BackgroundTrueType GX fonts contain advanced tables exploiting the typographic power of of QuickDraw GX, Apple's advanced model for printing, typography and graphics.
GX fonts (which are not necessarily TrueType) make fine typography much easier. Previously, one had to use multiple "expert" fonts, and position characters manually to obtain the typographic quality demanded by fine books and magazines. Now a single GX font may contain:
This behaviour depends on the declaration of intelligent font features in the font (such as swashes, ligatures, alternate glyphs), along with the contexts that in which they may appear (beginning of line, replacing the sequence "ffl", etc.).
TrueType GX fonts can also contain font variations similar in concept to Adobe multiple masters. (In fact multiple masters can be used in GX more easily than on other platforms.) Font variations, defined in the GX 'fvar' table, depend on consistent point numbering on the glyph variations, so that smooth "in-between" glyphs may be interpolated.
GX ToolsApple have made available a suite of tools for editing GX fonts (and normal TrueType fonts) - for free. These do a variety of things to fonts, such as GX-ifying normal fonts so that their ligatures become automatic in GX. Download them from Apple's font site.
TrueType GX on the Web