GSUB — Glyph Substitution Table

The Glyph Substitution (GSUB) table provides data for substition of glyphs for appropriate rendering of scripts, such as cursively-connecting forms in Arabic script, or for advanced typographic effects, such as ligatures.

Overview

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).

Glyphs for different positional forms of hah
Figure 1. Isolated, initial, medial, and final forms of the Arabic character HAH
Glyphs for f and i and an f-i ligature glyph
Figure 2. Two Latin glyphs and their associated ligature

OpenType fonts use character encoding standards, such as the Unicode Standard, that assumes a distinction between characters and glyphs: text is encoded as sequences of characters, and the 'cmap' table provides a mapping from that character to a single default glyph. Multiple characters are not directly mapped to a single glyph, as needed for ligatures; and a single character is not mapped directly to multiple glyphs, as may be needed for some complex-script scenarios. The GSUB table provides a way to describe such substititions, enabling applications to apply such substitions during text layout and rendering to achieve desired results.

To access substitute glyphs, GSUB maps from the glyph index or indices defined in a 'cmap' subtable to the glyph index or indices of the substitute glyphs. 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 from this one default 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 seven types of glyph substitutions that are widely used in international typography:

  • A single substitution replaces a single glyph with another single glyph. This is used to render positional glyph variants in Arabic and vertical text in the Far East (see Figure 3).

    Kanji ideograph with parentheses in horizontal and vertical layout
    Figure 3. Alternative forms of parentheses used when positioning Kanji vertically
  • A multiple substitution replaces a single glyph with more than one glyph. This is used to specify actions such as ligature decomposition (see Figure 4).

    An f-i ligature glyph decomposed to f and i glyphs
    Figure 4. Decomposing a Latin ligature glyph into its individual glyph components
  • An alternate substitution identifies functionally equivalent but different looking forms of a glyph. These glyphs are often referred to as aesthetic alternatives. For example, a font might have five different glyphs for the ampersand symbol, but one would have a default glyph index in the 'cmap' table. The client could use the default glyph or substitute any of the four alternatives (see Figure 5).

    Alternative ampersand glyphs
    Figure 5. Alternative ampersand glyphs in a font
  • A ligature substitution replaces several glyph indices with a single glyph index, as when an Arabic ligature glyph replaces a string of separate glyphs (see Figure 6). When a string of glyphs can be replaced with a single ligature glyph, the first glyph is substituted with the ligature. The remaining glyphs in the string are deleted, this does not include those glyphs that are skipped as a result of lookup flags.

    Sequence of three Arabic glyphs and the associated ligature glyph
    Figure 6. Three Arabic glyphs and their associated ligature glyph
  • Contextual substitution is an extension of the above lookup types, describing glyph substitutions in context — that is, a substitution of one or more glyphs within a certain pattern of glyphs. Each substitution describes one or more input glyph sequences and one or more substitutions to be performed on that sequence. Contextual substitutions can be applied to specific glyph sequences, glyph classes, or sets of glyphs.

  • Chaining contextual substitution, extends the capabilities of contextual substitution. With this, one or more substitutions can be performed on one or more glyphs within a pattern of glyphs (input sequence), by chaining the input sequence to a backtrack and/or lookahead sequence. Each such substitution can be applied in three formats to handle glyphs, glyph classes or glyph sets in the input sequence. Each of these formats can describe one or more of the backtrack, input and lookahead sequences.

  • Reverse Chaining contextual single substitution, allows one glyph to be substituted with another by chaining input glyph to a backtrack and/or lookahead sequence. The difference between this and other lookup types is that processing of input glyph sequence goes from end to start.

Note that the GSUB data formats used to implement the different types of substitution include an eighth type, extension substitution. This provides a format extension mechanism, allowing reference to subtables using 32-bit offsets rather than 16-bit offsets. It does not provide an additional type of substitution action, however.

GSUB Table and OpenType Font Variations

OpenType Font Variations allow a single font to support many design variations along one or more axes of design variation. For example, a font with weight and width variations might support weights from thin to black, and widths from ultra-condensed to ultra-expanded. For general information on OpenType Font Variations, see the chapter, OpenType Font Variations Overview.

In a variable font, it may be desirable to have different glyph-substitution actions used for different regions within the font’s variation space. For example, for narrow or heavy instances in which counters become small, it may be desirable to make certain glyph substitutions to use alternate glyphs with certain strokes removed or outlines simplified to allow for larger counters. Such effects can be achieved using a FeatureVariations table within the GSUB table. The FeatureVariations table is described in the chapter, OpenType Layout Common Table Formats. See also the Required Variation Alternates ('rvrn') feature in the OpenType Layout tag registry.

