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RE: position of baseline relative to em square

From: Greg Hitchcock <gregh@microsoft.com>
Date: Tue, 30 Nov 1999 14:01:27 -0800
Message-ID: <2DCBFADAFCBBD21189D400805F6FA1AB096B8487@RED-MSG-12>
To: "'David Lemon'" <typenerd@slip.net>, erik@netscape.com
Cc: www-font@w3.org
In general David is correct here. The Windows metrics otmAscent and
otmDescent map to the sTypoAscender and sTypoDescender in the OS/2 table of
the font. Typically otmAscent maps to the top of the l/c f (or sometimes d)
and the otmDescent maps to the bottom of the l/c g (or sometimes p) This is
usually very close to the em, although not quite. I believe that this
difference is sometimes caused by the bevel on the hot-metal slug that was
used for strengthening. One could reasonably take the difference between
otmAscent and otmDescent and evenly divide it above and below to determine
the baseline in the em.


-----Original Message-----
From: David Lemon [mailto:typenerd@slip.net]
Sent: Tuesday, November 30, 1999 8:15 AM
To: erik@netscape.com
Cc: www-font@w3.org
Subject: Re: position of baseline relative to em square

At 11:49 PM -0800 11/29/99, Erik van der Poel wrote:
> However, if CSS wants us to center in terms of the em square, we would
> need to know where the baseline lies in the em square.
> As far as I can tell, neither the Windows APIs nor the TrueType file
> give us this info. Is this true?

Although not very clearly specified, the (absolute values of the)
WindowsTypographicAscent and WindowsTypographicDescent in the OS/2 table
should sum to the em square height, positioning the baseline in the em
square. I can't guarantee the general case, but can state that Adobe's
OpenType fonts do exactly that.

- David Lemon
Received on Tuesday, 30 November 1999 17:02:49 UTC

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