table definition
0001 1 CoverageFormat, lists
000F 15 GlyphCount
0032 aGlyphID GlyphArray[0] in GlyphID order
0034 cGlyphID GlyphArray[1]
0036 eGlyphID GlyphArray[2]
003A iGlyphID GlyphArray[3]
003E mGlyphID GlyphArray[4]
003F nGlyphID GlyphArray[5]
0040 oGlyphID GlyphArray[6]
0043 rGlyphID GlyphArray[7]
0044 sGlyphID GlyphArray[8]
0045 tGlyphID GlyphArray[9]
0046 uGlyphID GlyphArray[10]
0047 vGlyphID GlyphArray[11]
0048 wGlyphID GlyphArray[12]
0049 xGlyphID GlyphArray[13]
004B zGlyphID GlyphArray[14]
  CoverageFormat1
DescenderCoverage
Coverage table definition
0001 1 CoverageFormat, lists
0005 5 GlyphCount
0038 gGlyphID GlyphArray[0] in GlyphID order
003B jGlyphID GlyphArray[1]
0041 pGlyphID GlyphArray[2]
0042 qGlyphID GlyphArray[3]
004A yGlyphID GlyphArray[4]

Example 10: ReverseChainSingleSubstFormat1 Subtable and SubstLookupRecord

Example 10 uses a ReverseChainSingleSubstFormat1 subtable for glyph sequences to glyph with the correct form that has a thick connection to the left (thick exit). This allow the glyph to correctly connect to the letter form to the left of it.

The ThickExitCoverage table is the listing of glyphs to be matched for substitution.

The LookaheadCoverage table, labeled ThickEntryCoverage, lists four GlyphIDs for the glyph following a substitution coverage glyph. This lookahead coverage attempts to match the context that will cause the substitution to take place.

The Substitute table maps the glyphs to replace those in the ThickConnectCoverage table.

Example 10

Hex Data Source Comments
  ReverseChainSingleSubstFormat1
ThickConnect
ReverseChainSingleSubstFormat1 subtable definition
0001 1 SubstFormat
0068 ThickExitCoverage offset to Coverage table
0000 0 BacktrackGlyphCount
0000 null - not used offset to BacktrackCoverage[0]
0001 1 LookaheadGlyphCount
0026 ThickEntryCoverage offset to LookaheadCoverage[0]
000C 12 GlyphCount
00A7 BEm2 Substitute[0], ordered by Coverage Index
00B9 BEi3 Substitute[1]
00C5 JIMm3 Substitute[2]
00D4 JIMi2 Substitute[3]
00EA SINm2 Substitute[4]
00F2 SINi2 Substitute[5]
00FD SADm2 Substitute[6]
010D SADi2 Substitute[7]
011B TOEm3 Substitute[8]
012B TOEi3 Substitute[9]
013B AINm2 Substitute[10]
0141 AINi2 Substitute[11]
  CoverageFormat1
ThickEntryCoverage
Coverage table definition
0001 1 CoverageFormat, lists
001F 31 GlyphCount
00A5 ALEFf1 GlyphArray[0], in GlyphID order
00A9 BEm4 GlyphArray[1]
00AA BEm5 GlyphArray[2]
00E2 DALf1 GlyphArray[3]
0167 KAFf1 GlyphArray[4]
0168 KAFfs1 GlyphArray[5]
0169 KAFm1 GlyphArray[6]
016D KAFm5 GlyphArray[7]
016E KAFm6 GlyphArray[8]
0170 KAFm8 GlyphArray[9]
0183 GAFf1 GlyphArray[10]
0184 GAFfs1 GlyphArray[11]
0185 GAFm1 GlyphArray[12]
0189 GAFm5 GlyphArray[13]
018A GAFm6 GlyphArray[14]
018C GAFm8 GlyphArray[15]
019F LAMf1 GlyphArray[16]
01A0 LAMm1 GlyphArray[17]
01A1 LAMm2 GlyphArray[18]
01A2 LAMm3 GlyphArray[19]
01A3 LAMm4 GlyphArray[20]
01A4 LAMm5 GlyphArray[21]
01A5 LAMm6 GlyphArray[22]
01A6 LAMm7 GlyphArray[23]
01A7 LAMm8 GlyphArray[24]
01A8 LAMm9 GlyphArray[25]
01A9 LAMm10 GlyphArray[26]
01AA LAMm11 GlyphArray[27]
01AB LAMm12 GlyphArray[28]
01AC LAMm13 GlyphArray[29]
01EC HAYf2 GlyphArray[30]
  CoverageFormat1
ThickExitCoverage
Coverage table definition
0001 1 CoverageFormat, lists
000C 12 GlyphCount
00A6 BEm1 GlyphArray[0], ordered by GlyphID
00B7 BEi1 GlyphArray[1]
00C3 JIMm1 GlyphArray[2]
00D2 JIMi1 GlyphArray[3]
00E9 SINm1 GlyphArray[4]
00F1 SINi1 GlyphArray[5]
00FC SADm1 GlyphArray[6]
010C SADi1 GlyphArray[7]
0119 TOEm1 GlyphArray[8]
0129 TOEi1 GlyphArray[9]
013A AINm1 GlyphArray[10]
0140 AINi1 GlyphArray[11]

This page was last updated 2 May 2014.

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