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Date: Tue, 16 Jul 2013 18:13:39 +0200 (CEST)
From: Roland Lutz <rlutz AT hedmen DOT org>
To: geda-user AT delorie DOT com
Subject: Re: [geda-user] gschem cairo text measure
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On Thu, 27 Jun 2013, Dave Curtis wrote:
> I'm not very familiar with Cairo, but I've managed to come up with 
> something that appears to work, but it relies heavily on a magic number 
> that I arrived at empirically.

> _gschemScalingConstant = 10000.0/555.0 # Magic number

Maybe you are interested in this constant from libgeda/src/o_text_basic.c:

/*! \brief Scale factor between legacy gschem font units and postscript points.
  *
  *  \par Description
  *  gschem fonts are nominally specified in points, however there is a
  *  difference in how the specified font size corresponds to the metrics of
  *  the font when compared to typical typographic usage.
  *
  *  The following factor was impirically determined to approximately match the
  *  cap-height between the legacy gschem font, and fonts rendered using pango.
  */
#define GEDA_FONT_FACTOR 1.3

This factor has been empirically determined, too; it's 1.3, not 18, 
though, so this is probably not exactly what you are looking for.

Roland