From f66c433e6c7b9853aff517325eaea442ec599128 Mon Sep 17 00:00:00 2001
From: Yorik van Havre <yorik@uncreated.net>
Date: Thu, 21 Feb 2013 19:36:24 -0300
Subject: [PATCH] 0000959: Arcs exported to DXF with wrong direction

---
 src/Mod/Draft/DraftGeomUtils.py | 16 ++++++++++++++++
 src/Mod/Draft/importDXF.py      | 18 ++++++++++++++----
 2 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/src/Mod/Draft/DraftGeomUtils.py b/src/Mod/Draft/DraftGeomUtils.py
index 9267d61aa..b1b834cea 100755
--- a/src/Mod/Draft/DraftGeomUtils.py
+++ b/src/Mod/Draft/DraftGeomUtils.py
@@ -400,6 +400,22 @@ def mirror (point, edge):
         return refl
     else:
         return None
+        
+def isClockwise(edge):
+    """Returns True if a circle-based edge has a clockwise direction"""
+    if not isinstance(edge.Curve,Part.Circle):
+        return True
+    v1 = edge.Curve.tangent(edge.ParameterRange[0])[0]
+    if DraftVecUtils.isNull(v1):
+        return True
+    # we take an arbitrary other point on the edge that has little chances to be aligned with the first one...
+    v2 = edge.Curve.tangent(edge.ParameterRange[0]+0.01)[0]
+    n = edge.Curve.Axis
+    if DraftVecUtils.angle(v1,v2,n) < 0:
+        return False
+    if n.z < 0:
+        return False
+    return True
 
 def findClosest(basepoint,pointslist):
     '''
diff --git a/src/Mod/Draft/importDXF.py b/src/Mod/Draft/importDXF.py
index 4de4a6bb9..27544f699 100644
--- a/src/Mod/Draft/importDXF.py
+++ b/src/Mod/Draft/importDXF.py
@@ -1258,13 +1258,23 @@ def getWire(wire,nospline=False):
             #    angle = -angle
             # polyline bulge -> negative makes the arc go clockwise
             bul = math.tan(angle/4)
+            
+            # OBSOLETE because arcs can have wrong normal
             # the next bit of code is for finding the direction of the arc
             # a negative cross product means the arc is clockwise
-            tang1 = edge.Curve.tangent(edge.ParameterRange[0])
-            tang2 = edge.Curve.tangent(edge.ParameterRange[1])
-            cross1 = Vector.cross(Vector(tang1[0][0],tang1[0][1],tang1[0][2]),Vector(tang2[0][0],tang2[0][1],tang2[0][2]))
-            if cross1[2] < 0:
+            #tang1 = edge.Curve.tangent(edge.ParameterRange[0])
+            #tang2 = edge.Curve.tangent(edge.ParameterRange[1])
+            #cross1 = Vector.cross(Vector(tang1[0][0],tang1[0][1],tang1[0][2]),Vector(tang2[0][0],tang2[0][1],tang2[0][2]))
+            #if DraftVecUtils.isNull(cross1):
+                # special case, both tangents are opposite, unable to take their cross vector
+                # we try again with an arbitrary point at a third of the arc length
+                #tang2 = edge.Curve.tangent(edge.ParameterRange[0]+(edge.ParameterRange[1]-edge.ParameterRange[0]/3))
+                #cross1 = Vector.cross(Vector(tang1[0][0],tang1[0][1],tang1[0][2]),Vector(tang2[0][0],tang2[0][1],tang2[0][2]))
+            #if cross1[2] < 0:
                 # polyline bulge -> negative makes the arc go clockwise 
+                #bul = -bul
+                
+            if not DraftGeomUtils.isClockwise(edge):
                 bul = -bul
             points.append((v1.x,v1.y,v1.z,None,None,bul))
         elif (isinstance(edge.Curve,Part.BSplineCurve)) and (not nospline):