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

From: Koji Ishii <kojiishi@gluesoft.co.jp>
Date: Mon, 31 Oct 2011 15:29:49 -0400
To: "www-style@w3.org" <www-style@w3.org>
Message-ID: <A592E245B36A8949BDB0A302B375FB4E0CF961FE25@MAILR001.mail.lan>
This is a follow up to CSS-ISSUE-195[1].

In East Asian layout, authors usually specify the width of text blocks by the number of full-width characters. This method is used regardless whether the font is fixed pitch or proportional pitch, or combined (fixed Kanji with proportional alphabets for example.)

Doing so gives the following benefits:

* Most fonts today have fixed pitch (full-width) glyphs for Kanji and Kana. When a line consists only of such full-width characters, justification doesn't kick in and therefore expansion is avoided. Avoiding the expansion produces the best result from typographic perspective, and such line is still the majority in most publications.

* In Roman scripts, authors often measure the amount of text by the number of words. In East Asia, authors use "#chars * #lines" instead. What "#chars" means here is a little ambiguous when the text contains non-full-width characters such as Roman alphabets, but it usually means the amount of text that fits in "#-full-width-chars * #lines" box, rather than counting the exact number of characters. It is therefore important for authors to specify the width of a box by the number of full-width characters.

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.


[1] http://www.w3.org/Style/CSS/Tracker/issues/195
[2] http://www.microsoft.com/typography/otspec/baselinetags.htm#ideoembox

Regards,
Koji
Received on Monday, 31 October 2011 19:32:33 GMT

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