Export mtl for obj without texture doesn't work

[clemense]

Hi!

I am very convinced that this recent change https://github.com/mikedh/trimesh/commit/5abdb9fcdf0f860667980bf1fa27338d1a86a19a#diff-b0fd95d2b4cd76b3a3ba885804d392e28913e1e78423883a0881cc88054160f8 (and the commit before) created a new problem: If a mesh with material (but without texture) is exported to obj, no mtl will be created. This also relates to what I observed here: #1499

I propose the following changes to fix this:

1. Return to the original condition: if include_texture and hasattr(mesh.visual, 'uv'): in obj.py.
2. A few lines afterwards add the condition: if len(np.shape(getattr(mesh.visual, 'uv', None))) == 2: to avoid writing an empty vt field in the obj file (which was the motivation for the original commit, see above).

Let me know if an MR is needed. Thanks!

[mikedh]

I'm not totally sure I understand this issue but PR's with a test very welcome! The test matrix/correct behavior looks like this (but one of these conditions isn't true?):

• mesh without UV coordinates, but has a material: should create MTL but no VT
• mesh without UV coordinates, no material: no MTL, no VT
• mesh with UV coordinates, no material: no MTL, VT in file
• mesh with UV but with material: MTL and VT in file

[clemense]

Correct, the following condition is not true:

mesh without UV coordinates, but has a material: should create MTL but no VT

This is because https://github.com/mikedh/trimesh/blob/4671ead1432819e8da2f595c4636df429ee3acaf/trimesh/exchange/obj.py#L854 in this case is False (it used to be True before https://github.com/mikedh/trimesh/pull/1488 because mesh without UV coordinates but with material has an empty uv variable). Because of that tex_data is None, which in turn will not enter the material writer https://github.com/mikedh/trimesh/blob/4671ead1432819e8da2f595c4636df429ee3acaf/trimesh/exchange/obj.py#L907

I can write a test, basically it would look like this:

sphere = trimesh.creation.uv_sphere()
sphere.visual = trimesh.visual.TextureVisuals(uv=None, material=trimesh.visual.material.empty_material())
output = sphere.export('sphere.obj')
assert('usemtl' in output)

[mikedh]

Oh thanks, I understand now. Yeah PR's welcome!