[MPEG-OTSPEC] Feature name parameters in multi-script and multi-language fonts

[John Hudson]

I posted this query on the MS typography-issues board back in March, but 
received no replies.
https://github.com/MicrosoftDocs/typography-issues/issues/1015

My understanding from reading the specification for GSUB feature 
parameters for Stylistic Sets and Character Variant features is that a 
single feature parameter can be applied to a feature tag, regardless of 
how that tag is used in terms of lookup contents for different script or 
language system tags. Is that understanding correct?

This seems to me to be contrary to the overall OTL model of script and 
langsys structures that are specifically designed to enable different 
writing systems to have different layout behaviours. So, for example, 
when making a font supporting Latin, Cyrillic and Greek script, it is 
perfectly feasible for a stylistic set feature such as ss01 to have 
entirely different behaviour appropriate to each script, and given the 
limited number of 20 registered stylistic set features this is a more 
efficient use of the features; also, it is more intuitive for the user 
of a particular script for accessible stylistic sets in the UI to begin 
as ss01, ss02, etc. rather than these be reserved to some other, unused 
writing system.

There are also cases where behaviour of stylistic set features might 
differ within a script, at the langsys level. I am currently looking at 
implementing a stylistic set within the Latin script that is specific to 
traditional Irish orthography (langsys IRT). Since it is the only 
stylistic set that would be meaningful within that langsys, my 
inclination would be to use ss01, but that feature has different 
behaviour for other Latin script languages, and different behaviour 
again for other scripts.

Can anyone see a route out of this, in terms of being able to link 
feature parameters to specific script/langsys combinations?

JH


-- 

John Hudson
Tiro Typeworks Ltd    www.tiro.com

Tiro Typeworks is physically located on islands
in the Salish Sea, on the traditional territory
of the Snuneymuxw and Penelakut First Nations.

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