Table Organization

The GSUB table begins with a header that defines offsets to a ScriptList, a FeatureList, a LookupList, and an optional FeatureVariations table (see Figure 7):

  • The ScriptList identifies all the scripts and language systems in the font that use glyph substitutes.
  • The FeatureList defines all the glyph substitution features required to render these scripts and language systems.
  • The LookupList contains all the lookup data needed to implement each glyph substitution feature.
  • The FeatureVariations table can be used to substitute alternate sets of lookup tables to use for any given feature under specified conditions. This is currently used only in variable fonts.

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

Block diagram of GSUB subtables
Figure 7. High-level organization of GSUB table

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 LangSys 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 table, and select the feature tables to apply to an input glyph string.
  5. If a Feature Variations table is present, evaluate conditions in the Feature Variation table to determine if any of the initially-selected feature tables should be substituted by an alternate feature table.
  6. Each Feature table provides an array of index numbers into the GSUB LookupList table. Assemble all lookups from the set of chosen features, and apply the lookups in the order given in the LookupList table.

For a detailed description of the Feature Variations table and how it is processed, see the FeatureVariations Table section in the OpenType Layout Common Table Formats chapter.

Lookup data is defined in Lookup tables, which are defined in the OpenType Layout Common Table Formats chapter. A Lookup table contains one or more Lookup Subtables that define the specific conditions, type, and results of a substitution action used to implement a feature. Specific Lookup subtable types are used for glyph substitution actions, and are defined in this chapter. All subtables within a Lookup table must be of the same lookup type, as listed in the following table for the GSUB LookupType Enumeration:

GSUB LookupType Enumeration

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 has one or more subtable 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 has a different array index 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 chaining contextual lookups (LookupType 6), 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 chapter illustrate the GSUB header and six of the eight LookupTypes, including the three formats available for contextual substitutions (LookupType 5).

GSUB Header

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

GSUB Header, Version 1.0

Type Name Description
uint16 majorVersion Major version of the GSUB table, = 1
uint16 minorVersion Minor version of the GSUB table, = 0
Offset16 scriptListOffset Offset to ScriptList table, from beginning of GSUB table
Offset16 featureListOffset Offset to FeatureList table, from beginning of GSUB table
Offset16 lookupListOffset Offset to LookupList table, from beginning of GSUB table

GSUB Header, Version 1.1

Type Name Description
uint16 majorVersion Major version of the GSUB table, = 1
uint16 minorVersion Minor version of the GSUB table, = 1
Offset16 scriptListOffset Offset to ScriptList table, from beginning of GSUB table
Offset16 featureListOffset Offset to FeatureList table, from beginning of GSUB table
Offset16 lookupListOffset Offset to LookupList table, from beginning of GSUB table
Offset32 featureVariationsOffset Offset to FeatureVariations table, from beginning of the GSUB table (may be NULL)

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. The deltaGlyphID is a constant value added to each input glyph index to calculate the index of the corresponding output glyph. Addition of deltaGlyphID is modulo 65536.

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
Offset16 coverageOffset Offset to Coverage table, from beginning of substitution subtable
int16 deltaGlyphID Add to original glyph ID to get substitute glyph ID

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 (substituteGlyphIDs) 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 substituteGlyphIDs array (glyphCount), as well as the list of the output glyph indices in the substitute array (substituteGlyphIDs).

The substituteGlyphIDs array must contain the same number of glyph indices as the Coverage table. To locate the corresponding output glyph index in the substituteGlyphIDs 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
Offset16 coverageOffset Offset to Coverage table, from beginning of substitution subtable
uint16 glyphCount Number of glyph IDs in the substituteGlyphIDs array
uint16 substituteGlyphIDs[glyphCount] Array of substitute glyph IDs — 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.

2.1 Multiple Substitution Format 1

The Multiple Substitution Format 1 subtable specifies a format identifier (substFormat), an offset to a Coverage table that defines the input glyph indices, a count of offsets in the sequenceOffsets array (sequenceCount), and an array of offsets to Sequence tables that define the output glyph indices (sequenceOffsets). 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 (substituteGlyphIDs).

