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	ð Coverage	Offset to Coverage table
		—from beginning of Substitution table
uint16	AlternateSetCount	Number of AlternateSet tables
Offset	ð AlternateSet	Array of offsets to AlternateSet tables
	[AlternateSetCount]	—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