Fix: ccw() floating point representation error.

Due to the way floating point numbers are represented, there is only
15-17 significant decimal digits of precision, so in some cases ccw
would evaluate 3 points as not collinear, when they should be.

This is fixed by rounding the area calculation to a fixed tolerance.
I have set this to 13. In the future may allow users to set this
through the API.

Resolves: #19
See also: #20

pyvisgraph/visible_vertices.py

@@ -26,7 +26,7 @@
 from pyvisgraph.graph import Point
 
 INF = 10000
-
+COLIN_TOLERANCE = 13
 
 def visible_vertices(point, graph, origin=None, destination=None, scan='full'):
     """Returns list of Points in graph visible by point.
@@ -283,7 +283,7 @@ def angle2(point_a, point_b, point_c):
 
 def ccw(A, B, C):
     """Return 1 if counter clockwise, -1 if clock wise, 0 if collinear """
-    area = (B.x - A.x) * (C.y - A.y) - (B.y - A.y) * (C.x - A.x)
+    area = round((B.x - A.x) * (C.y - A.y) - (B.y - A.y) * (C.x - A.x), COLIN_TOLERANCE)
     if area > 0: return 1
     if area < 0: return -1
     return 0

tests/test_pvg.py

@@ -26,7 +26,7 @@
 from pyvisgraph.visible_vertices import edge_intersect, point_edge_distance
 from pyvisgraph.visible_vertices import visible_vertices, angle
 from pyvisgraph.visible_vertices import intersect_point, edge_distance
-from math import pi, degrees
+from math import pi, degrees, cos, sin
 import pyvisgraph as vg
 
 '''
@@ -119,6 +119,7 @@ def test_point_edge_distance_function():
     assert point_edge_distance(point_f, point_g, edge3) == 1.4142135623730951
     assert point_edge_distance(point_h, point_g, edge3) == 0.9428090415820635
 
+
 def test_point_in_polygon():
     g = vg.VisGraph()
     point_a = Point(0,0)
@@ -128,6 +129,7 @@ def test_point_in_polygon():
     g.build([[point_a, point_b, point_c]])
     assert g.point_in_polygon(point_d) != -1
 
+
 class TestClosestPoint:
 
     def setup_method(self, method):
@@ -157,9 +159,9 @@ def test_closest_point_edge_point(self):
         p = Point(1,0.9)
         pid = g.point_in_polygon(p)
         cp = g.closest_point(p, pid, length=0.001)
-        print(cp)
         assert g.point_in_polygon(cp) == -1
 
+
 class TestCollinear:
 
     def setup_method(self, method):
@@ -199,3 +201,21 @@ def test_collin4(self):
                       [Point(2,4)]])
         visible = visible_vertices(Point(2,1), graph, None, None)
         assert visible == [Point(3,1), Point(2,2), Point(1,1)]
+
+    def test_collin5(self):
+        r = 0.2  # Radius of polygon
+        n = 4  # Sides of polygon
+        c = Point(1.0, 1.0)  # Center of polygon
+        verts = []
+        for i in range(n):
+            verts.append(Point(r * cos(2*pi * i/n - pi/4) + c.x,
+                               r * sin(2*pi * i/n - pi/4) + c.y))
+        g = vg.VisGraph()
+        g.build([verts])
+        s = Point(0, 0)
+        t = Point(1.7, 1.7)
+        shortest = g.shortest_path(s, t)
+        visible = visible_vertices(t, g.graph, s, None)
+        assert verts[3] not in visible
+        assert verts[1] not in shortest
+        assert verts[3] not in shortest