Can somebody please explain me more about .mtl file? I understand it describes how textures should be mapped back on the object, but here's what I don't understand: I have an object with materials. Materials are mainly textures connected into principled bsdf node, no any additional nodes in between. I export .obj together with .mtl, everything looks ok (at least to me). Below is a part with 1st material of .mtl file.
When the exported files are uploaded to sketchfab - I get the same model look I had in Blender (all looks identical and correct). But when I open the same .obj with Windows 3D viewer - there are some issues with roughness map (looks like viewer doesn't even see it). When this same file is opened with another viewer (someone asked me to make this model and opened it in their viewer, so I can not really tell what viewer it is), it also have issues with textures. Opacity and Base color are mapped, but again something wrong with others, roughness, normal and emission. They say that Ns parameter should be deleted from .mtl file, that Ks should be set to 0, illum set to 1... and some other recommendations.
So my question(s) is: is this normal that I have to manually change .mtl file? I would expect that there is a standard and Blender exports it in a way that every viewer should understand? Or maybe there are some settings I should change?