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—Nat
________________________________
From: Taro Yamamoto <tyamamot@adobe.com>
Sent: Saturday, April 15, 2023 11:29 PM
To: Nat McCully <nmccully@adobe.com>; Shinyu MURAKAMI <murakami@vivliostyle.org>; Makoto MURATA <eb2m-mrt@asahi-net.or.jp>
Cc: Koji Ishii <kojii@chromium.org>; 木田泰夫 <kida@mac.com>; 敏 小林 <binn@k.email.ne.jp>; JLReq TF 日本語 <public-i18n-japanese@w3.org>
Subject: RE: CJKフォントのpalt（詰め組）とkernの関係

I understand the ‘Latin-only kern’ mode works also for Japanese true-proportional fonts. Correct?


  *   Nat: Latin only means we apply kerning and likely unnecessary palt only to certain Unicodes in the non-CJK ranges that normally expect kerning to be on by default, for any font.

As a Western proportional or Japanese true-proportional font lacks the ‘palt’ information usually, the ‘kern’ should work, if it has ‘kern’ information. Correct?


  *   Nat: yes

But for a Japanese font without ‘palt’ information, neither the ‘palt’ nor the ‘kern’ feature doesn’t work. Correct?


  *   Nat: correct


  *   Nat: we generally do not check the font for the presence of a feature to make decisions about other features. So, the engine will apply palt if the user chooses "proportional widths". If the user chooses a kerning mode, the engine applies palt and kern both. Engines that do not apply palt whenever they apply kern are not correct according to the standard and to our understanding of kern amounts set in the kern feature for cjk glyph pairs.

Yes, but when a font lacks any ‘palt’ information, the ‘palt’ feature just doesn’t work, and the ‘kern’ feature is applied. Correct?


  *   Nat: correct

And, as mainstream Japanese fonts with the alternative width capability always have ‘palt’ information, the ‘palt’ feature only or both the ‘palt’ and ‘kern’ features should work always for those fonts. Right?


  *   Nat: yes

If so, the kind of fonts to which the ‘kern’ feature only is applied (without the ‘palt’ feature) is limited to Latin or Japanese true-proportional font. Right?


  *   Nat: yes

If so, I think the understanding mentioned by Mr. Murata is valid and suffices.

(3) When a font specifies the 'kern' feature as well as the 'palt' feature for a given glyph, the abstract font engine shall not use the 'kern' feature for rendering this glyph without using the 'palt' feature as well.

Font makers will need to just decide:
For Japanese fonts:
            (1) No ‘palt’, no’kern’
            (2) ‘palt’ only
            (3) both ‘palt’ and ‘kern’
For Latin or Japanese proportional fonts:
(4) No ‘palt’, No ‘kern’
(5) ‘kern’ only

I think the interaction between (3) above and these possible options on the font side seems reasonable and simple enough.
What do you think?


  *   Nat: seems fine to me

Regards,

--Taro