Glyph Class Definition Table

The GSUB and GPOS tables use the Glyph Class Definition table (GlyphClassDef) to identify which glyph classes to adjust with lookups.

The table uses the same format as the Class Definition table (for details, see the chapter, Common Table Formats). However, the GlyphClassDef table uses class values already defined in the GlyphClassDef Enumeration list:

GlyphClassDef Enumeration List

Class	Description
1	Simple glyph (single character, spacing glyph)
2	Ligature glyph (multiple character, spacing glyph)
3	Mark glyph (non-spacing combining glyph)
4	Component glyph (part of single character, spacing glyph)

The font developer does not have to classify every glyph in the font, but any glyph not assigned a class value falls into Class zero (0). For instance, class values might be useful for the Arabic glyphs in a font, but not for the Latin glyphs. Then the GlyphClassDef table will list only Arabic glyphs, and—by default—the Latin glyphs will be assigned to Class 0.

Example 2 at the end of this chapter defines a GlyphClassDef table with a sample glyph for each of the assigned classes.