Why can GetPrimAtPath func get the mesh parts in visuals and collisions?

[shengyin1224]

Thanks for your wonderful work! I want to simulate the lidar sensor of Unitree G1, so I create a lidar cfg like this:

height_scanner = RayCasterCfg( prim_path="/World/envs/env_1/Robot/mid360_link", update_period=1/60, attach_yaw_only=False, max_distance=40.0, pattern_cfg=patterns.LidarPatternCfg( channels=40, vertical_fov_range=(-52, 7), # 59° 垂直 horizontal_fov_range=(0.0, 360.0), # 全向雷达 horizontal_res=0.072 # 由点频反推出 ), debug_vis=True, mesh_prim_paths=["/World/envs/env_1/Bottle"], #mesh_prim_paths=["/World/defaultGroundPlane"], )

I have already imported a bottle like this:

AssetBaseCfg(class_type=None, prim_path='{ENV_REGEX_NS}/Bottle', spawn=UsdFileCfg(func=<function spawn_from_usd at 0x739964d30e50>, visible=True, semantic_tags=None, copy_from_source=True, mass_props=None, deformable_props=None, rigid_props=RigidBodyPropertiesCfg(rigid_body_enabled=None, kinematic_enabled=None, disable_gravity=False, linear_damping=None, angular_damping=None, max_linear_velocity=None, max_angular_velocity=None, max_depenetration_velocity=1000.0, max_contact_impulse=None, enable_gyroscopic_forces=None, retain_accelerations=None, solver_position_iteration_count=None, solver_velocity_iteration_count=None, sleep_threshold=None, stabilization_threshold=None), collision_props=None, activate_contact_sensors=False, scale=None, articulation_props=ArticulationRootPropertiesCfg(articulation_enabled=None, enabled_self_collisions=None, solver_position_iteration_count=None, solver_velocity_iteration_count=None, sleep_threshold=None, stabilization_threshold=None, fix_root_link=False), fixed_tendons_props=None, joint_drive_props=None, visual_material_path='material', visual_material=None, usd_path='/home/nerv/shengyin/EmPlan-Matrix-LowLevel/assets/3678/mobility_scale_isaacsim.usd', variants=None), init_state=AssetBaseCfg.InitialStateCfg(pos=(0.8, 0.0, 0.68), rot=(1.0, 0.0, 0.0, 0.0)), collision_group=0, debug_vis=False)

But I found that the lidar can not be used to track the bottle, because the func get_all_matching_child_prims in IsaacLab/source/isaaclab/isaaclab/sim/utils.py can not find the children prims in visuals and collisions of my asset which is shown in the below image:

The bottle is normal in the GUI but in the GUI stage the children of visuals are all in very light gray and they can not be found by prim = stage.GetPrimAtPath(prim_path) in get_all_matching_child_prims. What is the core key of this problem ?

I will be very happy if you can help me!

[RandomOakForest]

Thank you for posting this. The core issue behind your RayCaster/LiDAR sensor in Isaac Lab failing to detect imported assets like your bottle is almost certainly due to the asset's mesh prims being instance proxies in the USD stage. In Omniverse/Isaac Sim, light gray (and sometimes padlock icons) in the stage tree indicate instanced or referenced prims, sometimes called "instance proxies".¹

Why this breaks RayCaster (LiDAR):

• RayCaster and ray sensors in Isaac Lab require, for performance and technical reasons, access to the actual mesh data of the objects they scan (referred to as "warp meshes").²
• In your current workflow, the imported bottle's mesh (in visuals and/or collisions) is an instance proxy. That means, from the perspective of the USD API, it's not a full, directly editable mesh—a reference is made to a mesh defined elsewhere, and it is flagged as not directly editable or traversable in certain USD/Omniverse APIs (such as the ones used by get_all_matching_child_prims).¹
• Because the RayCaster initialization walks these children to build a list of mesh prims, if they are instance proxies, the search fails (the RayCaster can't load the geometry for ray-casting).²
• This is why the children cannot be found, the mesh is not included as a warp mesh, and the LiDAR sensor yields no detections for that object.
• From the docs and code for RayCasterCfg and its initialization confirm: "The ray-caster then ray-casts against these warp meshes using the ray pattern provided... only static meshes are supported. Extending the warp mesh to support dynamic meshes is a work in progress."²
• Meshes that are "gray" and instance proxies cannot be loaded as warp meshes—they must be non-instanced, non-proxy.

Solution: Make the asset's mesh prims non-instanced

You need to "flatten" or "make unique" the referenced mesh prims before using the asset for ray-casting. Here’s what works in practice in Isaac Sim:

• In Omniverse/Isaac Sim Stage, right-click the referenced (gray) prim and use "Make Instanceable → Off" or "Flatten to" (Omniverse's version depends on version and USD tooling).
• Alternatively, if assembling assets via script or command line, you need to convert instance proxies to non-instance prims—see Isaac Lab asset import documentation.³
• After flattening, the mesh prims will appear as editable, solid black icons, and LiDAR sensors should detect the geometry as expected.

Related Links:

• Ray Caster sensor explanation²
• Isaac Lab Asset Import Guide³
• Underlying code for ray-caster child prim collection⁴

Key Takeaways

• Your imported asset’s mesh must NOT be an instanced or reference proxy USD prim.
• Use "flatten" or "make instanceable → off" for the mesh prims so that the RayCaster can see them.
• Only true mesh prims (not instance proxies) are supported for LiDAR/ray-casting sensors in Isaac Lab as of Isaac Sim 4.5 (and Isaac Lab 2025 releases).

This is a common workflow pitfall when working with imported or referenced USD assets and Omniverse sensor simulations. If you convert your bottle mesh to a non-proxy prim, your RayCaster/LiDAR will detect it as expected.¹³²

Footnotes

1. https://forums.developer.nvidia.com/t/unable-to-change-collision-mesh/306430 ↩ ↩² ↩³

2. https://isaac-sim.github.io/IsaacLab/main/source/overview/core-concepts/sensors/ray_caster.html ↩ ↩² ↩³ ↩⁴ ↩⁵

3. https://isaac-sim.github.io/IsaacLab/main/source/how-to/import_new_asset.html ↩ ↩² ↩³

4. https://isaac-sim.github.io/IsaacLab/main/source/api/lab/isaaclab.sensors.html ↩