Apple Advanced Typography fonts, which make use of layout capabilities of the QuickDraw GX Layout model, are an alternative solution to OpenType fonts. Both GX and OpenType use the cmap table to map character code points to glyphs indices, after which all processing is done using glyph indices. There are several differences between the GX model and OpenType:
- GX provides support for bidirectional processing and reordering (for some South Asian scripts). With OpenType, Uniscribe or the text-processing application performs these activities.
- GX uses a property table in the font to ascertain direction and to determine any other property for each glyph in the font. OpenType infers properties based on the Unicode point to which the glyph is mapped.
- GX uses the metamorphosis (also called the "mort") table for substitutions that involve building up ligatures from their component glyphs, using noncontextual substitutions (such as a swash variant), or making contextual changes in the form of a letter, which occurs in Arabic. OpenType lists all substitutions in the GSUB table.
- GX uses finite state machines to identify context. OpenType uses string matching.