similar to OpenType.

Apple only has incomplete support for OpenType (such as not supporting South Asian scripts), making the use of AAT necessary for some fonts.

We know of the following incompatibilities between AAT and OT:

• Apple extends the kern table with a backwards incompatible version 1 format that changes the sizes of existing fields and introduces two new subtable formats. Harfbuzz supports this new version, but Microsoft does not.

  One of the new subtable formats, format 1, has a field that can act as either an offset or an initial state value. The differentiation is made based on the value of the offset: if it is the “expected” value (presumably if the data pointed to directly follows the table), it is an offset, else it is a value. This contradicts the purpose of offset fields, which explicitly allow data to be positioned somewhere else.

  Apple also introduced the kerx table, which is similar to kern, but with oddities such as the previously mentioned one removed. It is our understanding that modern Apple fonts rely on the kerx table instead of a version 1 kern table.

AAT includes the following tables that are absent from OpenType 1.9:

• acnt
• ankr
• bdat
• bhed
• bloc
• bsln
• fdsc
• feat
• fmtx
• fond
• gcid
• gvar
• just
• kerx
• lcar
• ltag
• mort
• morx
• opbd
• prop
• trak
• xref
• Zapf