URDF gazebo material tags incorrectly translated

[barulicm]

Environment

• OS Version: Ubuntu 20.04
• Source or binary build? Binary, apt, 9.4.0-1

Description

• Expected behavior:
  Gazebo material tags in the URDF that are already valid SDF material tags should be translated unchanged.
• Actual behavior:
  The translation process breaks the material tag by attempting to convert it to a reference to a Gazebo material script.

Steps to reproduce

1. Create a URDF including a gazebo material tag. Example:

   <?xml version="1.0"?>
   <robot name="example">
   <link name="link1">
       <collision>
           <geometry>
               <box size="1 1 1"/>
           </geometry>
       </collision>
       <visual>
           <geometry>
               <box size="1 1 1"/>
           </geometry>
       </visual>
       <inertial>
           <mass value="1"/>
           <inertia ixx="1.0" ixy="0.0" ixz="0.0" iyy="1.0" iyz="0.0" izz="1.0"/>
       </inertial>
   </link>
   <gazebo reference="link1">
       <material>
           <ambient>0 0 1 1</ambient>
           <diffuse>0 0 1 1</diffuse>
           <specular>0 0 1 1</specular>
       </material>
   </gazebo>
   </robot>
   

2. Convert the URDF file to SDF

   ign sdf -p example.urdf
   

Output

The generated SDF with the now broken material tag.

<sdf version='1.7'>
  <model name='example'>
    <link name='link1'>
      <inertial>
        <pose>0 0 0 0 -0 0</pose>
        <mass>1</mass>
        <inertia>
          <ixx>1</ixx>
          <ixy>0</ixy>
          <ixz>0</ixz>
          <iyy>1</iyy>
          <iyz>0</iyz>
          <izz>1</izz>
        </inertia>
      </inertial>
      <collision name='link1_collision'>
        <pose>0 0 0 0 -0 0</pose>
        <geometry>
          <box>
            <size>1 1 1</size>
          </box>
        </geometry>
        <surface>
          <contact>
            <ode/>
          </contact>
          <friction>
            <ode/>
          </friction>
        </surface>
      </collision>
      <visual name='link1_visual'>
        <pose>0 0 0 0 -0 0</pose>
        <geometry>
          <box>
            <size>1 1 1</size>
          </box>
        </geometry>
        <material>
          <script>
            <name>ambient</name>
            <uri>file://media/materials/scripts/gazebo.material</uri>
          </script>
        </material>
      </visual>
      <gravity>1</gravity>
      <velocity_decay/>
    </link>
  </model>
</sdf>

[peci1]

There's much more ign sdf -p messes with materials when generating files for Ignition Gazebo. I'm really curious if someone will do something about it, because this way, materials are just unusable. They can't even be passed as blobs to the final SDF because <material> is one of the unfortunate tags which get touched by ign sdf which mangle them like you posted. This is a real blocker for keeping your robot description written in Xacro while using it in Ignition Gazebo - yet this approach has so many benefits that I still choose it over all the other approaches. It seems that so far you have to make friends with sed to polish the result of ign sdf -p.