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Maniphest T83565

Subdivision surface modifier causes distorted UVs
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Description

System Information
OS: Windows10PRO 64bit v2004

Blender Version
Broken version: 2.90.1, 2.91.0 (official releases); 2.92.0 (alpha)
Worked: unknown

Short description of error
subdivision surface modifier causes distorted weird output

Exact steps for others to reproduce the error

  • load this blend file:
    subdiv_test.zip109 KBDownload
  • toggle the subdivision surface modifier
  • UVs on the upper edge of the faces is distorted

Workaround (so far):
when the faces on top are removed the UVs are correct again. (however, they are separated by a "Sharp Edge")

Event Timeline

Unknown Object (User) created this task.Dec 8 2020, 11:53 PM
Robert Guetzkow (rjg) renamed this task from subdivision surface modifier causes distorted UVs to Subdivision surface modifier causes distorted UVs.Dec 9 2020, 11:16 AM
Robert Guetzkow (rjg) added projects: BF Blender, Modeling.
Eugene Du (APEC) added a subscriber: Eugene Du (APEC).EditedDec 9 2020, 6:06 PM

Works as expected with such UV map unwrapped.
Tested in other app, got exact distortion.

You have a loops at the same position at the top and at the bottom.
At the bottom you did not got distortion cos on UV it also at the same position, but the top edges is separated on UV, thats why you got distortion only at the top.

Unknown Object (User) added a comment.Dec 9 2020, 9:37 PM

sorry, I don't fully understand your answer. I noticed, that the UVs for the top edges have different locations; yes, but subdivision is per face like tesselation in DirectX 11, isn't it ?
When I use Edge->Subdivide (with all faces selected) the result is the expected one. Shouldn't the modifier provide the same output ?

Eugene Du (APEC) added a comment.EditedDec 9 2020, 9:51 PM

if simple to explain, Subdivide - just cuts faces, Subdivision - cut faces and bend/stretch (smooth) surface, tessellation it's a different.
Modifier can provide same result as Subdivide if click to "Simple" instead of "Catmull-Clark", but this method do not smooth surface

Unknown Object (User) added a comment.EditedDec 9 2020, 10:21 PM

ok, so Catmull-Clark in this case obviously fails on the UVs by design, although the surface shape succeeds. thank you.

Germano Cavalcante (mano-wii) closed this task as Archived.Dec 10 2020, 9:55 PM
Germano Cavalcante (mano-wii) added a subscriber: Germano Cavalcante (mano-wii).

I opened the file and I can confirm that this is not a bug.
The apparently distorted UV is actually the small range of faces at the top of the geometry that is magnified with a modifier.
So the result is somewhat expected.

Thanks for the report anyway.