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. The glyphCount value must 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
Offset16 coverageOffset Offset to Coverage table, from beginning of substitution subtable
uint16 sequenceCount Number of Sequence table offsets in the sequenceOffsets array
Offset16 sequenceOffsets[sequenceCount] Array of offsets to Sequence tables. Offsets are from beginning of substitution subtable, ordered by Coverage index

Sequence table

Type Name Description
uint16 glyphCount Number of glyph IDs in the substituteGlyphIDs array. This must always be greater than 0.
uint16 substituteGlyphIDs[glyphCount] String of glyph IDs 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.

3.1 Alternate Substitution Format 1

The Alternate Substitution Format 1 subtable contains a format identifier (substFormat), an offset to a Coverage table containing the indices of glyphs with alternative forms (coverageOffset), a count of offsets to AlternateSet tables (alternateSetCount), and an array of offsets to AlternateSet tables (alternateSetOffsets).

For each glyph, an AlternateSet subtable contains a count of the alternative glyphs (glyphCount) and an array of their glyph indices (alternateGlyphIDs). 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
Offset16 coverageOffset Offset to Coverage table, from beginning of substitution subtable
uint16 alternateSetCount Number of AlternateSet tables
Offset16 alternateSetOffsets[alternateSetCount] Array of offsets to AlternateSet tables. Offsets are from beginning of substitution subtable, ordered by Coverage index

AlternateSet table

Type Name Description
uint16 glyphCount Number of glyph IDs in the alternateGlyphIDs array
uint16 alternateGlyphIDs[glyphCount] Array of alternate glyph IDs, 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 has one format: LigatureSubstFormat1.

4.1 Ligature Substitution Format 1

It contains a format identifier (substFormat), a Coverage table offset (coverageOffset), a count of the ligature sets defined in this table (ligatureSetCount), and an array of offsets to LigatureSet tables (ligatureSetOffsets). The Coverage table specifies only the index of the first glyph component of each ligature set.

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

LigatureSubstFormat1 subtable: All ligature substitutions in a script

Type Name Description
uint16 substFormat Format identifier: format = 1
Offset16 coverageOffset Offset to Coverage table, from beginning of substitution subtable
uint16 ligatureSetCount Number of LigatureSet tables
Offset16 ligatureSetOffsets[ligatureSetCount] Array of offsets to LigatureSet tables. Offsets are from beginning of substitution subtable, 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 (ligatureSetOffsets) to Ligature tables, which define the glyphs in each 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
Offset16 ligatureOffsets[LigatureCount] Array of offsets to Ligature tables. Offsets are from beginning of LigatureSet table, ordered by preference.

For each ligature in the set, a Ligature table specifies the glyph ID of the output ligature glyph (ligatureGlyph); a count of the total number of component glyphs in the ligature, including the first component (componentCount); and an array of glyph IDs for the components (componentGlyphIDs). The array starts with the second component glyph in the ligature (glyph sequence index = 1, componentGlyphIDs array index = 0) because the first component glyph is specified in the Coverage table.

Note: The componentGlyphIDs array lists glyph IDs according to the writing direction — that is, the logical order — 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.

Ligature table: Glyph components for one ligature

Type Name Description
uint16 ligatureGlyph glyph ID of ligature to substitute
uint16 componentCount Number of components in the ligature
uint16 componentGlyphIDs[componentCount - 1] Array of component glyph IDs — start with the second component, ordered in writing direction

LookupType 5: Contextual Substitution Subtable

A Contextual Substitution subtable describes glyph substitutions in context that replace one or more glyphs within a certain pattern of glyphs.

Contextual substitution subtables can use any of three formats that are common to the GSUB and GPOS tables. These define input sequence patterns to be matched against the text glyph sequence, and then actions to be applied to glyphs within the input sequence. The actions are specified as “nested” lookups, and each is applied to a particular sequence position within the input sequence.

Each sequence position + nested lookup combination is specified in a SequenceLookupRecord. Examples 7, 8, and 9 at the end of this chapter illustrate use of sequence lookup records within the GSUB table.

While the subtable formats are common between the GSUB and GPOS tables, the lookups referenced by sequence lookup records within the GSUB table are referenced by index into the GSUB LookupList table. In this way, actions specified by a GSUB contextual lookup can only be substitutions.

An input sequence pattern is matched against the current glyph sequence before any substitution actions are performed. The substitutions may change the current glyph sequence, but that has no affect on the initial matching operation. For a given lookup subtable, there may be multiple sequence lookup records, and these are processed in a specified order. Each substitution action on the glyph sequence applies to the results from the preceding sequence lookup records. Note in particular that the sequence position index in each sequence lookup record is relative to the glyph sequence as modified by the preceding actions.

