Ensure offset faces have same normal Issue #564

src/build123d/operations_generic.py

@@ -618,7 +618,10 @@ def offset(
                     inner_wires.append(offset_wire)
             except:
                 pass
-        new_faces.append(Face(outer_wire, inner_wires))
+        new_face = Face(outer_wire, inner_wires)
+        if (new_face.normal_at() - face.normal_at()).length > 0.001:
+            new_face = -new_face
+        new_faces.append(new_face)
     if edges:
         if len(edges) == 1 and edges[0].geom_type == GeomType.LINE:
             new_wires = [

tests/test_build_generic.py

@@ -626,6 +626,16 @@ def test_offset_failure(self):
             with self.assertRaises(RuntimeError):
                 offset(amount=-2, openings=topf)
 
+    def test_flipped_faces(self):
+        box = Box(10, 10, 10)
+        original_faces = box.faces()
+        offset_faces = [offset(face, amount=-3).face() for face in original_faces]
+
+        for original_face, offset_face in zip(original_faces, offset_faces):
+            self.assertTupleAlmostEquals(
+                tuple(original_face.normal_at()), tuple(offset_face.normal_at()), 3
+            )
+
 
 class PolarLocationsTests(unittest.TestCase):
     def test_errors(self):