Fix materials exported in URDF file

[traversaro]

Currently all the models exported using the creo --> simmechanics --> urdf workflow do not properly export materials.
For example if you load of today any URDF in iDynTree, you get the following warnings:

`[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material green not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material green not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material black not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material red not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material blue not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material yellow not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material pink not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material cyan not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material green not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material white not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material dblue not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material dgreen not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material gray not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material green not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material black not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material red not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material blue not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material yellow not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material pink not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material cyan not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material green not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material white not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material dblue not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material dgreen not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material gray not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material green not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material black not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, material red not found in model database.
[ERROR]  :: parseURDFMaterial : Impossible to parse URDF material, setting material to white.

[traversaro]

When we fix this issue, we can revert this commit: robotology/idyntree@5e760ab .

[traversaro]

We checked this problem with @diegoferigo and apparently this line of simmechanics-to-urdf seems related: https://github.com/robotology/simmechanics-to-urdf/blob/8c23cbcda83687dcb32d83f98a9fbb6b5f221349/simmechanics_to_urdf/firstgen.py#L989 .
It could be interesting to check upstream (on https://github.com/ros/urdfdom) if you are allowed to refer by name a material that is only defined in a link, rather than in the robot tag.

[traversaro]

I am a bit confused. By a quick check it seems that indeed this kind of "global scope for link material names" is not supported by the urdfdom reference implementation :
https://github.com/ros/urdfdom/blob/8b27e0b6fbddbb4cc670156961bd52b4d5ea632b/urdf_parser/src/model.cpp#L146 .

However, the iCub models seem to be correctly parsed by urdf_to_graphviz, so I am a bit confused.

[nunoguedelha]

ciao @traversaro , do you agree with moving this issue to the backlog for now?
cc @diegoferigo

[traversaro]

Yes, I think it already easy is in the backlog de facto.

[nunoguedelha]

ok, updated the pipeline.

[traversaro]

To be honest, I would simply suppress the warning in iDynTree until we found a way to fix the problem at the generator level.

[diegoferigo]

This is somehow related to #62 if I remember correctly. If colors are properly exported from the pipeline, the generator would not fill the model with these random colors.

[traversaro]

Another way to fix this is to make sure that the simmechanics-to-urdf converter does not emits malformed URDF files (referencing to non-existing colors) if the MaterialProp element is not present.