Let’s say we have two polygons on our mesh in Blender scene, and we want to check if their normals are facing the same direction and flip one of them if they are facing different directions. We can do this using the Blender Python API.
.blend file on Patreon
First, create a BMesh object and transfer the geometry of the currently active object into it.
If we’re in edit mode, we can do this with a single command.
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bm = bmesh.from_edit_mesh(bpy.context.object.data) |
Recalculate indices of its polygons.
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bm.faces.ensure_lookup_table() |
Let’s take two polygons and compare their normals.
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face1 = bm.faces[0] # <BMFace(0x000001A909562C50), index=0, totverts=4> face2 = bm.faces[1] # <BMFace(0x000001A909562C88), index=1, totverts=4> |
Calculate the dot product of their normal vectors.
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dot_product = face1.normal.dot(face2.normal) # -1.0 |
If the dot product of two vectors is less than zero, then they point in opposite directions. Therefore, the normals of our polygons also point in opposite directions.
To align both normals, flip the normal on the second polygon.
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if dot_product < 0: face2.normal_flip() |
Update the normals data in BMesh.
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bm.normal_update() |
Now we can return the modified geometry from the BMesh to the object in the scene. Also free the BMesh object we no longer need.
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bmesh.update_edit_mesh(bpy.context.object.data) bm.free() |
This way, we’ve checked the normal direction of two polygons of the active mesh. Since their normals point in opposite directions, we’ve flipped the normal on the second polygon, so both normals now point in the same direction.