glue code for becoming an inkscape plugin started...

centerline-trace.inx

+<?xml version="1.0" encoding="UTF-8"?>
+<inkscape-extension xmlns="http://www.inkscape.org/namespace/inkscape/extension">
+  <_name>Centerline Trace</_name>
+  <id>com.github.fablabnbg.inskscape-centerline-trace</id>
+  <dependency type="extension">org.inkscape.output.svg.inkscape</dependency>
+  <dependency type="executable" location="extensions">inkex.py</dependency>
+  <dependency type="executable" location="extensions">centerline-trace.py</dependency>
+
+  <!-- Keep in sync with chain_paths.py line 16 __version__ = ... -->
+  <param name="about_version" type="description">
+  
+  
+  https://github.com/fablabnbg/inkscape-centerline-trace
+  Version 0.1</param>
+
+  <effect needs-live-preview="false" >
+    <object-type>path</object-type>
+    <effects-menu>
+      <submenu _name="Images"/>
+    </effects-menu>
+  </effect>
+  <script>
+      <command reldir="extensions" interpreter="python">centerline-trace.py</command>
+  </script>
+</inkscape-extension>

centerline-trace.py

+#!/usr/bin/env python
+#
+# Inkscape extension to vectorize bitmaps by tracing along the center of lines
+# (C) 2016 juewei@fabmail.org
+#
+# code snippets visited to learn the extension 'effect' interface:
+# - convert2dashes.py
+# - http://github.com/jnweiger/inkscape-silhouette
+# - http://github.com/jnweiger/inkscape-gears-dev
+# - http://sourceforge.net/projects/inkcut/
+# - http://code.google.com/p/inkscape2tikz/
+# - http://code.google.com/p/eggbotcode/
+#
+# 2016-05-11 jw, V0.1 -- initial draught
+
+__version__ = '0.1'	# Keep in sync with chain_paths.inx ca line 22
+__author__ = 'Juergen Weigert <juewei@fabmail.org>'
+
+import sys, os, tempfile, base64
+
+debug = True
+#debug = False
+
+# search path, so that inkscape libraries are found when we are standalone.
+sys_platform = sys.platform.lower()
+if sys_platform.startswith('win'):	# windows
+  sys.path.append('C:\Program Files\Inkscape\share\extensions')
+elif sys_platform.startswith('darwin'):	# mac
+  sys.path.append('/Applications/Inkscape.app/Contents/Resources/extensions')
+else:   				# linux
+  # if sys_platform.startswith('linux'):
+  sys.path.append('/usr/share/inkscape/extensions')
+
+# inkscape libraries
+import inkex
+import cubicsuperpath
+
+inkex.localize()
+
+from optparse import SUPPRESS_HELP
+
+def uutounit(self,nn,uu):
+  try:
+    return self.uutounit(nn,uu)		# inkscape 0.91
+  except:
+    return inkex.uutounit(nn,uu)	# inkscape 0.48
+
+class TraceCenterline(inkex.Effect):
+  """
+  Inkscape Extension make long continuous paths from smaller parts
+  """
+  def __init__(self):
+    # Call the base class constructor.
+    inkex.Effect.__init__(self)
+
+    self.dumpname= os.path.join(tempfile.gettempdir(), "trace-centerline.dump")
+
+    try:
+      self.tty = open("/dev/tty", 'w')
+    except:
+      try:
+        self.tty = open("CON:", 'w')	# windows. Does this work???
+      except:
+        self.tty = open(os.devnull, 'w')  # '/dev/null' for POSIX, 'nul' for Windows.
+    if debug: print >>self.tty, "__init__"
+
+    self.OptionParser.add_option('-V', '--version',
+          action = 'store_const', const=True, dest='version', default=False,
+          help='Just print version number ("'+__version__+'") and exit.')
+
+  def version(self):
+    return __version__
+  def author(self):
+    return __author__
+
+  def calc_unit_factor(self, units='mm'):
+        """ return the scale factor for all dimension conversions.
+            - The document units are always irrelevant as
+              everything in inkscape is expected to be in 90dpi pixel units
+        """
+        dialog_units = uutounit(self, 1.0, units)
+        self.unit_factor = 1.0 / dialog_units
+        return self.unit_factor
+
+  def effect(self):
+    if self.options.version:
+      print __version__
+      sys.exit(0)
+
+    self.calc_unit_factor()
+
+    if not len(self.selected.items()):
+      inkex.errormsg(_("Please select an image."))
+      return
+
+    for id, node in self.selected.iteritems():
+      if debug: print >>self.tty, "id="+str(id), "tag="+str(node.tag)
+      if node.tag != inkex.addNS('image','svg'):
+        inkex.errormsg(_("Object "+id+" is not an image. Try\n  - Object->Ungroup"))
+        return
+      # handle two cases. Embedded and linked images
+      # <image .. xlink:href="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAT8AA ..." preserveAspectRatio="none" height="432" width="425" transform="matrix(1,0,-0.52013328,0.85408511,0,0)"/>
+      # <image  .. xlink:href="file:///home/jw/schaf.png"
+
+      # 
+      # 
+      # dump the entire svg to file, so that we can examine what an image is.
+      href=str(node.get(inkex.addNS('href','xlink')))
+      f=open(self.dumpname, 'w')
+      f.write(href)
+      f.close()
+      print >>self.tty, "Dump written to "+self.dumpname
+      if href[:7] == 'file://':
+        filename=href[7:]
+      elif href[:15] == 'data:image/png;':
+        png=base64.decodestring(href[15+7:]) 
+	f=tempfile.NamedTemporaryFile(mode="wb", suffix=".png", delete=False)
+	f.write(png)
+	filename=f.name
+      else:
+        inkex.errormsg(_("Neither file:// nor data:image/png; prefix. Cannot parse PNG image href "+href))
+      print >>self.tty, "filename="+filename
+      #
+      # do something
+      #
+      #if href[:5] == 'data:':
+      #  os.unlink(filename)
+        
+
+
+if __name__ == '__main__':
+        e = TraceCenterline()
+
+        e.affect()
+        sys.exit(0)    # helps to keep the selection