For example, consider a contextual lookup specifying an input glyph sequence of four glyphs. Suppose that no substitution is performed on the first glyph, but that the middle two glyphs will be replaced with a ligature, and a single glyph will replace the fourth glyph. Suppose also that the actions are listed in that order.

  • The first glyph is at sequence position 0. No SequenceLookupRecord is specified for sequence index 0.
  • The first SequenceLookupRecord specifies sequence position 1, and gives a LookupList index referencing a ligature substitution lookup. The first glyph specified in the nested lookup will be the glyph at sequence position 1; the second glyph specified in the nested lookup will be the glyph at sequence position 2. After the nested substitution has been performed, there will be three glyphs in the sequence context, not four.
  • The last SequenceLookupRecord must be defined in terms of the modified sequence context, specifying sequence position 2, not position 3. The nested single-substitution lookup will specify the glyph at position 2 as its input glyph.

5.1 Context Substitution Format 1: Simple Glyph Contexts

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.

For example, suppose the glyph string <abc> is to be replaced with its reverse glyph string <cba>. The input context would be defined as the glyph sequence, <abc>. Two single-substitution actions can be specified: the “a” at sequence position 0 is substituted by “c”, and the “c” at sequence position 2 is substituted by “a”.

Format 1 contextual substitutions are implemented using a SequenceContextFormat1 table. See Sequence Context Format 1: simple glyph contexts in the OpenType Layout Common Table Formats chapter for complete details.

Example 7 at the end of the chapter uses a SequenceContextFormat1 table to replace a sequence of three glyphs with a sequence preferred for the French language system.

5.2 Context Substitution Format 2: Class-based Glyph Contexts

Format 2 defines contexts for glyph substitutions as input sequence patterns, with patterns expressed in terms of glyph classes. The glyph classes are defined using a Class Definition table. Several sequence patterns may be specified, with each pattern specifying a class of glyphs for each input sequence position.

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 as a pattern of one glyph from Class 3, followed by one glyph from Class 1, followed by one glyph from Class 2.

Format 2 contextual substitutions are implemented using a SequenceContextFormat2 table. See Sequence Context Format 2: class-based glyph contexts in the OpenType Layout Common Table Formats chapter for complete details.

Example 8 at the end of this chapter uses a SequenceContextFormat2 table to substitute Arabic mark glyphs for base glyphs of different heights.

5.3 Context Substitution Format 3: Coverage-based Glyph Contexts

Format 3 defines a context for glyph substitutions as an input sequence pattern, with the pattern expressed in terms of Coverage tables. A different Coverage table is defined for each sequence position.

Format 3 is like format 2 in that patterns are defined using sets of glyphs. However, with the glyph classes used in format 2, each glyph is in exactly one class. With format 3, any glyph can occur in multiple Coverage tables.

Unlike Formats 1 and 2, however, this format can define only one context.

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:

  • For position 0, the Coverage table lists the set of lowercase glyphs.
  • For position 1, the Coverage table lists the set of uppercase glyphs.
  • For position 2, the Coverage table lists the set of lowercase and numeral glyphs, a superset of the glyphs defined in the Coverage table for position 0.
  • For position 3, the Coverage table lists the set of lowercase and uppercase vowels, a subset of the glyphs defined in the Coverage tables for both positions 0 and 1.

Format 3 contextual substitutions are implemented using a SequenceContextFormat3 table. See Sequence Context Format 3: coverage-based glyph contexts in the OpenType Layout Common Table Formats chapter for complete details.

Example 9 at the end of this chapter uses SequenceContextFormat3 to substitute swash glyphs for two out of three glyphs in a sequence.

LookupType 6: Chained Contexts Substitution Subtable

A Chained Contexts Substitution subtable describes glyph substitutions in context with an ability to look back and/or look ahead in the sequence of glyphs. The design of the Chained Contexts Substitution subtable is parallel to that of the Contextual Substitution subtable, including the availability of three formats. Each format can describe one or more chained backtrack, input, and lookahead sequence combinations, and one or more substitutions for glyphs in each input sequence.

Note: Substitutions can be specified only for the input sequence context, not for backtrack and lookahead sequences.

See the introduction to the Contextual Substitution Subtable section for general remarks regarding contextual substitutions, which also apply to Chained Contexts Substitutions.

Note that backtrack sequences are specified in reverse logical order. See the Chained Sequence Context Format 1 section in the OpenType Layout Common Table Formats chapter for details regarding chained backtrack, input, and lookahead sequences.

