Added flip extension

extensions/fablabchemnitz/flip.inx

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<inkscape-extension xmlns="http://www.inkscape.org/namespace/inkscape/extension">
+	<name>Flip</name>
+	<id>fablabchemnitz.de.Flip</id>
+	<effect>
+		<object-type>path</object-type>
+		<effects-menu>
+            <submenu name="FabLab Chemnitz">
+                <submenu name="Modify existing Path(s)"/>
+            </submenu>
+		</effects-menu>
+	</effect>
+	<script>
+	<command reldir="inx" interpreter="python">flip.py</command>
+	</script>
+</inkscape-extension>

extensions/fablabchemnitz/flip.py

@@ -0,0 +1,38 @@
+import math
+from inkex import EffectExtension, PathElement, transforms as T
+
+# https://en.wikipedia.org/wiki/Rotations_and_reflections_in_two_dimensions
+def reflection_matrix(theta):
+    theta2 = 2 * theta
+    return [
+        [math.cos(theta2),  math.sin(theta2), 0],
+        [math.sin(theta2), -math.cos(theta2), 0],
+    ]
+
+def svg_matrix_order(mat):
+    ((a, c, e), (b, d, f)) = mat
+    return a, b, c, d, e, f
+
+class FlipPath(EffectExtension):
+    """Extension to flip a path about the line from the start to end node"""
+
+    def effect(self):
+        for node in self.svg.selection.filter(PathElement).values():
+            points = list(node.path.end_points)
+            if len(points) < 2 or points[0] == points[-1]:
+                continue
+            start = points[0]
+            end = points[-1]
+            v = end - start
+            theta = math.atan2(v.y, v.x)
+
+            # transforms go in reverse order
+            mat = T.Transform()
+            mat.add_translate(start)
+            mat.add_matrix(*svg_matrix_order(reflection_matrix(theta)))
+            mat.add_translate(-start)
+
+            node.path = node.path.transform(mat)
+
+if __name__ == '__main__':
+    FlipPath().run()

extensions/fablabchemnitz/parabola.inx

@@ -11,8 +11,7 @@
                         <option value="cross">Cross</option>
                         <option value="square">Square</option>
                         <option value="triangle">Triangle</option></param>
-            <label> </label>
-            <label> </label>
+            <spacer/>
             <label>v1.1.0</label>
         </page>
         <page name="corners" gui-text="Corners">
@@ -34,4 +33,4 @@
     <script>
         <command location="inx" interpreter="python">parabola.py</command>
     </script>
-</inkscape-extension>
+</inkscape-extension>