inkscape-centerline-trace.py

@@ -22,7 +22,8 @@
 # The svg that has the longest path but the least number of
 # segments is returned.
 #
-# 
+# Requires: 
+# apt-get install autotrace
 
 import sys, os, re, math, tempfile, subprocess
 import xml.etree.ElementTree as ET
@@ -97,7 +98,14 @@ for i in range(num_attempts):
   fp.write("P4\n%d %d\n" % (bw.size[0], bw.size[1]))
   fp.write(bw.tobytes())
   fp.close()
-  p = subprocess.Popen(autotrace_cmd + [fp.name], stdout=subprocess.PIPE)
+  try:
+    p = subprocess.Popen(autotrace_cmd + [fp.name], stdout=subprocess.PIPE)
+  except Exception as e:
+    print '+ '+' '.join(autotrace_cmd)
+    print e
+    print "Try:\n  sudo apt-get install autotrace"
+    sys.exit(1)
+
   cand['svg'] = p.communicate()[0]
   os.unlink(fp.name)
   # <?xml version="1.0" standalone="yes"?>\n<svg width="86" height="83">\n<path style="stroke:#000000; fill:none;" d="M36 15C37.9219 18.1496 41.7926 19.6686 43.2585 23.1042C47.9556 34.1128 39.524 32.0995 35.179 37.6034C32.6296 40.8328 34 48.1105 34 52M36 17C32.075 22.4565 31.8375 30.074 35 36M74 42L46 38C45.9991 46.1415 46.7299 56.0825 45.6319 64C44.1349 74.7955 23.7094 77.5566 16.044 72.3966C7.27363 66.4928 8.04426 45.0047 16.2276 38.7384C20.6362 35.3626 27.7809 36.0006 33 36M44 37L45 37"/>\n</svg>