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Re: [csswg-drafts] [css-inline-3][css-writing-modes] Baseline Alignment of Mixed Vertical Orientations (#5424)

From: fantasai via GitHub <sysbot+gh@w3.org>
Date: Thu, 13 Aug 2020 04:36:00 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-673250397-1597293359-sysbot+gh@w3.org>
To make a concrete example, let's say the vertical baseline table of the font places alphabetic and central baselines at 50/100, and the horizontal baseline table places alphabetic at 10/100 and central at 40/100.

It's clear, I think, that in typical vertical typesetting, the upright runs and the sideways runs should be aligned to each other by the central baseline. But it's less clear what should happen if alignment by alphabetic baseline is requested. If we follow the proposal above:

* Suppose we have an inline box with sideways text inside an inline box with upright text. We are aligning sideways inside upright, so the context is sideways. Because the orientations mismatch, we create a hypothetical vertical baseline table for the sideways text: overlay the vertical baselines on the rotated horizontal baseline table, matching the central baselines. Since alphabetic and central coincide in the vertical table, this means alphabetic and central baseline will both be positioned at the rotated horizontal central coordinate, 40/100, and this point of the sideways will coincide with the alphabetic

* Next, consider the opposite situation: upright text in a sideways context. In this case we create a hypothetical rotated-horizontal baseline table for the upright text, matching at the central baseline. Since the alphabetic baseline in the horizontal baseline table is offset 30/100 from the central baseline, this means that the central baseline will be at 50/100 (to match the vertical baseline table) and the alphabetic baseline will be at 20/100.

I think this will work reasonably when aligning child text with an orientation mismatched to its parent. However, it is not a symmetrical operation: the result of aligning two runs depends on which orientation is considered the context and which is considered the intruder. For aligning a box within another box, there is a natural answer to this question. But for mixed orientation boxes: is the context sideways or upright?

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