Exported OBJ can have invalid face format

[timhutton]

This file was produced by Wings3D: knotty01.zip

Opening it in ParaView gives this error:

ERROR: In C:\bbd\7cc78367\build\superbuild\paraview\src\VTK\IO\Geometry\vtkOBJReader.cxx, line 590
vtkOBJReader (00000159E9F42C60): Error reading file near line 1352 while processing the 'f' command

The offending line is:

f 1// 6// 1343// 1296//

(the first 'f' face line)

It seems that this should be:

f 1 6 1343 1296

since this is a face without texture coordinates or normals. See:

• https://en.wikipedia.org/wiki/Wavefront_.obj_file#Face_elements
• http://www.martinreddy.net/gfx/3d/OBJ.spec

The same thing seems to have caused issues with other software previously: http://www.wings3d.com/forum/showthread.php?tid=865

Thoughts?

Context: I'm not a Wings3D users but have had this issue reported as a downstream user of VTK, which ParaView also uses.

[Micheus]

Hi @timhutton thanks for report that.

As far as I know, the exporter tries to be in accordance with the OBJ specification once it states that (in the file you referenced - as well as me in that forum's thread):

If there are only vertices and vertex normals for a face element (no
texture vertices), you would enter two slashes (//). For example, to
specify only the vertex and vertex normal reference numbers, you would
enter:

f 1//1 2//2 3//3 4//4

And - because others 3D Apps can read it without any problem - I can not think it would be wrong in that approach. (I could open it in these: ZBrush, Blender, MeshLab, InstantMesh, MS Paint 3D, 3D Converter, Assimp Viewer).

So, I tend to think that these small portion of apps that don't process those "// " as face without texture and normal information may have a different understanding of the specification.

Something that I suggest in that thread can help the users in these cases:

In the export dialog you can set Export normals/smoothing group

I just tried it with ParaView and the object was successfully loaded.

[dgud]

I would not classify this a bug in wings, IMHO that syntax should be valid.
Though I agree that it unnecessary to output them, have not looked at the code to see hard
it would be to fix though.

Also wings3d have exported OBJ files in this format since 2000 and we have only have had two importers that could not handle trailing '//'. So I would classify this as a bug in the importer lib.

[dgud]

So I discussed this with Björn and trough the years there have only been complaint on two
occasions, and if we change the behavior now we might break more applications.
So we decided to not change this until more complaints appear.