[XeTeX] AAT feature codes

[Will Robertson]

Jonathan Kew wrote:
> 
> Well.... sort of.... it shows the "registered" features, but font 
> developers are free to support additional features, and many AAT fonts 
> do in fact use features and selectors that aren't in Apple's list. 

Yeah, I noticed this after sending the post. Zapfino uses 128 for its 
"avoid d collisions", for example. My code at the moment assumes no 
number greater than 25 :) but that's easily expanded...

> (For 
> this reason, BTW, I'd like to see your package allow users to pass 
> arbitrary feature names as options to typespec, not only a predefined set.)

That shouldn't be too hard I think. With an interface like
  \typespec[ArbitraryFeature = %
       {"Special Features = Avoid d Collisions",%
        "Something = Else"}]{Zapfino}
I can see that working fairly nicely (once I work out how to parse it in 
TeX - can't be too hard). But I'm sure you agree having a standard 
interface to the standard features is a lot more intuitive than the 
stupid names Apple comes up with :)

Good idea! Your wish is my command.

> I suppose I could make \XeTeXselectorname prefix the name with "!" in 
> this situation; currently, these commands are a very low-level interface 
> to the ATSUI font name access functions, but I could put a wrapper 
> around it to save you doing so in TeX macros. Perhaps....

No, I agree with your assessment that "Common Ligatures" is the name of 
the feature whether you're turning it on or off.

It would be unambiguous if you added a numerical system, but that's as 
easily done in a macro. I'll implement that as you suggest, 
automatically negating the featurename if the selector is odd and 
non-exclusive.

Thanks heaps!
Will