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	ð Coverage	Offset to Coverage table
		—from beginning of Substitution table
uint16	LigSetCount	Number of LigatureSet tables
Offset	ð 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	ð 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 LookupFlag bit that specifies 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 by LookupFlag direction bit