added vector_angle_with sign

trimesh/geometry.py

@@ -146,6 +146,58 @@ def vector_angle(pairs):
 
     return angles
 
+def vector_angle_with_sign(mesh):
+    """
+    Find the signed angles between between adjacent face normals in a mesh.
+
+    Parameters
+    ----------
+    mesh : Trimesh
+      Source geometry to alter in-place.
+
+    Returns
+    ----------
+    angles : (n,) float
+      Angles with sign between vectors in radians
+    """
+    # Get adjacent face indices and shared edges
+    face_adjacency = mesh.face_adjacency
+
+    # Normals of adjacent triangles
+    normals = mesh.face_normals
+    pairs = normals[face_adjacency]
+    n1 = normals[face_adjacency[:, 0]]
+    n2 = normals[face_adjacency[:, 1]]
+
+    # COMs (centroids) of adjacent triangles
+    centers = mesh.triangles_center
+    c1 = centers[face_adjacency[:, 0]]
+    c2 = centers[face_adjacency[:, 1]]
+
+    pairs = np.asanyarray(pairs, dtype=np.float64)
+    if len(pairs) == 0:
+        return np.array([])
+    elif util.is_shape(pairs, (2, 3)):
+        pairs = pairs.reshape((-1, 2, 3))
+    elif not util.is_shape(pairs, (-1, 2, (2, 3))):
+        raise ValueError("pairs must be (n,2,(2|3))!")
+
+    # do the dot product between vectors
+    dots = util.diagonal_dot(pairs[:, 0], pairs[:, 1])
+    # clip for floating point error
+    dots = np.clip(dots, -1.0, 1.0)
+    # do cos and remove arbitrary sign
+    angles = np.abs(np.arccos(dots))
+
+    # Use geometric heuristic to determine sign
+    convex_vec = (c1 + n1) - (c2 + n2)
+    concave_vec = (c1 - n1) - (c2 - n2)
+
+    is_concave = np.linalg.norm(convex_vec, axis=1) < np.linalg.norm(concave_vec, axis=1)
+    angles[is_concave] *= -1  # Make angle negative for concave cases
+
+    return angles
+
 
 def triangulate_quads(quads, dtype=np.int64) -> NDArray:
     """