Coverage Table

Each subtable in a lookup references a Coverage table (Coverage), which specifies all the glyphs affected by a substitution or positioning operation described in the subtable. The GSUB, GPOS, and GDEF tables rely on this notion of coverage. If a glyph does not appear in a Coverage table, the client can skip that subtable and move immediately to the next subtable.

A Coverage table identifies glyphs by glyph indices (GlyphIDs) either of two ways:

As a list of individual glyph indices in the glyph set.

As ranges of consecutive indices. The range format gives a number of start-glyph and end-glyph index pairs to denote the consecutive glyphs covered by the table.

In a Coverage table, a format code (CoverageFormat) specifies the format as an integer: 1 = lists, and 2 = ranges.

A Coverage table defines a unique index value (Coverage Index) for each covered glyph. This unique value specifies the position of the covered glyph in the Coverage table. The client uses the Coverage Index to look up values in the subtable for each glyph.

Coverage Format 1

Coverage Format 1 consists of a format code (CoverageFormat) and a count of covered glyphs (GlyphCount), followed by an array of glyph indices (GlyphArray). The glyph indices must be in numerical order for binary searching of the list. When a glyph is found in the Coverage table, its position in the GlyphArray determines the Coverage Index that is returned—the first glyph has a Coverage Index = 0, and the last glyph has a Coverage Index = GlyphCount -1.

Example 5 at the end of this chapter shows a Coverage table that uses Format 1 to list the GlyphIDs of all lowercase descender glyphs in a font.

CoverageFormat1 table: Individual glyph indices

Type	Name	Description
uint16	CoverageFormat	Format identifier
		—format = 1
uint16	GlyphCount	Number of glyphs in the GlyphArray
GlyphID	GlyphArray[GlyphCount]	Array of GlyphIDs
		—in numerical order

Coverage Format 2

Format 2 consists of a format code (CoverageFormat) and a count of glyph index ranges (RangeCount), followed by an array of records (RangeRecords). Each RangeRecord consists of a start glyph index (Start), an end glyph index (End), and the Coverage Index associated with the range's Start glyph. Ranges must be in GlyphID order, and they must be distinct, with no overlapping.

The Coverage Indexes for the first range begin with zero (0), and the Start Coverage Indexes for each succeeding range are determined by adding the length of the preceding range (End GlyphID - Start GlyphID + 1) to the array Index. This allows for a quick calculation of the Coverage Index for any glyph in any range using the formula: Coverage Index (GlyphID) = StartCoverageIndex + GlyphID - Start GlyphID.

Example 6 at the end of this chapter shows a Coverage table that uses Format 2 to identify a range of numeral glyphs in a font.

CoverageFormat2 table: Range of glyphs

Type	Name	Description
uint16	CoverageFormat	Format identifier
		—format = 2
uint16	RangeCount	Number of RangeRecords
struct	RangeRecord[RangeCount]	Array of glyph ranges
		—ordered by Start GlyphID

RangeRecord

Type	Name	Description
GlyphID	Start	First GlyphID in the range
GlyphID	End	Last GlyphID in the range
uint16	StartCoverageIndex	Coverage Index of first GlyphID in range