[css-inline] Define how to synthesize ideographic-over and ideographic-under

[kojiishi]

The text-box-edge property has text-edge as its value.

It defines that:

• ideographic: Use the ideographic-over baseline/ideographic-under baseline as the over/under edge.
• ideographic-ink: Use the ideographic-ink-over baseline/ideographic-ink-under baseline as the over/under edge.

Which goes to these definitions:

• ideographic-over: Corresponds to the line-over design edge of CJK (Han/Hangul/Kana) text. Assigned to idtp in OpenType.
• ideographic-under: Corresponds to the line-under design edge of CJK (Han/Hangul/Kana) text. Assigned to ideo in OpenType.
• ideographic-ink-over: Corresponds to the line-over ink edge of CJK (Han/Hangul/Kana) text. Assigned to icft in OpenType.
• ideographic-ink-under: Corresponds to the line-under ink edge of CJK (Han/Hangul/Kana) text. Assigned icfb in OpenType.

Questions:

1. Most fonts, including many CJK fonts, don't have these tables. How should this be synthesized when these are missing?
2. When the first font in the cascade doesn't have these tables but the second one does, should UA synthesize from the first font, or use the table from the second font?

@fantasai @nt1m @vitorroriz @Clqsin45 @bfgeek @jfkthame @kidayasuo

[nt1m]

cc @alanbaradlay who implemented this in WebKit. Maybe he can help answer?

[fantasai]

This is currently defined in https://www.w3.org/TR/css-inline-3/#baseline-synthesis ; though if it needs some improvement, we can improve it.

CC @MurakamiShinyu

[kojiishi]

This is currently defined in https://www.w3.org/TR/css-inline-3/#baseline-synthesis ; though if it needs some improvement, we can improve it.

Thank you for the pointer, I didn't know the section. I read it, but I'm not sure if I understand the algorithm in this section. Can you help?

1. The central baseline is defined to be halfway between the ideographic-over and ideographic-under baselines, so any two of these determines the third.

and the central baseline:

Corresponds to the ideographic central baseline, halfway between the ideographic-under and ideographic-over baselines. Assigned to bsln value 1 in TrueType AAT.

Am I reading this correctly that this is defined from each other? I'm having difficulty to understand what it suggests when all of them are missing. If it means to fallback to the item 3:

3. In CJK fonts the ascent and descent typically match the ideographic-over and ideographic-under baselines, so can be used as a fallback when both are missing.

Is this suggesting to use ascent/descent as the synthesized ideographic-over and ideographic-under? For text-box-trim, using ascent/descent means that it's the same as text. This Japanese article and another Japanese article indicates that using ascent/descent doesn't help for the vertical centering and alignment purposes for CJK fonts. For Meiryo, Hiragino, and Noto (Chrome/Firefox/Safari default Japanese fonts), ascent/descent don't match the desired position of ideographic-over and ideographic-under, and I'm failing to read what to do when ascent/descent don't match.