Don't merge UV vertices by default when they are overlapping #78414

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Daniel Bystedt · 2020-06-29T01:07:57+02:00

Daniel Bystedt commented

2020-06-29 01:07:57 +02:00

IMPORTANT: Let me do some more investigation around this. I think the issue might be the wrong workflow and not how Blender is handling UV's. Likely the correct workflow is making sure that all uv faces are separate before overlapping them (when working with trim sheets for instance)

If the following is true in the same mesh

two vertices are sharing a position in uv space
the two vertecies share the same mesh island.
the two vertecies share the same uv island

then blender merges the uv islands automatically.

This is very problematic when working with Trims and overlapped uvs as things are merged in ways that were not intended

The problem seems to be caused by blender not having a concept of UV islands, but rather each face having its own uvs that are handled as shared if they are close together in uv space.

There was a fix done in 2015 that was reverted in 2017 because it caused another bug.
https://developer.blender.org/T44320

No uv vertex merge on non connected uv islands
Merged UV’s on overlapping UV islands are not an issue when the UV islands are not connected the same way the mesh is. The square in the center Y position in the UV editor represents the mesh connection faces in the 3d viewport

Uv vertex merge on connected uv islands
Merged UV’s on overlapping UV islands is an issue when the UV islands are connected the same way the mesh is.The mesh connection faces in the 3d viewport is connected to the monkeys facial uv island in the UV editor (circled red in example)

Trim sheet example

|
Extremely simple trimsheet image for weighted normals
|

Stretched cube with weighted normals. The image to the left is used as normal map

|

Starting point. UV's on each rectangular face is overlapping |

When one of the faces are moved in UV space, the face next to it is deformed. The desired behaviour is that the deformed face should not be deformed at all

IMPORTANT: Let me do some more investigation around this. I think the issue might be the wrong workflow and not how Blender is handling UV's. Likely the correct workflow is making sure that all uv faces are separate before overlapping them (when working with trim sheets for instance) If the following is true in the same mesh - two vertices are sharing a position in uv space - the two vertecies share the same mesh island. - the two vertecies share the same uv island then blender merges the uv islands automatically. This is very problematic when working with Trims and overlapped uvs as things are merged in ways that were not intended The problem seems to be caused by blender not having a concept of UV islands, but rather each face having its own uvs that are handled as shared if they are close together in uv space. There was a fix done in 2015 that was reverted in 2017 because it caused another bug. https://developer.blender.org/T44320 **No uv vertex merge on non connected uv islands** Merged UV’s on overlapping UV islands are not an issue when the UV islands are not connected the same way the mesh is. The square in the center Y position in the UV editor represents the mesh connection faces in the 3d viewport ![image](https://archive.blender.org/developer/F8652400/no_issue_overlap.gif) **Uv vertex merge on connected uv islands** Merged UV’s on overlapping UV islands is an issue when the UV islands are connected the same way the mesh is.The mesh connection faces in the 3d viewport is connected to the monkeys facial uv island in the UV editor (circled red in example) ![image](https://archive.blender.org/developer/F8652398/uv_merge_-_one_mesh_island_HAS_issue.gif) **Trim sheet example** |![image](https://archive.blender.org/developer/F8708508/image.png) Extremely simple trimsheet image for weighted normals |![image](https://archive.blender.org/developer/F8708510/image.png) Stretched cube with weighted normals. The image to the left is used as normal map| | -- | -- | |![image](https://archive.blender.org/developer/F8708514/image.png) 1. Starting point. UV's on each rectangular face is overlapping | ![image](https://archive.blender.org/developer/F8708516/image.png) When one of the faces are moved in UV space, the face next to it is deformed. The desired behaviour is that the deformed face should not be deformed at all | -- | -- |

Daniel Bystedt commented

2020-06-29 01:07:58 +02:00

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

Daniel Bystedt commented

2020-06-29 01:07:58 +02:00

Added subscriber: @DanielBystedt

Eugene Du commented

2020-06-29 10:03:00 +02:00

Added subscriber: @APEC

Serafim Chayka commented

2021-03-12 19:40:35 +01:00

Added subscriber: @Serafim-Chayka

hufeng commented

2021-07-02 08:45:35 +02:00

Added subscriber: @hufeng

Daniel Bystedt added this to the Modeling project 2023-03-14 08:51:02 +01:00

Daniel Bystedt commented

2023-03-21 23:47:29 +01:00

At some point in time this issue seems to have been fixed. I tested both connected and non connected mesh and investigated uv overlapping described above.

Tested blender version 3.5 2023-03-18

At some point in time this issue seems to have been fixed. I tested both connected and non connected mesh and investigated uv overlapping described above. Tested blender version 3.5 2023-03-18

Daniel Bystedt closed this issue

2023-03-21 23:47:50 +01:00

Blender Bot added

and removed

labels 2023-03-21 23:47:51 +01:00

ainex commented

2023-03-22 14:39:12 +01:00

@DanielBystedt Hello, I think it's not fixed.

I aligned two islands and random vertices has been merged.

Tested in blender for windows: 3.5 2023-03-21 and 3.6 2023-03-22

@DanielBystedt Hello, I think it's not fixed. I aligned two islands and random vertices has been merged. Tested in blender for windows: 3.5 2023-03-21 and 3.6 2023-03-22 ![ezgif-2-56c65ec945.gif](/attachments/e275d49f-4c73-45a7-8b3c-4eb9dda92dd4)

ezgif-2-56c65ec945.gif

1.4 MiB

Christophe commented

2024-01-23 14:10:26 +01:00

I can confirm that in 3.6.5 it is still not fixed. Some vertices can be merged, here is on an unwrapping process with the seam the vertex should be separated

I can confirm that in 3.6.5 it is still not fixed. Some vertices can be merged, here is on an unwrapping process with the seam the vertex should be separated ![Capture_14_07_19.gif](/attachments/92b268f4-08a8-494e-89fd-dd5f55953262)

Capture_14_07_19.gif

3.7 MiB

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Don't merge UV vertices by default when they are overlapping #78414