Add ability to specify a material assignment to a mesh asset.

This extends both the spec and the XML definition to allow specification of a material name on a mesh. This material acts as a fallback in the case that the referencing geom element does not specify a material of it's own.

PiperOrigin-RevId: 798232378
Change-Id: Iaf300c727732744ea1b613e64334ee505ac6e209

python/mujoco/introspect/structs.py

@@ -9353,6 +9353,13 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=ValueType(name='mjsPlugin'),
                  doc='sdf plugin',
              ),
+             StructFieldDecl(
+                 name='material',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjString'),
+                 ),
+                 doc='name of material',
+             ),
              StructFieldDecl(
                  name='info',
                  type=PointerType(

python/mujoco/specs_test.py

@@ -1478,6 +1478,34 @@ class SpecsTest(absltest.TestCase):
     self.assertEqual(mj_model.sensor_dim[0], 4)
     self.assertEqual(mj_model.sensor_dim[1], 1)
 
+  def test_mesh_material(self):
+    spec = mujoco.MjSpec()
+
+    spec.add_material(name='red', rgba=(1, 0, 0, 1))
+    spec.add_material(name='green', rgba=(0, 1, 0, 1))
+
+    mesh = spec.add_mesh(name='sphere')
+    mesh.make_sphere(subdivision=1)
+    mesh.material = 'red'
+
+    geom = spec.worldbody.add_geom()
+    geom.type = mujoco.mjtGeom.mjGEOM_MESH
+    geom.meshname = 'sphere'
+
+    geom_2 = spec.worldbody.add_geom()
+    geom_2.type = mujoco.mjtGeom.mjGEOM_MESH
+    geom_2.meshname = 'sphere'
+    geom_2.material = 'green'
+
+    model = spec.compile()
+
+    self.assertEqual(model.geom_matid[0], 0)
+    self.assertEqual(model.geom_matid[1], 1)
+
+    mesh.material = 'green'
+    model = spec.compile()
+
+    self.assertEqual(model.geom_matid[0], 1)
 
 if __name__ == '__main__':
   absltest.main()