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Subject: Re: [geda-user] gschem cairo text measure
From: Edward Hennessy <ehennes AT sbcglobal DOT net>
In-Reply-To: <51CCD179.7010903@sonic.net>
Date: Thu, 27 Jun 2013 18:48:10 -0700
Message-Id: <1E0F5AC4-88AB-4CCF-ABE3-439DBE79B907@sbcglobal.net>
References: <51CCD179 DOT 7010903 AT sonic DOT net>
To: geda-user AT delorie DOT com
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On Jun 27, 2013, at 4:57 PM, Dave Curtis <davecurtis AT sonic DOT net> wrote:

> 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
> 

Dave,

When I worked on text rendering, I wound up with a couple magic numbers too:

http://git.geda-project.org/gparts/tree/src/schgui-cairo-text.c

I looked through the code and could not find the magic constant to make the
text match up with the legacy renderer.

Cheers,
Ed