Adds clearer initialization errors for asset classes (isaac-sim#509)

# Description

Earlier, the error message for initializing asset classes (with invalid
prims) only checked "length != 1". However, the error messages are
clearer if we separate them for the null and many cases, respectively.
This MR fixes this for better error messages.

## Type of change

- New feature (non-breaking change which adds functionality)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] I have run all the tests with `./orbit.sh --test` and they pass
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

source/extensions/omni.isaac.orbit/omni/isaac/orbit/assets/articulation/articulation.py

@@ -546,15 +546,23 @@ def _initialize_impl(self):
         if template_prim is None:
             raise RuntimeError(f"Failed to find prim for expression: '{self.cfg.prim_path}'.")
         template_prim_path = template_prim.GetPath().pathString
+
         # find articulation root prims
         root_prims = sim_utils.get_all_matching_child_prims(
             template_prim_path, predicate=lambda prim: prim.HasAPI(UsdPhysics.ArticulationRootAPI)
         )
-        if len(root_prims) != 1:
+        if len(root_prims) == 0:
+            raise RuntimeError(
+                f"Failed to find an articulation when resolving '{self.cfg.prim_path}'."
+                " Please ensure that the prim has 'USD ArticulationRootAPI' applied."
+            )
+        if len(root_prims) > 1:
             raise RuntimeError(
-                f"Failed to find a single articulation root when resolving '{self.cfg.prim_path}'."
-                f" Found roots '{root_prims}' under '{template_prim_path}'."
+                f"Failed to find a single articulation when resolving '{self.cfg.prim_path}'."
+                f" Found multiple '{root_prims}' under '{template_prim_path}'."
+                " Please ensure that there is only one articulation in the prim path tree."
             )
+
         # resolve articulation root prim back into regex expression
         root_prim_path = root_prims[0].GetPath().pathString
         root_prim_path_expr = self.cfg.prim_path + root_prim_path[len(template_prim_path) :]

source/extensions/omni.isaac.orbit/omni/isaac/orbit/assets/rigid_object/rigid_object.py

@@ -300,25 +300,35 @@ def _initialize_impl(self):
         if template_prim is None:
             raise RuntimeError(f"Failed to find prim for expression: '{self.cfg.prim_path}'.")
         template_prim_path = template_prim.GetPath().pathString
+
         # find rigid root prims
         root_prims = sim_utils.get_all_matching_child_prims(
             template_prim_path, predicate=lambda prim: prim.HasAPI(UsdPhysics.RigidBodyAPI)
         )
-        if len(root_prims) != 1:
+        if len(root_prims) == 0:
+            raise RuntimeError(
+                f"Failed to find a rigid body when resolving '{self.cfg.prim_path}'."
+                " Please ensure that the prim has 'USD RigidBodyAPI' applied."
+            )
+        if len(root_prims) > 1:
             raise RuntimeError(
                 f"Failed to find a single rigid body when resolving '{self.cfg.prim_path}'."
                 f" Found multiple '{root_prims}' under '{template_prim_path}'."
+                " Please ensure that there is only one rigid body in the prim path tree."
             )
+
         # resolve root prim back into regex expression
         root_prim_path = root_prims[0].GetPath().pathString
         root_prim_path_expr = self.cfg.prim_path + root_prim_path[len(template_prim_path) :]
         # -- object view
         self._root_physx_view = self._physics_sim_view.create_rigid_body_view(root_prim_path_expr.replace(".*", "*"))
+
         # log information about the articulation
         carb.log_info(f"Rigid body initialized at: {self.cfg.prim_path} with root '{root_prim_path_expr}'.")
         carb.log_info(f"Number of instances: {self.num_instances}")
         carb.log_info(f"Number of bodies: {self.num_bodies}")
         carb.log_info(f"Body names: {self.body_names}")
+
         # create buffers
         self._create_buffers()
         # process configuration

source/extensions/omni.isaac.orbit/test/assets/test_rigid_object.py

@@ -35,12 +35,13 @@
 from omni.isaac.orbit.utils.math import default_orientation, random_orientation
 
 
-def generate_cubes_scene(num_cubes: int = 1, height=1.0) -> tuple[RigidObject, torch.Tensor]:
+def generate_cubes_scene(num_cubes: int = 1, height=1.0, has_api: bool = True) -> tuple[RigidObject, torch.Tensor]:
     """Generate a scene with the provided number of cubes.
 
     Args:
         num_cubes: Number of cubes to generate.
         height: Height of the cubes.
+        has_api: Whether the cubes have a rigid body API on them.
 
     Returns:
         RigidObject: The rigid object representing the cubes.
@@ -52,10 +53,21 @@ def generate_cubes_scene(num_cubes: int = 1, height=1.0) -> tuple[RigidObject, t
     for i, origin in enumerate(origins):
         prim_utils.create_prim(f"/World/Table_{i}", "Xform", translation=origin)
 
+    # Resolve spawn configuration
+    if has_api:
+        spawn_cfg = sim_utils.UsdFileCfg(
+            usd_path=f"{ISAAC_NUCLEUS_DIR}/Props/Blocks/DexCube/dex_cube_instanceable.usd",
+        )
+    else:
+        # since no rigid body properties defined, this is just a static collider
+        spawn_cfg = sim_utils.CuboidCfg(
+            size=(0.1, 0.1, 0.1),
+            collision_props=sim_utils.CollisionPropertiesCfg(),
+        )
     # Create rigid object
     cube_object_cfg = RigidObjectCfg(
         prim_path="/World/Table_.*/Object",
-        spawn=sim_utils.UsdFileCfg(usd_path=f"{ISAAC_NUCLEUS_DIR}/Props/Blocks/DexCube/dex_cube_instanceable.usd"),
+        spawn=spawn_cfg,
         init_state=RigidObjectCfg.InitialStateCfg(pos=(0.0, 0.0, height)),
     )
     cube_object = RigidObject(cfg=cube_object_cfg)
@@ -99,6 +111,23 @@ def test_initialization(self):
                             # update object
                             cube_object.update(sim.cfg.dt)
 
+    def test_initialization_with_no_rigid_body(self):
+        """Test that initialization fails when no rigid body is found at the provided prim path."""
+        for num_cubes in (1, 2):
+            for device in ("cuda:0", "cpu"):
+                with self.subTest(num_cubes=num_cubes, device=device):
+                    with build_simulation_context(device=device, auto_add_lighting=True) as sim:
+                        cube_object, _ = generate_cubes_scene(num_cubes=num_cubes, has_api=False)
+
+                        # Check that boundedness of rigid object is correct
+                        self.assertEqual(ctypes.c_long.from_address(id(cube_object)).value, 1)
+
+                        # Play sim
+                        sim.reset()
+
+                        # Check if object is initialized
+                        self.assertFalse(cube_object._is_initialized)
+
     def test_external_force_on_single_body(self):
         """Test application of external force on the base of the object.