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Re: What's an em

From: Jelle Bosma <jelleb@euronet.nl>
Date: Wed, 2 Feb 2000 04:52:14 -0500 (EST)
Message-ID: <01bf6d62$dcbc1700$64646464@jel-nt.jelle.com>
To: <www-style@w3.org>, <www-font@w3.org>, "Just van Rossum" <just@letterror.com>
Hear, hear!

In Digital outline fonts the EM is a square which is a large as the point
size at any size.

Type designers will position the outlines in the EM in such a way
to make the point size the smallest possible line to line
distance in which the font should be used.

Actual character heights may vary within a family in
order to look as if they are the same.

The Baseline is at y coordinate 0.
The TypoAscender of the OS/2 table gives the ascender
The TypoDescender the descender.
The TypoLinegap together with TypoAscender and TypoDescender
is used to specify the recommended line to line distance
in relation to the point size.

Windows uses a separate WinAscend and WinDescend value
as a clipping zone and default line to line distance.

For non-Latin fonts where the original definition of these
values do not make sense, they are used to set recommended
and minimum line to line distance. This can be the EM, but
often larger in order to be able to mix with latin, whithout
having to change point size all the time.

Jelle Bosma

-----Oorspronkelijk bericht-----
Van: Just van Rossum <just@letterror.com>
Aan: Karlsson Kent - keka <keka@im.se>; www-style@w3.org <www-style@w3.org>;
'www-font@w3.org' <www-font@w3.org>
Datum: dinsdag 1 februari 2000 23:07
Onderwerp: RE: What's an em

>You can discuss this 'till you drop, you're not going to get a definitive
>answer. The fact that you already found so many (contradicting) definitions
>only means one thing: there is no true definition.
>However, the only *useful* definition is that of point size, since it's the
>only one that's exact: eg. TrueType has a value called unitsPerEm. If a
>font has 1000 units per em, it means that when set at 12 points, 1000 units
>in design space are 12 points in user space. That's all there is to it.
>(Don't ever start measuring letters: it's a dead end, since it's not exact
>by definition. Take two simple lowercase letters 'o', one light, one very
>heavy. Scale them so they are exactly the same height. View at small size.
>The bold one looks smaller.)
Received on Thursday, 3 February 2000 10:24:54 UTC

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