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OBJ Import omits 50% of Tris
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Description

System Information
Operating system: Win10
Graphics card: GTX 1660 Ti

Blender Version
2.79 portable
friend tried version 2.83 with same result

Short description of error
OBJ import works not as expected. 50% Tris are ommited during OBJ import.

Exact steps for others to reproduce the error
Import the attached .obj with 532 Tris. But Blender only imports 266 Tris. It looks like double sided tris are ignored by Blender OBJ import.

PS:
The .obj is custom made by an own tool, but I do not see any violation to .obj specification.


simpler file:

Event Timeline

A friend found this:
https://developer.blender.org/T57965

Looks like Blender OBJ import omites Tris by Verts only and is not looking at all for different Normals. Maybe also different Texture UVs are ignored, but I did not try.

Ankit Meel (ankitm) changed the task status from Needs Triage to Needs Information from User.Tue, May 4, 9:28 AM

Could you create a smaller file, preferably 4-6 faces ?

No problem. The attached .obj has two double sided Tris. In total 4 Tris. Import results in only 2 Tris.

Ankit Meel (ankitm) changed the task status from Needs Information from User to Needs Triage.Tue, May 4, 7:29 PM

Thanks
I have not seen any code that indicates that different triangles are possible from the same set of vertices. Or that a triangle can have two different sets of normals/ UV coordinates. Such a check is not done in the exporter either.
T57965#556987 With Bastien's comment, I guess it's safe to say that it's expected behavior.
I'll merge the reports.