[MPEG-OTSPEC] Shared GSUB/GPOS notes, was Re: dmap proposal

[Jonathan Kew]

On 28/12/2023 01:49, John Hudson wrote:
> Given the multivalency inherent in that definition of what is meant by 
> language system, it is difficult to see exactly /how/ software vendors 
> are meant to ‘correctly’ implement support. Personally, I think a 
> proper implementation is one that provides the user with a mechanism 
> to explicitly apply a particular OTL langsys to text, independent of 
> all other language or region tagging, i.e. to be able to invoke 
> particular GSUB and GPOS behaviour as grouped within a given font 
> under langsys tags in a way that overrides any algorithmic application 
> of the tags.

As a note in passing: examples of this kind of support would be the CSS 
'font-language-override' property[1], or the 'language' option to the 
\font command in XeTeX[2]. (Arguably they're both somewhat misnamed, 
using "language" as a shorthand for "OpenType language system", which 
may be a bit misleading. Sorry; that's my fault.)

Unfortunately, only a minority of systems provide users with such a 
facility, as far as I'm aware.

JK

[1] https://www.w3.org/TR/css-fonts-4/#font-language-override-prop
[2] https://mirrors.ctan.org/info/xetexref/xetex-reference.pdf
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