AAT Layout
AAT first requires the text to be turned entirely into glyphs before text layout occurs. Operations on the text take place entirely within the glyph layer.
The core table used in the AAT layout process is the 'morx' table. The 'morx' table is divided into a series of chains, and each chain is further divided into subtables. The chains and subtables are processed in order. When each subtable is encountered, the layout engine compares flags in the subtable against control flags, generally derived from user settings. This determines whether or not the subtable is processed.
The set of available features in the font is made accessible to the user via the 'feat' table. The 'feat' table provides pointers to the localizable strings which can be used to describe a feature to the end user and the appropriate flags to send to the text engine if the feature is selected. Features can be made invisible to the user by the simple expedient of not including entries in the 'feat' table for them. Apple uses this approach, for example, to support required ligatures.
Subtables may do non-contextual glyph substitutions, contextual glyph substitutions, glyph rearrangements, glyph insertions, and ligature formation. Contextual actions are sensitive to the surrounding text. They can be used, for example, to automatically turn an s into a medial s anywhere in a word except at its end.
The 'morx' subtables for non-contextual glyph substitutions are simple mapping tables between the glyph substituted and its substitute. The others all involve the use of finite state machines.
For purposes of processing the finite state machine, glyphs are organized into classes. The classes may be small, with only a single glyph (for something like ligature formation), or it may include dozens glyphs or even more. A special class is automatically defined for any glyph not included in any of the explicit classes. Special classes are also available for the end of the glyph stream and glyphs deleted from the glyph stream.
Beginning with a start-of-text state, the layout engine parses the text, glyph by glyph. Depending on its current state and the class of the glyph it encounters, it will switch to a new state and possibly perform an appropriate action. The process continues until the glyph stream is exhausted.
The use of finite state machines allows 'morx' tables to be relatively small and to be processed relatively quickly. They also provide considerable flexibility. Inasmuch, however, as Apple's font tools require the generation of 'morx' tables via raw state table information, they can be difficult to produce and debug. The font designer is also responsible to make sure that 'morx' subtables are ordered correctly for the desired effect.
Since AAT operates entirely with glyphs and never with characters, all the layout information necessary for producing the proper display resides within the font itself. This allows fonts to be added for new scripts without requiring any specific support from the OS. Third parties can produce fonts for scripts not officially supported by Apple, and they will work with Mac OS X. On the other hand, this also means that every font for a given script requires its own copy of the script's shaping information in its own 'morx' tables.
Other AAT tables (or AAT-specific extensions to standard TrueType tables) allow for context-sensitive kerning, justification, ligature-splitting. AAT also supports variation fonts, where a font's shape can vary depending on scaled value supplied by the user. Variation fonts are similar to Adobe's defunct Multiple master fonts, where the end-points are defined and any medial value is valid. With this, the user can then drag a slider in the user interface to make glyphs taller or shorter, drag another one to make them fatter or thinner, another one to increase the size of the serifs, etc. all independently of one another. Glyphs may even have their fundamental shapes radically altered. There is nothing like this in OpenType.
Other AAT tables the user can also have point-size dependent effects, for example at 12 pt the horizontal and vertical strokes can be of similar width, but at 300 pt, the stroke width variation could be quite great.
In practice, few AAT fonts use any features of the technology other than those available through the 'morx' table. Zapfino, Hoefler Text, and Skia are fonts shipping with Mac OS X which illustrate a variety of AAT's capabilities.
Read more about this topic: Apple Advanced Typography