6.1 Chained Contexts Substitution Format 1: Simple Glyph Contexts

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. Specific glyph sequences are used for input, backtrack or lookahead contexts.

Format 1 chained context substitutions are implemented using a ChainedSequenceContextFormat1 table. See Chained Sequence Context Format 1: simple glyph contexts in the OpenType Layout Common Table Formats chapter for complete details.

6.2 Chained Contexts Substitution Format 2: Class-based Glyph Contexts

Format 2 defines contexts for glyph substitutions as patterns expressed in terms of glyph classes. The glyph classes are defined using a Class Definition table. Several sequence patterns may be specified, with each pattern specifying a class of glyphs for each sequence position.

To chain contexts, three separate Class Definition tables are used for the backtrack sequence, input sequence, and lookahead sequence.

Format 2 contextual substitutions are implemented using a ChainedSequenceContextFormat2 table. See Chained Sequence Context Format 2: class-based glyph contexts in the OpenType Layout Common Table Formats chapter for complete details.

6.3 Chained Contexts Substitution Format 3: Coverage-based Glyph Contexts

Format 3 defines contexts for glyph substitutions as patterns expressed in terms of Coverage tables. A different Coverage table is defined for each position in a sequence. To chain contexts, three separate sets of Coverage tables are used for the backtrack sequence, input sequence, and lookahead sequence.

Format 3 is like format 2 in that patterns are defined using sets of glyphs. However, with the glyph classes used in format 2, each glyph is in exactly one class. With format 3, any glyph can occur in multiple Coverage tables.

Format 3 contextual substitutions are implemented using a ChainedSequenceContextFormat3 table. See Chained Sequence Context Format 3: coverage-based glyph contexts in the OpenType Layout Common Table Formats chapter for complete details.

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.

This subtable type uses one format: ExtensionSubstFormat1.

7.1 Extension Substitution Subtable Format 1

ExtensionSubstFormat1 subtable

Type Name Description
uint16 substFormat Format identifier. Set to 1.
uint16 extensionLookupType Lookup type of subtable referenced by extensionOffset (that is, the extension subtable).
Offset32 extensionOffset Offset to the extension subtable, of lookup type extensionLookupType, relative to the start of the ExtensionSubstFormat1 subtable.

The extensionLookupType field must be set to any lookup type other than 7. All subtables in a LookupType 7 lookup must have the same extensionLookupType. All offsets within 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:

  • Proceed as though the Lookup table’s lookupType field were set to the extensionLookupType of the subtables.
  • Proceed as though each extension subtable referenced by extensionOffset replaced the LookupType 7 subtable that referenced it.

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.

Compared to the Chaining Contextual Sustitution (lookup subtable type 6), this format is restricted to only a coverage-based subtable format, input sequences can contain only a single glyph, and only single substitutions are allowed on this glyph. This constraint is integrated into the 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.

8.1 Reverse Chaining Contextual Single Substitution Format 1: Coverage-based Glyph Contexts

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 for the Chained Sequence Context Format 1 table, in the OpenType Layout Common Table Formats chapter. 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 processing a reverse chaining substitution, i begins at the logical end of the string and moves to the beginning.

The subtable contains a Coverage table for the input glyph and Coverage table arrays for backtrack and lookahead sequences. It also contains an array of substitute glyph indices (substituteGlyphIDs), which are substititions for glyphs in the Coverage table, and a count of glyphs in the substituteGlyphIDs array. The substituteGlyphIDs array must contain the same number of glyph indices as the Coverage table. To locate the corresponding output glyph index in the substituteGlyphIDs array, this format uses the Coverage index returned from the Coverage table.

Example 10 at the end of this chapter uses ReverseChainSingleSubstFormat1 to substitute Arabic glyphs with a correct stroke thickness on the left (exit) to match the stroke thickness on the right (entry) of the following glyph (in logical order).

