Last year, I mentioned in a couple of threads that following OTWG discussions, I'd been asked to prepare revised definitions for the four OpenType Layout features Isolated Forms <isol>, Initial Forms <init>, Medial Forms <medi>, and Terminal Forms <fina>. This was to replace the very old and mostly speculative text, which suggested these features would be applied based on analysis of word position, with text that described how these features are actually applied by shaping engines, which is explicitly based on Unicode joining properties of the <ArabicShaping.txt>
In all the excitement about variable fonts, it may have slipped notice that as of OpenType v1.8, these revisions have been incorporated into the format. Anyone supporting these features in fonts, layout engines, or applications, should familiarise themselves with the new definitions. Note that the change means that these features should expressly not
be used for word-positional variants. In my proposal, I did discuss the possibility of defining new features for that purpose, but decided not to propose them at this time. Such independent word-positional features would have the benefit of being available to all scripts, including those that also have joining behaviours, but present challenges for layout engines that need to be worked out before the features would be viable.
Thank you to the people who reviewed and improved my proposal, especially Jonathan Kew, Vladimir Levantovsky, and Peter Constable.
Updated registered feature descriptions:<isol> Isolated Forms<init> Initial Forms<medi> Medial Forms<fina> Terminal Forms
[Note that the three Syriac-specific Medial Forms #2 <med2>
, Terminal Forms #2
<fin2>, and Terminal Forms #3
> are not affected by this change.]