def normal_at_vertex(a):
normal = None
for vertex in vertex_to_vertices[a]:
if vertex.face.smooth_lit: # No smooth_lit means no vertex normal
face = vertex.face
if normal is None:
normal = Vector(face.normal.parent)
i = face.vertices.index(vertex)
a = face.vertices[i - 1]
if i + 1 == len(face.vertices):
b = face.vertices[0]
else:
b = face.vertices[i + 1]
normal.add_mul_vector((vertex >> a).angle_to(vertex >> b), face.normal)
normal.normalize()
return normal
