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.

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

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