Meshes vertex normals changed via Python API are not used by dupli vert instances #97018

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Arnaud Couturier · 2022-04-04T08:18:51+02:00

Arnaud Couturier commented

2022-04-04 08:18:51 +02:00

System Information
Operating system: Windows-10-10.0.19044-SP0 64 Bits
Graphics card: NVIDIA GeForce GTX 1080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 512.15

Blender Version
Broken: version: 3.1.2, branch: master, commit date: 2022-03-31 17:40, hash: cc66d1020c
Worked: 3.0.1

Short description of error
Changing the normals of a mesh via the Python API (with mesh.vertices- [x].normal) does not modify the orientation of dupli vert instances placed on each vertex of that mesh. It assumed there are no edges nor faces, only verts for instances placement.

Exact steps for others to reproduce the error

create a monkey object/mesh
scale it down 2x or 3x to better see the results later
create a plane object/mesh
delete all edges and faces in the plane mesh, only keep its verts
make the plane the parent object of monkey
enable dupli vert instances on the plane object, vertices mode, and enable "Align to vertex normal"
make sure you are in object mode, open a python console panel, and type:

m = bpy.data.meshes['Plane']
m.vertices- [x].normal = 1,0,0

Notice nothing changes in the viewport, despite vertex 0 has the new normal correctly affected. All verts point towards the local origin by default.demo.blend

This bug makes it impossible to place lots of instances objects in a scene using only vertices, I use it a lot.

**System Information** Operating system: Windows-10-10.0.19044-SP0 64 Bits Graphics card: NVIDIA GeForce GTX 1080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 512.15 **Blender Version** Broken: version: 3.1.2, branch: master, commit date: 2022-03-31 17:40, hash: `cc66d1020c` Worked: 3.0.1 **Short description of error** Changing the normals of a mesh via the Python API (with mesh.vertices- [x].normal) does not modify the orientation of dupli vert instances placed on each vertex of that mesh. It assumed there are no edges nor faces, only verts for instances placement. **Exact steps for others to reproduce the error** - create a monkey object/mesh - scale it down 2x or 3x to better see the results later - create a plane object/mesh - delete all edges and faces in the plane mesh, only keep its verts - make the plane the parent object of monkey - enable dupli vert instances on the plane object, vertices mode, and enable "Align to vertex normal" - make sure you are in object mode, open a python console panel, and type: m = bpy.data.meshes['Plane'] m.vertices- [x].normal = 1,0,0 - Notice nothing changes in the viewport, despite vertex 0 has the new normal correctly affected. All verts point towards the local origin by default.[demo.blend](https://archive.blender.org/developer/F12968327/demo.blend) This bug makes it impossible to place lots of instances objects in a scene using only vertices, I use it a lot.

Arnaud Couturier commented

2022-04-04 08:18:51 +02:00

Added subscriber: @piiichan

Pratik Borhade commented

2022-04-04 09:09:34 +02:00

Added subscriber: @PratikPB2123

Omar Emara commented

2022-04-04 10:48:41 +02:00

Added subscriber: @OmarEmaraDev

Omar Emara commented

2022-04-04 10:48:41 +02:00

Changed status from 'Needs Triage' to: 'Needs User Info'

Omar Emara commented

2022-04-04 10:48:41 +02:00

This doesn't seem to be working either on 3.0 using the blend file you provided. No matter what I assign to the normal, it is always Vector((-0.7070833444595337, -0.7070833444595337, 0.0)). Should I have done something differently?

This doesn't seem to be working either on 3.0 using the blend file you provided. No matter what I assign to the normal, it is always `Vector((-0.7070833444595337, -0.7070833444595337, 0.0))`. Should I have done something differently?

Arnaud Couturier commented

2022-04-04 11:36:20 +02:00

Here is a video that shows that for me assigning a normal does change its value, but not in the viewport.
demo.mp4

In Blender 3.0, true it is the same behavior using this method, but if you create a complete new mesh with Python, it works. Using the same demo .blend file, type in the Python console:

m = bpy.data.meshes.new("new_mesh")
m.from_pydata(vertices=[(1,0,0)], edges=- [ ], faces=[])
m.vertices- [x].normal = 0,0,1

Then swap the plane mesh with the newly created one, and the corresponding dupli vert instance will be rotated.

In the end, both ways should work.