ReverseChainSingleSubstFormat1 Subtable

Type Name Description
uint16 substFormat Format identifier: format = 1
Offset16 coverageOffset Offset to Coverage table, from beginning of substitution subtable.
uint16 backtrackGlyphCount Number of glyphs in the backtrack sequence.
Offset16 backtrackCoverageOffsets[backtrackGlyphCount] Array of offsets to coverage tables in backtrack sequence, in glyph sequence order.
uint16 lookaheadGlyphCount Number of glyphs in lookahead sequence.
Offset16 lookaheadCoverageOffsets[lookaheadGlyphCount] Array of offsets to coverage tables in lookahead sequence, in glyph sequence order.
uint16 glyphCount Number of glyph IDs in the substituteGlyphIDs array.
uint16 substituteGlyphIDs[glyphCount] Array of substitute glyph IDs — 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 major/minor 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 2 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: calculated output glyph indices
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
rangeRecords[0]
004E 78 Start glyph ID for numeral zero glyph
0058 87 End glyph ID 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 substituteGlyphIDs[0], ordered by Coverage index
0135 VerticalClosedBracketGlyph substituteGlyphIDs[1]
013E VerticalOpenParenthesisGlyph substituteGlyphIDs[2]
0143 VerticalClosedParenthesisGlyph substituteGlyphIDs[3]
CoverageFormat1
VerticalPunctuationCoverage
Coverage table definition
0001 1 coverageFormat: lists
0004 4 glyphCount
003C HorizontalOpenBracketGlyph glyphArray[0], ordered by glyph ID
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 sequenceOffsets[0] (offset to Sequence table 0)
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 alternateSetCount — equals glyphCount in Coverage table
000E AltAmpersandSet alternateSetOffsets[0] (offset to AlternateSet table 0)
CoverageFormat1
AltAmpersandCoverage
Coverage table definition
0001 1 coverageFormat: lists
0001 1 glyphCount
003A DefaultAmpersandGlyphID glyphArray[0]
AlternateSet
AltAmpersandSet
AlternateSet table definition
0002 2 glyphCount
00C9 AltAmpersand1GlyphID alternateGlyphIDs[0] — glyphs in arbitrary order
00CA AltAmpersand2GlyphID alternateGlyphIDs[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, ligatureSetCount specifies two LigatureSet tables, one for each covered glyph, and the ligatureSetOffsets 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 ligatureOffsets 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 ligatureSetCount
0014 ELigaturesSet ligatureSetOffsets[0] (offset to LigatureSet table 0) — LigatureSet tables in Coverage index order
0020 FLigaturesSet ligatureSetOffsets[1]
CoverageFormat2
LigaturesCoverage
Coverage table definition
0002 2 coverageFormat: ranges
0001 1 rangeCount
rangeRecords[0]
0019 eGlyphID Start, first glyph ID
001A fGlyphID End, last glyph ID in range
0000 0 startCoverageIndex: coverage index of start glyph ID = 0
LigatureSet
ELigaturesSet
LigatureSet table definition — all ligatures that start with e
0001 1 ligatureCount
0004 etcLigature ligatureOffsets[0] (offset to Ligature table 0)
Ligature
etcLigature
Ligature table definition
015B etcGlyphID ligatureGlyph — output glyph ID
0003 3 componentCount
0028 tGlyphID componentGlyphIDs[0] — second component in ligature
0017 cGlyphID componentGlyphIDs[1] — third component in ligature
LigatureSet
FLigaturesSet
LigatureSet table definition all ligatures start with f
0002 2 ligatureCount
0006 ffiLigature ligatureOffsets[0] — listed first because ffi ligature is preferred to fi ligature
000E fiLigature ligatureOffsets[1]
Ligature
ffiLigature
Ligature table definition
00F1 ffiGlyphID ligatureGlyph — output glyph ID
0003 3 componentCount
001A fGlyphID componentGlyphIDs[0] — second component in ligature
001D iGlyphID componentGlyphIDs[1] — third component in ligature
Ligature
fiLigature
Ligature table definition
00F0 fiGlyphID ligatureGlyph — output glyph ID
0002 2 componentCount
001D iGlyphID componentGlyphIDs[0] — second component in ligature

Example 7: Contextual Substitution Format 1

Example 7 illustrates format 1 contextual substitution, using a SequenceContextFormat1 subtable 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 SequenceContextFormat1 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 glyph IDs for the first glyphs in the SpaceGlyph and DashGlyph sequences. One SequenceRuleSet table is defined for each covered glyph.

