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Re: line-height: normal and multiple descendant font sizes

From: John Hudson <tiro@tiro.com>
Date: Fri, 18 Oct 2013 08:58:35 -0700
Message-ID: <52615AAB.2040307@tiro.com>
To: Glenn Adams <glenn@skynav.com>, Alan Gresley <alan@css-class.com>
CC: W3C Style <www-style@w3.org>
On 17/10/13 10:34 PM, Glenn Adams wrote:

> Actually, the spec means font size not font [family] when it says
> rendered in more than one font. Otherwise, it wouldn't say the largest
> font*size*. Font family is not related to line height computation.

It is if the line height is being computed from in-font metrics (OS/2 
WinAscent and WinDescent values, OS/2 Typo values, or corresponding hhea 
table values), which is what the line-height spec indicates. I presumed 
that the spec in this instance meant 'largest vertical metrics' when it 
says 'largest font size', but agree that this could be clearer.

This is a topic discussed at some length during development of the 
Composite Font Representation (ISO/IEC 14496-28:2012), and as in that 
case the only safe method is to use the largest individual metrics 
values from each of the fonts involved (bearing in mind that metrics may 
affect not only line height but also glyph clipping zones). This means 
that the appropriate above-baseline metric and below-baseline metric may 
come from different fonts in the line, especially if different writing 
systems are involved. For example, combining Vietnamese and Burmese text 
on the same line will almost certainly require taking the ascender 
height from the Vietnamese font and the descender depth from the Burmese 
font, because of the way in which these two writing systems make use of 
vertical space.

JH
Received on Friday, 18 October 2013 15:59:21 UTC

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