W3C home > Mailing lists > Public > www-style@w3.org > November 2011

Re: [css3-values] Unit for ideographic advance (ISSUE-195)

From: John Daggett <jdaggett@mozilla.com>
Date: Mon, 7 Nov 2011 18:18:03 -0800 (PST)
To: Koji Ishii <kojiishi@gluesoft.co.jp>
Cc: www-style@w3.org
Message-ID: <182520247.95494.1320718683587.JavaMail.root@zimbra1.shared.sjc1.mozilla.com>
Koji Ishii wrote:

> OpenType spec defines how to calculate Ideographic Em-Box[2].
> Historically speaking, East Asian font designers used to use
> head.unitsPerEm to store this information, so the logic described in
> the spec falls back to head.unitsPerEm if the information is missing.
> Authors use "em" units for this purpose today for the same historical
> reasons, which works for such fonts, but it can be different.
> What we'd like to propose is to make inline-direction length of the
> Ideographic Em-Box as a unit, so that authors can specify the number
> of Ideographic Em-Box for lengths such as box width, margins, column
> width, and column gaps, and so that the value works as authors expect
> for any fonts.

Isn't what you're describing something more akin to 'ideo-ch', the
advance of a full-width character in the inline direction?

Note that 'em', 'ex', 'ch' and 'rem' are all defined relative to the
font-size so you need to define something in a similar fashion. I think
what you're trying to define is the ratio of the width/height of the
ideographic em-box to the width/height of the em-box itself in the
inline direction, multiplied by the font-size.

What's the use case for this unit?  In other words, what are the cases
that using em-units won't solve?  Seems like the only situation is
compressed or expanded fonts.

Whatever definition you come up with, I think you need to test and
verify that your definition works with the actual fonts and layouts
you're interested in supporting.


John Daggett
Received on Tuesday, 8 November 2011 02:18:31 UTC

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