Merge pull request #253 from gntech/add-sagittaArc

Add sagittaArc function and test

cadquery/cq.py

@@ -1331,6 +1331,41 @@ def threePointArc(self, point1, point2, forConstruction=False):
 
         return self.newObject([arc])
 
+    def sagittaArc(self, endPoint, sag, forConstruction=False):
+        """
+        Draw an arc from the current point to endPoint with an arc defined by the sag (sagitta).
+
+        :param endPoint: end point for the arc
+        :type endPoint: 2-tuple, in workplane coordinates
+        :param sag: the sagitta of the arc
+        :type sag: float, perpendicular distance from arc center to arc baseline.
+        :return: a workplane with the current point at the end of the arc
+
+        The sagitta is the distance from the center of the arc to the arc base.
+        Given that a closed contour is drawn clockwise;
+        A positive sagitta means convex arc and negative sagitta means concave arc.
+        See "https://en.wikipedia.org/wiki/Sagitta_(geometry)" for more information.
+        """
+
+        startPoint = self._findFromPoint(False)
+        endPoint = self.plane.toWorldCoords(endPoint)
+        midPoint = endPoint.add(startPoint).multiply(0.5)
+
+        sagVector = endPoint.sub(startPoint).normalized().multiply(abs(sag))
+        if(sag > 0):
+            sagVector.x, sagVector.y = -sagVector.y, sagVector.x # Rotate sagVector +90 deg
+        else:
+            sagVector.x, sagVector.y = sagVector.y, -sagVector.x # Rotate sagVector -90 deg
+
+        sagPoint = midPoint.add(sagVector)
+
+        arc = Edge.makeThreePointArc(startPoint, sagPoint, endPoint)
+
+        if not forConstruction:
+            self._addPendingEdge(arc)
+
+        return self.newObject([arc])
+
     def rotateAndCopy(self, matrix):
         """
         Makes a copy of all edges on the stack, rotates them according to the
@@ -2080,7 +2115,7 @@ def sweep(self, path, sweepAlongWires=False, makeSolid=True, isFrenet=False, com
         :param path: A wire along which the pending wires will be swept
         :param boolean sweepAlongWires:
             False to create mutliple swept from wires on the chain along path
-            True to create only one solid swept along path with shape following the list of wires on the chain 
+            True to create only one solid swept along path with shape following the list of wires on the chain
         :param boolean combine: True to combine the resulting solid with parent solids if found.
         :param boolean clean: call :py:meth:`clean` afterwards to have a clean shape
         :return: a CQ object with the resulting solid selected.
@@ -2407,7 +2442,7 @@ def _sweep(self, path, sweepAlongWires=False, makeSolid=True, isFrenet=False):
         :param path: A wire along which the pending wires will be swept
         :param boolean sweepAlongWires:
             False to create mutliple swept from wires on the chain along path
-            True to create only one solid swept along path with shape following the list of wires on the chain 
+            True to create only one solid swept along path with shape following the list of wires on the chain
         :return:a FreeCAD solid, suitable for boolean operations
         """
 

examples/FreeCAD/Ex005_Extruded_Lines_and_Arcs.py

@@ -18,15 +18,23 @@
 #     half-way back to the origin in the X direction and 0.5 mm above where
 #     the last line ended at. The arc then ends at (0.0, 1.0), which is 1.0 mm
 #     above (in the Y direction) where our first line started from.
-# 5.  close() is called to automatically draw the last line for us and close
+# 5.  An arc is drawn from the last point that ends on (0.3, 0.2), the sag of 
+#     the curve 0.1 determines that the curve is concave with the midpoint 0.1 mm
+#     from the arc baseline. If the sag was -0.1 the arc would be convex.
+#     This convention is valid when the profile is drawn counterclockwise.
+#     The reverse is true if the profile is drawn clockwise.
+#     Clockwise:        +sag => convex,  -sag => concave
+#     Counterclockwise: +sag => concave, -sag => convex
+# 6.  close() is called to automatically draw the last line for us and close
 #     the sketch so that it can be extruded.
-# 5a. Without the close(), the 2D sketch will be left open and the extrude
+# 6a. Without the close(), the 2D sketch will be left open and the extrude
 #     operation will provide unpredictable results.
-# 6.  The 2D sketch is extruded into a solid object of the specified thickness.
+# 7.  The 2D sketch is extruded into a solid object of the specified thickness.
 result = cq.Workplane("front").lineTo(width, 0) \
                               .lineTo(width, 1.0) \
-                              .threePointArc((1.0, 1.5), (0.0, 1.0)) \
+                              .threePointArc((1.0, 2.5), (0.2, 2.6)) \
+                              .sagittaArc((0.3, 0.2), 0.1) \ 
                               .close().extrude(thickness)
-
+                              
 # Displays the result of this script
 show_object(result)

tests/TestCadQuery.py

@@ -867,6 +867,10 @@ def test2DDrawing(self):
                           r.vertices(selectors.NearestToPointSelector((0.0, 0.0, 0.0)))\
                           .first().val().Y))
 
+        # Test the sagittaArc functions
+        s = Workplane(Plane.YZ())
+        r = s.sagittaArc((10, 8), 1).close()
+
     def testLargestDimension(self):
         """
         Tests the largestDimension function when no solids are on the stack and when there are