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[MJX] accelerometer, framelinacc and frameangacc report -gravity on static and mocap bodies #3716
Description
mj_objectAcceleration in mujoco c returns zero when the object's body is welded to a dof-less body (world, static or mocap). mjx reads cacc directly, and cacc[0] is -gravity, so accelerometer, framelinacc and frameangacc report 9.81 instead of 0.
this shows up in the menagerie: google_barkour_v0 has <accelerometer site="origin"/> on a worldbody site, and mjx reports [0, 0, 9.81] where mujoco reports [0, 0, 0].
repro:
import mujoco, numpy as np, jax
from mujoco import mjx
XML = """
<mujoco>
<worldbody>
<site name="world_site" pos="0.1 0 0"/>
<body name="mocap" mocap="true" pos="0 0 1"><site name="mocap_site"/></body>
<body name="static" pos="0 1 1"><site name="static_site"/><geom size=".1"/></body>
<body name="free" pos="1 0 1"><freejoint/><geom size=".1"/><site name="free_site"/></body>
</worldbody>
<sensor>
<accelerometer site="world_site"/>
<accelerometer site="mocap_site"/>
<accelerometer site="static_site"/>
<framelinacc objtype="site" objname="mocap_site"/>
<frameangacc objtype="site" objname="static_site"/>
<accelerometer site="free_site"/>
</sensor>
</mujoco>
"""
m = mujoco.MjModel.from_xml_string(XML); d = mujoco.MjData(m)
mujoco.mj_forward(m, d)
mx = mjx.put_model(m); dx = mjx.put_data(m, d)
dx = jax.jit(mjx.forward)(mx, dx)
print("mujoco c:", d.sensordata.reshape(-1, 3))
print("mjx: ", np.asarray(dx.sensordata).reshape(-1, 3))output:
mujoco c: [[0. 0. 0.]
[0. 0. 0.]
[0. 0. 0.]
[0. 0. 0.]
[0. 0. 0.]
[0. 0. 0.]]
mjx: [[0. 0. 9.81]
[0. 0. 9.81]
[0. 0. 9.81]
[0. 0. 9.81]
[0. 0. 0. ]
[0. 0. 0. ]]
mujoco 3.16.0, mjx from main (mujoco nightly 3.16.1.dev), jax cpu.
the same bug in mujoco_warp: google-deepmind/mujoco_warp#1804, fix in google-deepmind/mujoco_warp#1805.
Activity
yuvaltassa commentedon Nov 7, 2026
I think this is no longer true at head? please check
dhairyajangir commentedon Nov 10, 2026
I reran the six-sensor model on Linux CPU with JAX 0.12.0.1. The MuJoCo and MJX 3.17.0 wheels reproduced all four discrepancies. With both wheels at 4.0.0, all six sensors agreed within
4.2e-7.I got the same result with the MJX Python source at
a937a4f916dc586660145973b015e56e04043377and the 4.0.0 native wheel. The four native readings that were zero in 3.17.0 now read +9.81 and agree with MJX. I used Codex to help run and check the comparison.I also checked the unmodified Barkour v0 MJX scene in Menagerie at
98a70c56. On the paired 3.17.0 wheels, its world-origin accelerometer reproduced the report in both the default pose and thestandingkeyframe. With the current MJX source and native 4.0.0, both read[0, 0, +9.81]in both poses. I initialized the MJX sensor output to NaN before each forward pass to verify it was recomputed.Applying just the production hunks from #3717 at
e1f96e85to current main made MJX read zero in both poses while native MuJoCo continued to read[0, 0, +9.81]. This comparison used current main with those hunks and the same release native engine.