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Date: Thu, 27 Jun 2013 16:57:45 -0700
From: Dave Curtis <davecurtis AT sonic DOT net>
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To: geda-user AT delorie DOT com
Subject: [geda-user] gschem cairo text measure
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Reply-To: geda-user AT delorie DOT com

I'm working on a symbol generator, and trying to match up text width 
measurement with gschem.  I've dug into the gschem code a little, and 
gschem seems to use Cairo these days.  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.

What is the correct way to match up text measurement?

Here is what I have:

class FontInfo(object):
      _gschemScalingConstant = 10000.0/555.0 # Magic number
     def __init__(self):
        blah blah blah
     def measure(self, aString):
         "Returns the layout length of aString in gschem distance."
         if self._csf == None:
             self._build_csf()
         width = self._csf.text_extents(aString)[2] # item 2 of extents 
is Cairo 'width'
         return width
     def _build_csf(self):
         "Builds and caches a Cairo Scaled Font."
         fontFace = cr.ToyFontFace(self.name) # default is Arial
         identityMatrix = cr.Matrix()
         fontOptions = cr.FontOptions() # get defaults
         scaling = self.size * self._gschemScalingConstant # 'size' is 
gschem font size from 'T' line
         scalingMatrix = cr.Matrix(xx = scaling, yy = scaling)
         self._csf = cr.ScaledFont(fontFace, scalingMatrix,
             identityMatrix, fontOptions)

Peter Clifton's name is all over the Cairo commits, but I see he hasn't 
posted to the list for about a week, so I'm guessing he's on holiday.  
Any help appreciated.

-dave