SpaceAndDashSubRuleSet lists all the contexts that begin with a SpaceGlyph. It contains an offset to one SequenceRule table (SpaceAndDashSubRule), which specifies two glyphs in the context sequence, the second of which is a DashGlyph. The SequenceRule table contains a SequenceLookupRecord 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 points to a SequenceRule table (DashAndSpaceSubRule), which specifies two glyphs in the context sequence, and the second one is a SpaceGlyph. The SequenceRule table contains a SequenceLookupRecord that 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
SequenceContextFormat1
DashSubtable
SequenceContextFormat1 subtable definition for Lookup[0], DashLookup
0001 1 format
000A DashCoverage offset to Coverage table
0002 2 seqRuleSetCount
0012 SpaceAndDashSubRuleSet seqRuleSetOffsets[0] (offset to SequenceRuleSet table 0) — SequenceRuleSets ordered by Coverage index
0020 DashAndSpaceSubRuleSet seqRuleSetOffsets[1]
CoverageFormat1
DashCoverage
Coverage table definition
0001 1 coverageFormat: lists
0002 2 glyphCount
0028 SpaceGlyph glyphArray[0] — glyphs in numeric order
005D DashGlyph glyphArray[1], dash glyph ID
SequenceRuleSet
SpaceAndDashSubRuleSet
SequenceRuleSet[0] table definition
0001 1 seqRuleCount
0004 SpaceAndDashSubRule seqRuleOffsets[0] (offset to SequenceRule table 0) — SequenceRule tables ordered by preference
SequenceRule
SpaceAndDashSubRule
SequenceRule[0] table definition
0002 2 glyphCount — number in input sequence
0001 1 seqLookupCount
005D DashGlyph inputSequence[0], starting with second glyph — SpaceGlyph, in Coverage table, is first glyph
seqLookupRecords[0]
0000 0 sequenceIndex — substitution at first glyph position (0)
0001 1 lookupListIndex — index for SpaceToThinSpaceLookup in LookupList
SequenceRuleSet
DashAndSpaceSubRuleSet
SequenceRuleSet[1] table definition
0001 1 seqRuleCount
0004 DashAndSpaceSubRule seqRuleOffsets[0] (offset to SequenceRule table 0) — SequenceRule tables ordered by preference
SequenceRule
DashAndSpaceSubRule
SequenceRule[0] table definition
0002 2 glyphCount — number in the input glyph sequence
0001 1 seqLookupCount
0028 SpaceGlyph inputSequence[0] — starting with second glyph
seqLookupRecords[0]
0001 1 sequenceIndex — substitution at second glyph position (glyph sequence index = 1)
0001 1 lookupListIndex — index for SpaceToThinSpaceLookup

Example 8: Contextual Substitution Format 2

Example 8 illustrates a format 2 contextual substitution using a SequenceContextFormat2 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 Definition table that defines four glyph classes: default mark glyphs (Class 1), high base glyphs (Class 2), very high base glyphs (Class 3), and all remaining glyphs, including medium-height base glyphs. 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 ClassSequenceRuleSet tables are defined, one for substituting high marks and one for very high marks. No ClassSequenceRuleSets are specified for Class 0 and Class 1 glyphs because no contexts begin with glyphs from these classes. The classSeqRuleSetOffsets lists offsets to the ClassSequenceRuleSet tables in class value order, so the offset for ClassSequenceRuleSet for class 2 precedes that for class 3.

Within each ClassSequenceRuleSet, a ClassSequencRule is defined. In SetMarksHighSubClassSet2, corresponding to contexts that begin with a glyph in class 2, the ClassSequenceRule table specifies an input sequence with two glyphs: the first glyph in Class 2 (a high glyph), and the second in Class 1 (a mark glyph). The SequenceLookupRecord specifies applying SubstituteHighMarkLookup at the second position in the sequence—that is, a high mark glyph will replace the default mark glyph.

