MjSpec.to_xml doesn't write inertial properties #2364

hyparxis · 2025-01-17T00:39:54Z

hyparxis
Jan 17, 2025

Intro

Hi!

I'm a researcher/SWE experimenting with model building using the MjSpec python API.

My setup

MuJoCo 3.2.7, Python API, Ubuntu 22.04

My question

I tried creating a scene using mjspec but I noticed that none of the inertial properties (what would usually be in the <inertial/> tag) get written to xml. Filing it as a question here since I wasn't sure if this is intended behavior but it seems unintuitive. I couldn't find another way of accomplishing this in docs/source code (e.g. an add_inertial function) either. I added a minimum reproducible example below.

Minimal model and/or code that explain my question

import mujoco

spec = mujoco.MjSpec()
body = spec.worldbody.add_body(mass=123.0, fullinertia=[4, 5, 6, 0, 0, 0])
body.add_joint(type=mujoco.mjtJoint.mjJNT_FREE)
spec.compile()
print(spec.to_xml())

model = mujoco.MjModel.from_xml_string(spec.to_xml())

yields

<mujoco model="MuJoCo Model">
  <compiler angle="radian"/>

  <worldbody>
    <body>
      <joint type="free" limited="false" actuatorfrclimited="false"/>
    </body>
  </worldbody>
</mujoco>

    model = mujoco.MjModel.from_xml_string(spec.to_xml())
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: Error: mass and inertia of moving bodies must be larger than mjMINVAL
Element name '', id 1, line 5

Confirmations

I searched the latest documentation thoroughly before posting.
I searched previous Issues and Discussions, I am certain this has not been raised before.

yuvaltassa · 2025-01-17T20:45:36Z

yuvaltassa
Jan 17, 2025
Maintainer

Looks like a bug.

Please open a bug (rather than a discussion).
Thanks!

0 replies

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

MjSpec.to_xml doesn't write inertial properties #2364

Uh oh!

{{title}}

Uh oh!

Uh oh!

{{editor}}'s edit

{{editor}}'s edit

Uh oh!

Replies: 1 comment

Uh oh!

{{title}}

Uh oh!

Select a reply

Uh oh!

MjSpec.to_xml doesn't write inertial properties #2364

Uh oh!

Uh oh!

hyparxis Jan 17, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 1 comment

Uh oh!

yuvaltassa Jan 17, 2025 Maintainer

hyparxis
Jan 17, 2025

yuvaltassa
Jan 17, 2025
Maintainer