Blender incorrectly allows 2 or more faces to share all same vertices #103482

New Issue

Harry McKenzie · 2022-12-27T07:32:36+01:00

Harry McKenzie commented

2022-12-27 07:32:36 +01:00

System Information
Operating system: Windows 7, 8, 9, 10, 11
Graphics card: NVIDIA GeForce GTX TITAN

Blender Version
Broken: 3.4/3.5
Worked: Never worked

Short description of error

Blender allows for 2 or more faces to share exactly all vertices. They should have essentially merged into 1 face. But if you try to dissolve one vertex on a plane of 4 vertices, then it deletes the other face and becomes a one-faced triangle. So one of these 2 scenarios is a bug; either (1) Don't allow the 2 or more faces to co-exist or (2) Do not merge all faces into one when dissolving one vertex. More information here https://blender.stackexchange.com/questions/282322/is-having-2-faces-with-all-shared-vertices-a-bug-or-valid-geometry

Exact steps for others to reproduce the error

You can easily reproduce this scenario by regular blender operations:

1.) Create a plane and go into Edit Mode
2.) Select a triangular part by selecting 3 vertices and pressing F
3.) Select the opposite triangle part by selecting another 3 vertices on the opposite side
4.) Press A and X > Limited Dissolve
5.) Notice that you have 2 faces now.
6.) Repeat this procedure and you can have as many faces as you like all sharing exactly the same 4 vertices

Then try to dissolve one vertex with X > Dissolve Vertex. One face gets deleted.

**System Information** Operating system: Windows 7, 8, 9, 10, 11 Graphics card: NVIDIA GeForce GTX TITAN **Blender Version** Broken: 3.4/3.5 Worked: Never worked **Short description of error** Blender allows for 2 or more faces to share exactly all vertices. They should have essentially merged into 1 face. But if you try to dissolve one vertex on a plane of 4 vertices, then it deletes the other face and becomes a one-faced triangle. So one of these 2 scenarios is a bug; either (1) Don't allow the 2 or more faces to co-exist or (2) Do not merge all faces into one when dissolving one vertex. More information here https://blender.stackexchange.com/questions/282322/is-having-2-faces-with-all-shared-vertices-a-bug-or-valid-geometry **Exact steps for others to reproduce the error** You can easily reproduce this scenario by regular blender operations: 1.) Create a plane and go into Edit Mode 2.) Select a triangular part by selecting 3 vertices and pressing `F` 3.) Select the opposite triangle part by selecting another 3 vertices on the opposite side 4.) Press `A` and `X` *> Limited Dissolve* 5.) Notice that you have 2 faces now. 6.) Repeat this procedure and you can have as many faces as you like all sharing exactly the same 4 vertices ![image.png](https://archive.blender.org/developer/F14096169/image.png) ![infinite.gif](https://archive.blender.org/developer/F14096171/infinite.gif) Then try to dissolve one vertex with `X` *> Dissolve Vertex*. One face gets deleted.

Harry McKenzie commented

2022-12-27 07:32:36 +01:00

Added subscriber: @Harry-Kunz

Germano Cavalcante commented

2022-12-27 21:14:24 +01:00

Added subscriber: @mano-wii

Germano Cavalcante commented

2022-12-27 21:14:24 +01:00

Changed status from 'Needs Triage' to: 'Archived'

Germano Cavalcante closed this issue

2022-12-27 21:14:24 +01:00

Germano Cavalcante commented

2022-12-27 21:14:24 +01:00

Creating an invalid mesh with Python in Blender has always been possible.
Validating a Mesh (Mesh.validate()) is a costly operation and therefore is not called automatically at the end of each operation.

Mesh operators and modifiers (merge_by_distance, Weld, among hundreds of others) work expecting for a non-invalid mesh and the result is a non-invalid mesh.

Unless you can tell me how to create an invalid mesh through standard Blender operations. Creating an invalid mesh using this Python code is not considered a bug.

Creating an invalid mesh with Python in Blender has always been possible. Validating a Mesh (`Mesh.validate()`) is a costly operation and therefore is not called automatically at the end of each operation. Mesh operators and modifiers (`merge_by_distance`, `Weld`, among hundreds of others) work expecting for a non-invalid mesh and the result is a non-invalid mesh. Unless you can tell me how to create an invalid mesh through standard Blender operations. Creating an invalid mesh using this Python code is not considered a bug.

Harry McKenzie commented

2022-12-28 00:19:01 +01:00

Thank you for looking into this. Yes you can create this mesh without python using regular blender operations. As a matter of fact I just discovered that you can create infinite faces. I updated this task and described how you can create as many faces as you like all sharing same 4 vertices with regular blender operations. This is definitely a bug.

Harry McKenzie changed title from ~~Blender incorrectly allows 2 faces to share all same vertices~~ to Blender incorrectly allows 2 or more faces to share all same vertices

2022-12-28 00:53:59 +01:00

Germano Cavalcante commented

2022-12-28 05:18:02 +01:00

Changed status from 'Archived' to: 'Needs Triage'

Germano Cavalcante reopened this issue

2022-12-28 05:18:02 +01:00

Germano Cavalcante commented

2022-12-28 14:40:18 +01:00

Thanks for the update. I can confirm the problem.
This appears to be a variation of the same issue reported in #70977

I'm not sure if that report should continue as a Known Issue, this issue deserves more attention in my opinion.

Thanks for the update. I can confirm the problem. This appears to be a variation of the same issue reported in #70977 I'm not sure if that report should continue as a `Known Issue`, this issue deserves more attention in my opinion.

Germano Cavalcante commented

2022-12-28 14:45:15 +01:00

Closed as duplicate of #70977

Germano Cavalcante closed this issue

2022-12-28 14:45:15 +01:00

Harry McKenzie commented

2022-12-28 15:36:47 +01:00

Thanks for the update as well. Fortunately there is an easy workaround fix for this issue using Mesh > Split > Faces by Edges and then M > Merge By Distance

Thanks for the update as well. Fortunately there is an easy workaround fix for this issue using *Mesh > Split > Faces by Edges* and then `M` *> Merge By Distance*

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Blender incorrectly allows 2 or more faces to share all same vertices #103482