tetraflexagon_diagram: do not use a fill_color for the template tiles

[flexagon-toolkit.git] / src / flexagon / trihexaflexagon.py

diff --git a/src/flexagon/trihexaflexagon.py b/src/flexagon/trihexaflexagon.py
index faf4d90..7bd4f72 100755 (executable)
--- a/src/flexagon/trihexaflexagon.py
+++ b/src/flexagon/trihexaflexagon.py
@@ -44,12 +44,22 @@ class Triangle(object):
     def get_angle_in_plan_relative_to_hexagon(self):
         """"Get the angle of the triangle in the plan relative to the rotation
         of the same triangle in the hexagon."""
-        # The meaning of the formula regarding the index is the following:
+        # The explicit formula for this angle would be:
+        #
+        #    pi + pi / 6 + (((self.index + 1) % 6) // 2) * pi * 2 / 3
+        #
+        # The meaning of the part regarding the index is the following:
         #   - rotate the indices by 1
         #   - group by 2 (because couples of triangles move together in the
         #     plan)
         #   - multiply the group by a rotation factor
-        return pi + pi / 6 + (((self.index + 1) % 6) // 2) * pi * 2 / 3
+        #
+        # The explicit formula shows clearly that triangles move in groups of
+        # 2 in the plan.
+        #
+        # However, use an implicit form for robustness, so that if the other
+        # angle functions change this one can be left untouched.
+        return self.get_angle_in_hexagon() - self.get_angle_in_plan()
 
     def get_angle_in_hexagon(self):
         """Get the angle of the triangle in the hexagons.