[XeTeX] How to get _third_ stylistic alternate?

[Jonathan Kew]

On 15 Apr 2008, at 8:56 am, Marcin Woliński wrote:

> BTW, do I understand correctly that the only OT features with numeric
> argument in XeTeX are  “alternate substitution” features where  
> in the
> OTspec parlance the aplication gets a group of GIDs as a result of a
> lookup?

Yes. Well, XeTeX doesn't really care; you could specify an argument  
to any OT feature, and it will be passed to the font engine. But it  
won't have any effect unless the feature is implemented in the font  
using an Alternate Substitution subtable (GSUB lookup type 3), which  
is a one-of-many lookup; in this case, the argument selects which of  
the (potentially) many alternates will be used.

The only features I've seen implemented this way, AFAIR, are 'aalt',  
'salt', and 'ssty', but there are others in the registry where it is  
recommended (such as 'falt'). And there's nothing to prevent a font  
developer using such a lookup for other features as well. (Whether  
other apps would use it correctly is another matter....)

JK