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Re: [csswg-drafts] [css-text][css-fonts] Optical bounds of a line (#5466)

From: Khaled Hosny via GitHub <sysbot+gh@w3.org>
Date: Fri, 28 Aug 2020 15:44:53 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-682739737-1598629492-sysbot+gh@w3.org>
It should be noted that `opdb` feature is not implemented anywhere (and probably unimplementable without hacks) since OpenType shapers (that take care of applying font features) work on a much lower level than what is needed for this kind of features. 

Also anything written in the OpenType feature registry should be taken with a big grain of salt since it is often outdated, confused, or contradicts actual implementations.

Apple also has [`opbd` table](https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6opbd.html) (standalone font table not an OTL font feature), that might be more implementable (simple data structure, can be applied by applications after shaping without having to re0implenet considerable chunks of the OTL tables), but I don’t know if Core Text implements it, there are only two fonts that have it in an up to date macOS system: Apple Chancery and Hoefler Text.

See also https://github.com/harfbuzz/harfbuzz/issues/1891

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