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

From: fantasai via GitHub <sysbot+gh@w3.org>
Date: Thu, 13 Aug 2020 02:18:27 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-673208925-1597285106-sysbot+gh@w3.org>
> Currently IIUC Ascent and Descent Metrics is used only for Canvas TextMetrics

No, they are used in CSS layout also, e.g. for line height calculation. em-top/em-bottom are only used for Canvas, though.

> When text-orientation: mixed, glyphs may be rendered upright or rotated sideways. Should Ascent and Descent Metrics use horizontal metrics for rotated sideways glyphs, or vertical metrics for upright glyphs?

@kojiishi oh, that's tricky question. In general we [use the appropriate metrics per run based on orientation](https://www.w3.org/TR/css-writing-modes-3/#vertical-font-features). But you're right that for getting the metrics of a box, we have to decide whether `mixed` should be considered upright or sideways.

I think I'm leaning towards using vertical metrics, unless they're missing, then use rotated horizontal metrics. I think that will work best for CJK, but I'm unsure about Mongolian--I think internally it's treated as rotated even though such characters are "upright" for the reader? That used to be the case in the past, I think; I'm unsure about current practices.

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