truerig 0.1.1

Robot-model conversion (URDF/MJCF/USD) you can trust — with a fidelity report of everything it changed.

TrueRig

Robot-model conversion you can trust. Convert robot descriptions between URDF ↔ MJCF ↔ USD through a lossless canonical IR — and get a fidelity report of everything that changed.

The thesis is the point: existing converters fail silently. TrueRig's product is knowing what you lost.

pip install truerig

Why

Every open-source URDF→MJCF converter today fails on real robots, silently. We reproduced this against the current urdf2mjcf (MuJoCo 3.10) on two standard models:

Defect	Robot	Existing tool behaviour
Off-diagonal inertia dropped on 32 links (up to 31.5% of the diagonal)	Unitree G1	one buried "Warning" line
Closed-chain coupling (5 mimic joints → 0 constraints)	Robotiq 2F-85	no warning at all
package:// mesh URIs passed through unresolved / doubled paths	both	hard load error
Invalid MJCF emitted (empty-name material)	Robotiq	MuJoCo refuses the file

Out of the box, neither converted model even loads in MuJoCo without manual fixing — and once you fix the plumbing, the physics is still silently wrong.

3 of the 4 defects are pure engineering diligence. The 4th (closed chains) is genuinely lossy — so TrueRig's job isn't to pretend loss doesn't happen, it's to preserve everything it can and report the rest honestly.

How it works

 URDF ─┐                          ┌─ MJCF
 MJCF ─┼─► frontend ─► IR ─► backend ─┼─ URDF      + validate(IR)  → physics checks
 USD  ─┘   (parse)   (lossless)  (emit) └─ USD      + report(loss_log) → what changed

The IR is frozen (truerig/ir.py) and is lossless by construction: frontends record all source data; backends drop only what the target can't express and log a LossEvent for each drop. See module docstrings for per-component contracts.

What works today

• URDF ↔ MJCF — full bidirectional conversion
• URDF / MJCF → USD — experimental (UsdPhysics articulation, inertia diagonalized losslessly); pip install truerig[usd]
• validate · report · simcheck — physics sanity checks, a fidelity report you can gate CI on, and a behavioural round-trip check in simulation

Field-tested on 12 diverse robots — Unitree G1/H1/A1/B1/Go2, ANYmal C, Franka Panda, KUKA iiwa14, Allegro Hand, Cassie, Baxter, Fetch, Solo — plus the G1/Robotiq/Z1 golden set. All convert and load in MuJoCo with full inertia and closed-chain coupling preserved; degenerate source inertia is repaired and reported, never silently. 51 tests, CI on Python 3.10–3.12.

Usage

truerig convert robot.urdf robot.mjcf --report text --strict   # convert + fidelity report
truerig validate robot.urdf                                    # physics sanity checks
truerig simcheck robot.urdf robot.mjcf                          # behavioural round-trip in sim

Roadmap

Embedding USD mesh geometry (currently referenced by path) and a hosted demo at truerig.xyz. Early and open (MIT) — issues and PRs welcome.