Re: Supporting stylistic sets

[Yuri Khan <yuri.v.khan@gmail.com>]

On Sat, 24 Sept 2022 at 23:15, Eli Zaretskii <eliz@gnu.org> wrote:

> I'm sorry, I still don't understand.  (I'm not an expert on fonts, so
> I'd appreciate a slightly more detailed explanations.)  Are you saying
> that each tag can be used by a single character in a font?  IOW, no
> feature tag can ever be shared by two or more characters in a font?

As I understand it, the font has instructions like “if stylistic set X
is in effect, use glyphs Y1, Y2, …, Yn for characters Z1, Z2, …, Zn”.
Depending on the font designer’s whim, there could be separate
stylistic sets for each character, sets for multiple characters
similar in shape, and/or sets for a group of characters where the user
is unlikely to want to control each character separately.

For example, Fira Code has a stylistic set ‘onum’ that applies to all
digits. By enabling this, the user signals that they want old-style,
aka minuscule, numerals, where certain digits are seated lower
relative to the baseline, and/or have a smaller glyph height. (On
Windows, see the Georgia font for an example of minuscule digits.) It
is a choice that is normally made for all digits as a set, not
individually “I want odd digits to be set as minuscule but even digits
as default”.

For another example, the stylistic set ‘cv31’ discussed above applies
to both opening and closing parentheses, as it is unlikely that a user
would want different styles for those.