Inconsistency between index values ​​in SingleArticulation._articulation_view._metadata

[jmake]

I was looking for an error related to my grips. For this, I printed the joints/DOF indices using

SingleArticulation.get_dof_index and SingleArticulation._articulation_view._metadata

Both of the print correctly

{'joint_2_Revolute': 0, 'joint_3_Revolute': 1, 'joint_4_Revolute': 2, 'joint_5_Revolute': 3, 'joint_6_Revolute': 4, 'joint_7_Revolute': 5, 'joint_l_Prismatic': 6, 'joint_r_Prismatic': 7}

Further on, when printing the action returned by .

...
self.ik_solver = ArticulationKinematicsSolver(...)
...
action,success = self.ik_solver.compute_inverse_kinematics(position, orient)
...

I get

{'joint_positions': [0.06999675929546356, 0.00041565806593899787, -5.164096450496006e-07, -0.0010897454344648114, 0.0004211838709412905, 0.0004656123427189517, 0.06992676109075546, -0.00011432283922414872], 'joint_velocities': None, 'joint_efforts': None}

This showed me that the indices did not match, since in my case, I was trying to assign a value of 0.07 to my grips.

According to get_dof_index, the indices to be modified are 6 and 7; however, joint_positions shows that the modification was applied to indices 0 and 6.

After searching, I found that the above matches the order of cspace in my .yaml file (Automatically created by me).

cspace:

• joint_r_Prismatic
• joint_3_Revolute
• joint_7_Revolute
• joint_5_Revolute
• joint_4_Revolute
• joint_6_Revolute
• joint_l_Prismatic
• joint_2_Revolute

Once it is rearranged, everything is correct.

cspace:

• joint_2_Revolute
• joint_3_Revolute
• joint_4_Revolute
• joint_5_Revolute
• joint_6_Revolute
• joint_7_Revolute
• joint_l_Prismatic
• joint_r_Prismatic

The reason for the error is clear—since cspace is defined within the YAML file—but it is counterintuitive to resolve.
At the very least, there should be an additional way to verify that all indices are correct across all files.

[vick-yu]

Hi @jmake,

This is by design, but I agree it's confusing. Here's what's happening:

Why the indices differ

There are two independent joint orderings at play:

1. USD articulation order -- what get_dof_index() returns. Determined by PhysX's traversal of the USD prim hierarchy.

2. Lula cspace order -- what ArticulationKinematicsSolver.compute_inverse_kinematics() returns in joint_positions. Determined by the order of joints in your robot description YAML's cspace: list.

The ArticulationAction returned by the IK solver uses the cspace YAML order. Internally, ArticulationSubset handles the mapping between these two orderings when applying actions to the robot -- so if you call articulation.apply_action(action) through the proper API, the mapping is done for you.

The problem surfaces when you inspect the raw joint_positions array and assume it matches get_dof_index() -- it doesn't.

How to verify the mapping

You can inspect the index mapping that ArticulationSubset computes:

joints_subset = ik_solver.get_joints_subset()
print("Kinematics joint order:", joints_subset.joint_names)
print("Mapped DOF indices:", joints_subset.joint_indices)

This will show you exactly which cspace index maps to which USD DOF index.

Your feature request

Agree that a built-in validation check would help -- something that warns when the cspace YAML order doesn't match the USD articulation order. In the meantime, adding an assertion at setup time catches this early:

ik_joint_names = ik_solver.get_joints_subset().joint_names
for i, name in enumerate(ik_joint_names):
    dof_idx = robot.get_dof_index(name)
    if dof_idx != i:
        print(f"Warning: joint '{name}' is index {i} in cspace but DOF {dof_idx} in articulation")

We'll bring this up with the team as a potential improvement.

[vick-yu]

An internal ticket (6133612) has been created to track this feature request.