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Multiresolution modifier: Using 'Unsubdivide' while the mesh has faces hidden in edit mode results in Blender freezing completely #126633

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opened 2024-08-22 10:18:17 +02:00 by SleepyEngi · 8 comments
SleepyEngi commented 2024-08-22 10:18:17 +02:00
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System Information
Operating system: Windows 10
Graphics card: Nvidia rtx 2060

Blender Version
Broken: 4.1, 4.2
Worked: ~
Short description of error
If you have a subdivided mesh, and select and hide faces in edit mode, attempting to un-subdivide the mesh with the multiresolution modifier results in blender freezing completely for ever.

Exact steps for others to reproduce the error
Based on the default startup or an attached .blend file (as simple as possible).
From scratch

  1. Add any object
  2. Subdivide it using either a subdivision surface or multiresolution modifier, and apply it.
  3. Go into edit mode and select some faces, and hide them.
  4. Go into object mode, add a multiresolution modifier, and click the un-subdivide button
  5. Blender will now freeze

From blender file

  1. Click the un-subdivide button on the multiresolution modifier

Extra notes
One time the bug showed up differently for me in 4.2, instead of freezing, it completed the operation, but when I looked at the model, all the hidden faces had merged into one face, making a section of the model vanish. I couldn't get this to happen again on the example file, but it might give some insight.

**System Information** Operating system: Windows 10 Graphics card: Nvidia rtx 2060 **Blender Version** Broken: 4.1, 4.2 Worked: ~ **Short description of error** If you have a subdivided mesh, and select and hide faces in edit mode, attempting to un-subdivide the mesh with the multiresolution modifier results in blender freezing completely for ever. **Exact steps for others to reproduce the error** Based on the default startup or an attached .blend file (as simple as possible). **From scratch** 1. Add any object 2. Subdivide it using either a subdivision surface or multiresolution modifier, and apply it. 3. Go into edit mode and select some faces, and hide them. 4. Go into object mode, add a multiresolution modifier, and click the un-subdivide button 5. Blender will now freeze **From blender file** 1. Click the un-subdivide button on the multiresolution modifier **Extra notes** One time the bug showed up differently for me in 4.2, instead of freezing, it completed the operation, but when I looked at the model, all the hidden faces had merged into one face, making a section of the model vanish. I couldn't get this to happen again on the example file, but it might give some insight.
MultiresUnsubdivide_Bug1.blend
189 KiB
SleepyEngi added the
Severity
Normal
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 labels 2024-08-22 10:18:21 +02:00
Alaska commented 2024-08-25 04:45:29 +02:00
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I can confirm the issue with the provided file.

Profiling this it seems like Blender gets stuck in BM_edge_share_quad_check. I will add the Modeling module to this, sorry if this is the wrong module.

It's not a crash, but the behaviour is just as bad as a crash, so I will set to high severity like the triaging module has been asked to do for crashes.

I can confirm the issue with the provided file. Profiling this it seems like Blender gets stuck in `BM_edge_share_quad_check`. I will add the Modeling module to this, sorry if this is the wrong module. It's not a crash, but the behaviour is just as bad as a crash, so I will set to high severity like the triaging module has been asked to do for crashes.
Sean Kim commented 2024-08-28 02:59:03 +02:00
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Just confirming that this behavior extends all the way back to 2.93 when this feature was introduced.

Just confirming that this behavior extends all the way back to 2.93 when this feature was introduced.
Sean Kim commented 2024-08-28 03:34:09 +02:00
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Here's a file that demonstrates the behavior mentioned in the original post in the Extra Notes section

I think the key takeaway here is that the operator is taking visibility into consideration somewhere which either causes the infinite (?) loop seen in some meshes or in the fact that hidden faces are merged oddly

Here's a file that demonstrates the behavior mentioned in the original post in the `Extra Notes` section I think the key takeaway here is that the operator is taking visibility into consideration somewhere which either causes the infinite (?) loop seen in some meshes or in the fact that hidden faces are merged oddly
minimal-unsubdiv.blend
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Sean Kim commented 2024-08-28 21:52:37 +02:00
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Should have a fix for this up for PR shortly.

Should have a fix for this up for PR shortly.
Sean Kim self-assigned this 2024-08-28 21:52:45 +02:00
Sean Kim commented 2024-08-28 23:56:33 +02:00
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Related older issue here: #99887

Related older issue here: https://projects.blender.org/blender/blender/issues/99887
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 labels 2024-08-29 06:48:38 +02:00
Pratik Borhade commented 2024-08-29 08:05:46 +02:00
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@ideasman42 hi, safe to backport the fix?

@ideasman42 hi, safe to backport the fix?
Campbell Barton commented 2024-08-29 08:17:46 +02:00
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@PratikPB2123 yes, added along with: 86f2c120f2

@PratikPB2123 yes, added along with: 86f2c120f2c9549217aa68dc6b2f75c2dcfe37f5
Sean Kim commented 2024-08-29 22:56:22 +02:00
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@ideasman42 - Do you think this should be considered for 3.6 as well, possibly 3.3 (unsure since our final LTS release for that version is only in a few weeks)?

@ideasman42 - Do you think this should be considered for 3.6 as well, possibly 3.3 (unsure since our final LTS release for that version is only in a few weeks)?
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Reference: blender/blender#126633
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