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Distorted mesh after "Vertices -> Remove Doubles"
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Description

System Information
Macbook 13" late 2008 with 4GB RAM, I see this issue on Windows machine too

Blender Version
2.75a, 2.76b

Short description of error
In the attached file I have an object and a duplicate of this object. The object was created by exporting fraom CATIA to STEP, then importing the STEP to Glovius and exporting to 3DS and finally creating the BLEND file from the 3DS.
In the process the object is created with a lot of duplicate vertices (I don't know why...) but with a satisfactory geometry. Without modifying the geometry, I can apply Smooth without problems. The result is OK (object "_object")
If I apply "Vertices -> Remove Doubles" the mesh becomes distorted (see object "_object.001") which results as well in a distorted render result (see areas around holes in render result). If I render the object with Flat geometry and with duplicates removed, the result is OK - but not smooth...

Exact steps for others to reproduce the error
In attached file:

  • Select object "_object" (original object after import of 3DS)
  • best make a duplicate of object _object
  • Go into edit mode
  • Apply "Vertices -> Remove Doubles"

Details

Type
Bug

Event Timeline

Wolfgang Geithner (wgeithner) updated the task description. (Show Details)
Wolfgang Geithner (wgeithner) raised the priority of this task from to Needs Triage by Developer.
Sergey Sharybin (sergey) claimed this task.

This is just the way shading works. The inner part of holes (tube-shaped) was a separate geometry from the top cover, which made it so no normals interpolation happens inside of the hole. With removed doubles you just merged all the geometry into a single manifold, which makes it so normals are getting interpolated inside of the hole, causing artifacts for you.

Either don't merge those parts of the mesh, or use autosplit or add an edge split modifier.

Thanks for the report, but it's not a bug at all, just the way shading works.

Thanks for the report, but there is no bug here.

Issue comes from the fact that your object has 'sharp' edges, i.e. edges which vertices have different normals for each of their faces. Since this is not supported of hand by many application, your exporter at some point “split” you mesh such that you get one vertex per face, and hence expected normals.

If you can export your object directly to .obj or .fbx formats, Blender can now import those while preserving custom normals (and you should not have duplicated normals anymore either) - provided your exporter supports those custom normals.

Re-generating custom normals when un-splitting a mesh is a nice TODO tool, but definitively not a bug. ;)