fix(face): Implement final fix for Face3D normal calculation

I have used the method here: https://stackoverflow.com/questions/32274127/how-to-efficiently-determine-the-normal-to-a-polygon-in-3d-space And it seems more robust than anything else that I have tried so far. I can't believe it took me this long to find this method in my searches.
ladybug-tools · Jul 30, 2021 · e6abc10 · e6abc10
1 parent 1f6c03a
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Showing 4 changed files with 196 additions and 23 deletions.
diff --git a/ladybug_geometry/geometry3d/face.py b/ladybug_geometry/geometry3d/face.py
@@ -1915,26 +1915,26 @@ def _plane_from_vertices(verts):
             verts: The vertices to be used to extract the normal.
         """
         try:
-            # walk around the shape and get cross products and magnitudes
-            cprod1, d1 = Face3D._normal_from_3pts(verts[-2], verts[-1], verts[0])
-            cprod2, d2 = Face3D._normal_from_3pts(verts[-1], verts[0], verts[1])
-            cprods, ds = [cprod1, cprod2], [d1, d2]
+            # walk around the shape and get cross products
+            cprods, base_vert = [], verts[0]
             for i in range(len(verts) - 2):
-                cprodx, dx = Face3D._normal_from_3pts(*verts[i:i + 3])
-                cprods.append(cprodx)
-                ds.append(dx)
-            # sum together the cross products of any vertices that are not colinear
-            min_d = (sum(ds) / len(ds)) * 0.05  # rel tolerance for colinear vertices
+                verts_3 = (base_vert, verts[i + 1], verts[i + 2])
+                cprods.append(Face3D._normal_from_3pts(*verts_3))
+            # sum together the cross products
             normal = [0, 0, 0]
-            for cprodx, dx in zip(cprods, ds):
-                if dx > min_d:
-                    normal[0] += cprodx[0] / dx
-                    normal[1] += cprodx[1] / dx
-                    normal[2] += cprodx[2] / dx
+            for cprodx in cprods:
+                normal[0] += cprodx[0]
+                normal[1] += cprodx[1]
+                normal[2] += cprodx[2]
+            # normalize the vector
+            if normal != [0, 0, 0]:
+                ds = math.sqrt(normal[0] ** 2 + normal[1] ** 2 + normal[2] ** 2)
+                normal_vec = Vector3D(normal[0] / ds, normal[1] / ds, normal[2] / ds)
+            else:  # zero area Face3D; default to positive Z axis
+                normal_vec = Vector3D(0, 0, 1)
         except Exception as e:
             raise ValueError('Incorrect vertices input for Face3D:\n\t{}'.format(e))
-        return Plane(Vector3D(normal[0], normal[1], normal[2]), verts[0]) \
-            if normal != [0, 0, 0] else Plane(o=verts[0])
+        return Plane(normal_vec, verts[0])
 
     @staticmethod
     def _normal_from_3pts(pt1, pt2, pt3):
@@ -1949,12 +1949,9 @@ def _normal_from_3pts(pt1, pt2, pt3):
         v1 = (pt2.x - pt1.x, pt2.y - pt1.y, pt2.z - pt1.z)
         v2 = (pt3.x - pt1.x, pt3.y - pt1.y, pt3.z - pt1.z)
         # get the cross product of the two edge vectors
-        c_prod = (v1[1] * v2[2] - v1[2] * v2[1],
-                  -v1[0] * v2[2] + v1[2] * v2[0],
-                  v1[0] * v2[1] - v1[1] * v2[0])
-        # normalize the vector to make it a unit vector
-        d = math.sqrt(c_prod[0] ** 2 + c_prod[1] ** 2 + c_prod[2] ** 2)
-        return c_prod, d
+        return (v1[1] * v2[2] - v1[2] * v2[1],
+                -v1[0] * v2[2] + v1[2] * v2[0],
+                v1[0] * v2[1] - v1[1] * v2[0])
 
     @staticmethod
     def _corner_pt_verts(corner_pt, verts3d, verts2d):

diff --git a/tests/face3d_test.py b/tests/face3d_test.py
@@ -723,7 +723,28 @@ def test_normal_with_jagged_vertices():
 
     assert 0.99 < face_geo.normal.z < 1.01
     assert not face_geo.check_planar(0.000001, False)
-    face_geo.remove_colinear_vertices(0.01)  # correct plan ensures removal of verts
+    face_geo.remove_colinear_vertices(0.01)  # correct plane ensures removal of verts
+
+
+def test_jagged_l_face():
+    """Test that shapes with colinear vertices have a relatively close normal."""
+    geo_file = './tests/json/jagged_l.json'
+    with open(geo_file, 'r') as fp:
+        geo_dict = json.load(fp)
+    face_geo = Face3D.from_dict(geo_dict)
+
+    assert -0.99 > face_geo.normal.z > -1.01
+
+
+def test_face_with_tough_normal():
+    """Test that shapes with perfect symmetry to undermine normal calc."""
+    geo_file = './tests/json/face_with_tough_normal.json'
+    with open(geo_file, 'r') as fp:
+        geo_dict = json.load(fp)
+    face_geo = Face3D.from_dict(geo_dict)
+
+    assert 0.99 < face_geo.normal.z < 1.01
+    face_geo.remove_colinear_vertices(0.01)  # correct plane ensures removal of verts
 
 
 def test_flip():

diff --git a/tests/json/face_with_tough_normal.json b/tests/json/face_with_tough_normal.json
@@ -0,0 +1,45 @@
+{
+    "type": "Face3D", 
+    "boundary": [
+        [
+            107.25242534512725, 
+            17.866487910895195, 
+            39.250000000000021
+        ], 
+        [
+            113.3683215703075, 
+            17.866487910895195, 
+            39.250000000000028
+        ], 
+        [
+            113.3683215703075, 
+            31.759087049324272, 
+            39.25
+        ], 
+        [
+            114.2470012451289, 
+            33.880407411265018, 
+            39.25
+        ], 
+        [
+            116.36832157030744, 
+            34.759087049324265, 
+            39.25
+        ], 
+        [
+            104.25242534512725, 
+            34.758771519357481, 
+            39.250000000000057
+        ], 
+        [
+            106.37374570706803, 
+            33.880091844536032, 
+            39.250000000000057
+        ], 
+        [
+            107.25242534512725, 
+            31.75877151935746, 
+            39.250000000000057
+        ]
+    ]
+}
diff --git a/tests/json/jagged_l.json b/tests/json/jagged_l.json
@@ -0,0 +1,110 @@
+{
+    "type": "Face3D", 
+    "boundary": [
+        [
+            -437.67470345554875, 
+            448.83356213545198, 
+            0.98880353287442802
+        ], 
+        [
+            -437.67470345554881, 
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+        ], 
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+            0.98880353287477618
+        ], 
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+        ], 
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+            425.631026065789, 
+            0.98880353287477618
+        ], 
+        [
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+            425.63102606581083, 
+            0.9888035328746021
+        ], 
+        [
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+            0.98880353287477618
+        ], 
+        [
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+            0.98880353287477618
+        ], 
+        [
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+        [
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+        [
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+        [
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+        [
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+        [
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+            448.83356213545198, 
+            0.98880353287442802
+        ]
+    ]
+}