W3C home > Mailing lists > Public > www-style@w3.org > November 1999

Re: font-size and accents, again

From: Erik van der Poel <erik@netscape.com>
Date: Mon, 29 Nov 1999 15:00:28 -0800
Message-ID: <3843058C.B192C005@netscape.com>
To: Walter Ian Kaye <walter@natural-innovations.com>
CC: www-style@w3.org
Walter Ian Kaye wrote:
> 
> Erik van der Poel wrote:
> >
> >David Perrell wrote:
> >>
> >> Following is some selected data from the AFM file for Adobe's
> >> Times Roman. The coordinate system for an Adobe font starts on
> >> the baseline with up and right positive and is 1000 units per
> >> em.
> >>
> >>   UnderlinePosition -98
> >>   UnderlineThickness 54
> >>   FontBBox -167 -252 1004 904
> >>   CapHeight 673
> >>   XHeight 445
> >>   Descender -219
> >>   Ascender 686
> >
> >Yes, this shows that the bbox is taller than the font size, and that
> >ascender + descender is smaller than the font size (in this case). Is
> >there also a field that indicates where the baseline lies in the em
> >square?
> 
> Doesn't that kind of depend upon how you're defining the em square? :-)
> How is zero relative to m, when m is relative to zero?

Well, presumably the glyphs in outline fonts are defined in terms of
coordinates in the same units as the "unitsPerEm" field. Some of those
coordinates are positive, others are negative, right? So there must be a
baseline running through the em square. My question is: Where does this
baseline lie? How many units from the top? Or bottom?

Once I have the answer for AFM and Type 1, my next question will be: How
do I find this out for TrueType? (I found some interesting fields called
sTypoAscender, sTypoDescender and sTypoLineGap, but if you add the
absolute values of the ascender and descender, you don't get the
unitsPerEm number.)

  http://www.microsoft.com/typography/tt/ttf_spec/ttch02.doc

Erik
Received on Monday, 29 November 1999 18:03:20 GMT

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