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Re: [csswg-drafts] [css-inline-3] Ascent and Descent Metrics in vertical flow (#5381)

From: 梁海 Liang Hai via GitHub <sysbot+gh@w3.org>
Date: Mon, 17 Aug 2020 03:11:56 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-674632663-1597633915-sysbot+gh@w3.org>
> Is there a preferred way to calculate the baseline offset from origin in vertical layout? The CJK baseline for Japanese vertical being embox center means layout engines calculate the center baseline as an offset from the glyph origin in order to properly position glyph runs of different sizes in vertical. What are the conventions for Mongolian?

There isn’t a non-heuristic way. The whole idea of “baseline for aligning glyphs of different font sizes” is just a rather marginal case for any script, which generally ends with arbitrary decisions to fulfill the layout architecture’s need. In theory, Mongolian vertical runs would like to be center aligned as well, similar to how CJK ones do, but as Mongolian glyphs are not horizontally symmetrical in vertical lines like how CJK ones do, there isn’t a straightforward way to calculate the alignment line.

Mongolian’s connected stem (that continuous vertical stroke in vertical lines, which may be used to calculate an alignment line in some way) is placed away from the Roman baseline, according to the need of visually aligning a whole glyph (not the connected stem) with both Roman and CJK glyphs. And it’s a pain to make sure Mongolian glyphs align with CJK glyphs in vertical lines, because there isn’t a standard for layout engines to place the Roman baseline in relation to CJK em-boxes, which further contributes to the lack of convention for where Mongolian’s connected stem goes in a glyph.

Btw, I find the “baseline” term confusing and very awkward when it’s used to refer to various kinds of alignment line in a layout architecture context. There’s really nothing of the so called “CJK baseline” comparable with the Roman baseline, besides both being considered the default alignment line. And the so called “hanging baseline” for Indic scripts is just a bizarre idea, when those scripts already have a typographically baseline highly comparable to the Roman baseline, while the headstroke only suggests a not-necessarily-stronger preference for alignment.

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