Here is a video that shows that for me assigning a normal does change its value, but not in the viewport. [demo.mp4](https://archive.blender.org/developer/F12968888/demo.mp4) In Blender 3.0, true it is the same behavior using this method, but if you create a complete new mesh with Python, it works. Using the same demo .blend file, type in the Python console: m = bpy.data.meshes.new("new_mesh") m.from_pydata(vertices=[(1,0,0)], edges=- [ ], faces=[]) m.vertices- [x].normal = 0,0,1 Then swap the plane mesh with the newly created one, and the corresponding dupli vert instance will be rotated. In the end, both ways should work.

Omar Emara commented

2022-04-04 13:02:18 +02:00

Changed status from 'Needs User Info' to: 'Needs Developer To Reproduce'

Omar Emara commented

2022-04-04 13:02:18 +02:00

I can replicate a change in the behavior across versions. In 3.0, changes to vertex normals take effect but are overwritten later, when going to edit mode for instance. In 3.1, changes to normals are written, but do not take effect at all.
Tried BKE_mesh_vertex_normals_clear_dirty(mesh); in BKE_mesh_vertex_normals_clear_dirty(mesh);, but that didn't work. The original behavior is suspicious to me, so tagging the module for more information.

I can replicate a change in the behavior across versions. In 3.0, changes to vertex normals take effect but are overwritten later, when going to edit mode for instance. In 3.1, changes to normals are written, but do not take effect at all. Tried `BKE_mesh_vertex_normals_clear_dirty(mesh);` in `BKE_mesh_vertex_normals_clear_dirty(mesh);`, but that didn't work. The original behavior is suspicious to me, so tagging the module for more information.

Arnaud Couturier commented

2022-04-04 13:23:40 +02:00

Yes going to edit mode erases all vertex normals set via Python. This behavior goes way back to... I don't know... blender 2.49 at least. The best solution would be: custom vertex normals take effect AND going into edit mode doesn't erase them.

Evan Wilson commented

2022-04-04 15:01:20 +02:00

Added subscribers: @HooglyBoogly, @EAW

Evan Wilson commented

2022-04-04 15:01:20 +02:00

This was changed in 3.1 in the moving of normals out of MVert. See:
cfa53e0fbe

See the Python_API release notes here:
https://wiki.blender.org/wiki/Reference/Release_Notes/3.1/Python_API

In b7fe27314b, there is the note:

Defines a read-only vector type since normals should not be modified manually.

@HooglyBoogly do you have a suggested workaround?

This was changed in 3.1 in the moving of normals out of MVert. See: cfa53e0fbe See the Python_API release notes here: https://wiki.blender.org/wiki/Reference/Release_Notes/3.1/Python_API In b7fe27314b, there is the note: >Defines a read-only vector type since normals should not be modified manually. @HooglyBoogly do you have a suggested workaround?

Hans Goudey commented

2022-04-04 15:22:48 +02:00

Changed status from 'Needs Developer To Reproduce' to: 'Archived'

Hans Goudey closed this issue

2022-04-04 15:22:48 +02:00

Hans Goudey commented

2022-04-04 15:22:48 +02:00

Added subscriber: @Hans-6

Hans Goudey commented

2022-04-04 15:22:48 +02:00

Setting the normal was always problematic and should never have been possible, since many operations would overwrite them anyway. Vertex normals are derived data, not something that's meant to be editable by the user.

@Hans-6 Goudey (HooglyBoogly) do you have a suggested workaround?

I'd suggest using a vector attribute and the geometry nodes instance on points node for creating instances.

So, I don't think this is a bug, just a weird part of the Python API.

Setting the normal was always problematic and should never have been possible, since many operations would overwrite them anyway. Vertex normals are derived data, not something that's meant to be editable by the user. >@Hans-6 Goudey (HooglyBoogly) do you have a suggested workaround? I'd suggest using a vector attribute and the geometry nodes instance on points node for creating instances. So, I don't think this is a bug, just a weird part of the Python API.

Arnaud Couturier commented

2022-04-09 10:28:24 +02:00

I see, I'll try the geometry nodes alternative, thank you.

The vertex normal attribute should be made read-only in the Python API, to avoid confusion for users.

I see, I'll try the geometry nodes alternative, thank you. The vertex normal attribute should be made read-only in the Python API, to avoid confusion for users.

Hans Goudey commented

2022-04-28 19:16:45 +02:00

Thanks, good idea. I did that here: 891268aa82

Thanks, good idea. I did that here: 891268aa82

Arnaud Couturier commented

2022-04-29 10:41:36 +02:00

Great! :)

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Meshes vertex normals changed via Python API are not used by dupli vert instances #97018