Add a utility to select vertices whose normals are front-facing.

utilities/blender/select_front_facing.py

@@ -0,0 +1,38 @@
+import bpy
+from mathutils import Vector
+from math import degrees
+# bpy.ops.wm.tool_set_by_id(name="builtin.scale")
+#clear scene, make mesh
+#bpy.ops.object.mode_set(mode = 'OBJECT')
+#bpy.ops.object.select_all(action='SELECT')
+#bpy.ops.object.delete(use_global=False)
+#bpy.ops.mesh.primitive_cube_add(size=2, enter_editmode=False, align='WORLD', location=(0, 0, 0), rotation=(1.5708, 1.5708, 0))
+#obj = bpy.data.objects["Cube"]
+obj = bpy.context.object
+
+#select vertex
+obj = bpy.context.active_object
+bpy.ops.object.mode_set(mode = 'EDIT')
+bpy.ops.mesh.select_mode(type="VERT")
+#bpy.ops.mesh.select_mode(type="EDGE")
+# bpy.ops.mesh.select_mode(type="FACE")
+bpy.ops.mesh.select_all(action = 'DESELECT')
+bpy.ops.object.mode_set(mode = 'OBJECT')
+# for i in range(len(bpy.context.object.data.vertices)):
+front_normal = Vector((0, 0, 1))
+# ^ I'm not sure why this works, but it is probably
+#   from using diff_quat oddly below.
+for i in range(len(obj.data.vertices)):
+    obj.data.vertices[i].select = False
+    v = obj.data.vertices[i]  # MeshVertex
+    vn = v.normal
+    # v.select_set(False)  # deprecated
+    diff_quat = front_normal.rotation_difference(vn)
+    # ^ returns a quaternion (magnitude 1)
+    axis, rad = diff_quat.to_axis_angle()
+    deg = degrees(rad)
+    if deg > 90.0:
+        continue
+    obj.data.vertices[i].select = True
+
+bpy.ops.object.mode_set(mode = 'EDIT')