"Class 0" in GSUB/GPOS

The spec says:

A font developer can assign any glyph to any class, each identified with an integer called a class value. A Class Definition table (ClassDef) groups glyph indices by class, beginning with Class 1, then Class 2, and so on. All glyphs not assigned to a class fall into Class 0.

Does "all glyphs not assigned to a class" literally mean every other glyph in the font or every other glyph in the coverage table or something else?


  • Answering my own question after a bit of experimentation, it does appear to mean “every other glyph in the font”. Which is a) cool and potentially very useful, b) extremely unwieldy to express in feature language, and therefore c) probably the reason why my font has been building for the past thirty minutes.
  • Every other glyph in the font.
  • Yes, every other glyph in the font: a ClassDef creates a partition of glyphs for the entire font.
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