In SetMarksVeryHighSubClassSet3, , corresponding to contexts that begin with a glyph in class 3, the ClassSequencRule specifies an input sequence with two glyphs: the first in Class 3 (a very high glyph), and the second in Class 1 (a mark glyph). The SequenceLookupRecord 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
SequenceContextFormat2
SetMarksHighSubtable
SequenceContextFormat2 subtable definition
0002 2 format
0010 SetMarksHighCoverage offset to Coverage table
001C SetMarksHighClassDef offset to Class Def table
0004 4 classSeqRuleSetCount
0000 NULL classSeqRuleSetOffsets[0] — NULL: no contexts that begin with Class 0 glyphs are defined
0000 NULL classSeqRuleSetOffsets[1] — no contexts that begin with Class 1 glyphs are defined
0032 SetMarksHighSubClassSet2 classSeqRuleSetOffsets[2] — offset to ClassSequencRuleSet table for contexts that begin with Class 2 glyphs (high base glyphs)
0040 SetMarksVeryHighSubClassSet3 classSeqRuleSetOffsets[3] — offset to ClassSequencRuleSet 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 classFormat: ranges
0003 3 classRangeCount
classRangeRecords[0] ClassRangeRecords ordered by startGlyphID; record 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: 2
classRangeRecords[1] ClassRangeRecord 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: 3
ClassRange[2] for Class 1, mark gyphs
classRangeRecords[2] ClassRangeRecord for Class 1, mark glyphs
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: 1
ClassSequencRuleSet
SetMarksHighSubClassSet2
ClassSequencRuleSet[2] table definition— all contexts that begin with Class 2 glyphs
0001 1 classSeqRuleCount
0004 SetMarksHighSubClassRule2 classSeqRuleOffsets[0] (offset to ClassSequenceRule table 0) — ClassSequenceRule tables ordered by preference
ClassSequenceRule
SetMarksHighSubClassRule2
ClassSequenceRule[0] table definition, Class 2 glyph (high base) glyph followed by a Class 1 glyph (mark)
0002 2 glyphCount
0001 1 seqLookupCount
0001 1 inputSequence[0] — input sequence beginning with the second Class in the input context sequence; Class 1, mark glyphs
seqLookupRecords[0] seqLookupRecords array in design order
0001 1 sequenceIndex — apply substitution to position 2, a mark
0001 1 lookupListIndex
ClassSequencRuleSet
SetMarksVeryHighSubClassSet3
ClassSequencRuleSet[3] table definition — all contexts that begin with Class 3 glyphs
0001 1 classSeqRuleCount
0004 SetMarksVeryHighSubClassRule3 classSeqRuleOffsets[0]
ClassSequenceRule
SetMarksVeryHighSubClassRule3
ClassSequenceRule[0] table definition — Class 3 glyph (very high base glyph) followed by a Class 1 glyph (mark)
0002 2 glyphCount
0001 1 seqLookupCount
0001 1 inputSequence[0] — input sequence beginning with the second Class in the input context sequence; Class 1, mark glyphs
seqLookupRecords[0] seqLookupRecords array in design order
0001 1 sequenceIndex — apply substitution to position 2, second glyph class (mark)
0002 2 lookupListIndex

Example 9: Contextual Substitution Format 3

Example 9 illustrates a format 3 contextual substitution, using a SequenceContextFormat3 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 SequenceLookupRecords: 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
SequenceContextFormat3
SwashSubtable
SequenceContextFormat3 subtable definition
0003 3 format
0003 3 glyphCount — number in input glyph sequence
0002 2 seqLookupCount
0014 AscenderDescenderCoverage coverageOffsets[0] — offsets to Coverage tables, in context sequence order
0030 XheightCoverage coverageOffsets[1]
0052 DescenderCoverage coverageOffsets[2]
seqLookupRecords[0] SequenceLookupRecords in glyph position order
0000 0 sequenceIndex
0001 1 lookupListIndex — single substitution to output ascender or descender swash
seqLookupRecords[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] — glyphs in glyph ID 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]
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]
003B jGlyphID glyphArray[1]
0041 pGlyphID glyphArray[2]
0042 qGlyphID glyphArray[3]
004A yGlyphID glyphArray[4]

Example 10: ReverseChainSingleSubstFormat1 Subtable

Example 10 uses a ReverseChainSingleSubstFormat1 subtable to substitute glyphs with a form that has a thick connection to the left (thick exit). This allows 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 glyph IDs 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 substituteGlyphIDs array provides the glyphs to replace glyphs that correspond in order in the ThickExitCoverage 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 backtrackCoverageOffsets[0]
0001 1 lookaheadGlyphCount
0026 ThickEntryCoverage lookaheadCoverageOffsets[0]
000C 12 glyphCount
00A7 BEm2 substituteGlyphIDs[0] — substitute glyphs ordered by Coverage index
00B9 BEi3 substituteGlyphIDs[1]
00C5 JIMm3 substituteGlyphIDs[2]
00D4 JIMi2 substituteGlyphIDs[3]
00EA SINm2 substituteGlyphIDs[4]
00F2 SINi2 substituteGlyphIDs[5]
00FD SADm2 substituteGlyphIDs[6]
010D SADi2 substituteGlyphIDs[7]
011B TOEm3 substituteGlyphIDs[8]
012B TOEi3 substituteGlyphIDs[9]
013B AINm2 substituteGlyphIDs[10]
0141 AINi2 substituteGlyphIDs[11]
CoverageFormat1
ThickEntryCoverage
Coverage table definition
0001 1 coverageFormat: lists
001F 31 glyphCount
00A5 ALEFf1 glyphArray[0] — glyphs in glyph ID 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]
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]