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RE: [ft] scalable font metrics, ascent/descent/height, metrics struct


From: Turner, David
Subject: RE: [ft] scalable font metrics, ascent/descent/height, metrics struct
Date: Wed, 1 Jun 2005 14:14:57 +0200

> 1. There is discrepancy between the calculated metrics (under DIY) and
> those retrieved from the OpenType OS2 Table.  i.e.  If I compare
> winAscender/winDescender with Latin-1 in the Times New Roman, or the
> ascender under OS2 table and DIY in the New Courier.  Do you know why?
> 
> 2. This _really_ puzzles me.  I call FT_Set_Pixel_Sizes to set the
> character dimensions of the desired face object (in the way shown in
> the attached source code.)  But if I sum up any pairs of
> ascender/descender none of them are close to the set size.  Do you
> know why?
> 
Yes:

 - first, the ascender/descender values embedded in the fonts are
   most of the time incorrect. We do some magic within FreeType
   to re-compute some of them in order to match Windows behaviour,
   since this was asked by users.

 - second, even when they're correct, the value returned in the
   FT_Size metrics are only scaled version of the global values
   found in the font. But processes like hinting (automatic or
   not) generally make them differ from the real ones.


- David Turner
- The FreeType Project  (www.freetype.org)

> > > 2. What is "AFM file" as mentioned in sTypoAscender and 
> sTypoDescender
> > > documentation?
> > 
> > Adobe Font Metrics files are text files which accompany Type 1
> > PostScript fonts (which don't contain metrics information).  On
> > Windows box you'll sometimes fine PFM files which are something
> > similar.
> > 
> > 
> >    Werner
> 
> Thanks very much again for your help Werner.  Your responses have been
> very helpful.
> 
> Anson
> 




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