RE: CJKフォントのpalt（詰め組）とkernの関係

Nat,


  *   Nat: the above seems correct to me.

Fine.


  *   Nat: this is the key understanding all font makers should have about how much kern they set on pairs of Kana etc that are normally monospaced by default. I hope they understand the engines assume kern pair amounts are deltas off the proportional widths not the monospaced widths. The engines first apply palt so the kern amounts set in the kern pairs of Kana etc are correct.

Yes, you are right.


  *    Nat: the above seems correct to me.

Fine.


  *   Nat: to be clear, this is in the case of auto kern everything and not the default behavior for cjk.

Yes, I think you are right. The default behavior should be without ‘palt’ and without ‘kern’.


  *   Nat: in Adobe apps "auto" or "metrics" kern means apply both palt and kern. "Latin-only kern" mode means apply palt and kern only to non-CJK (non-SJIS 由来の全角). With those features applied we ask the font engine for glyphs and positions. If the font lacks one of those features it is ignored.

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

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

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

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?

Regards,

--Taro