ur_description: remove all "LightGrey" <material> tags from URDF files

[ManifoldFR]

This triggered Pinocchio's URDF parser to set geometry_object.overrideMaterial to true on all loaded GeometryObjects, although the loaded meshes have perfectly serviceable materials.

cc @nim65s

[nim65s]

Thanks !

[cmastalli]

@ManifoldFR -- have you checked that Rviz is working properly? I think Rviz might parse or not Collada files. This depends on how they have been generated.

[ManifoldFR]

I do not use RViz. But @nim65s told me he would look at the result inside other viewers before merging.

What I know is that materials for Collada files only show up correctly in the Meshcat visualizer because it is broken and an internal bug somehow compensates for it. I think the material override is not properly applied because of a bug in Meshcat's Collada loader (or at least, the one we call through the DaeMeshGeometry class we had to implement ourselves because Collada support is bugged in the released PyPI version of Meshcat). meshcat-python itself is kind of a complete mess and our visualizer stacks some weird patches on top of it.

If RViz doesn't know how to properly load materials from Collada and we have to 'hack' our URDF files to force-load a bogus material just for it, and break the workflow for viewers that aren't themselves broken, then I'm not sure what to do lol

[cmastalli]

I do not use RViz. But @nim65s told me he would look at the result inside other viewers before merging.

What I know is that materials for Collada files only show up correctly in the Meshcat visualizer because it is broken and an internal bug somehow compensates for it. I think the material override is not properly applied because of a bug in Meshcat's Collada loader (or at least, the one we call through the DaeMeshGeometry class we had to implement ourselves because Collada support is bugged in the released PyPI version of Meshcat). meshcat-python itself is kind of a complete mess and our visualizer stacks some weird patches on top of it.

Thanks for the explanation, @ManifoldFR !

If RViz doesn't know how to properly load materials from Collada and we have to 'hack' our URDF files to force-load a bogus material just for it, and break the workflow for viewers that aren't themselves broken, then I'm not sure what to do lol

I hope we do not have to worry about this. We can discuss this further if needed.