Loading Cassie model into Brax

[Icepomel0]

Which model is the issue affecting? agility_cassie

What is the issue? When I use the code:

from brax.io import mjcf
path = '~/agility_cassie/scene.xml'
sys = mjcf.load(path)

it returns such error message:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/icepomelo/.local/lib/python3.10/site-packages/brax/io/mjcf.py", line 512, in load
    mj = load_mjmodel(path)
  File "/home/icepomelo/.local/lib/python3.10/site-packages/brax/io/mjcf.py", line 506, in load_mjmodel
    mj = mujoco.MjModel.from_xml_string(xml, assets=assets)
ValueError: XML Error: Schema violation: unique element 'inertial' found 2 times

Element 'body', line 0`

Is there any additional context you can provide (e.g., a spec sheet or a URDF to show a value mismatch)? I looked up the model to find the duplicate 'inertial' define for the model but I did not find anything. How to fix such issue? Thanks in advance!

[kevinzakka]

Hi @Icepomel0, can you post this on the brax github?

[Icepomel0]

Hi Kevin,

Thanks for the reply. I think the issue is not from Brax but it is from MJX, where it does support some of the existing function of origin MuJoCo. I tried to load the model google_barkour by loading a specific defined in MJX one and it does cost any issue. Therefore I think the issue was raised from the model compatibility itself.

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---

From: Kevin Zakka @.> Sent: Wednesday, March 27, 2024 10:44:31 AM To: google-deepmind/mujoco_menagerie @.> Cc: Yiming Xie @.>; Mention @.> Subject: Re: [google-deepmind/mujoco_menagerie] Loading Cassie model into Brax (Issue #49)

Hi @Icepomel0https://github.com/Icepomel0, can you post this on the brax github?

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[btaba]

Hi @Icepomel0

The model is not compatible with MJX, feel free to make a PR for a compatible version. Here is what I tried:

import mujoco
from mujoco import mjx
m = mujoco.MjModel.from_xml_path('agility_cassie/scene.xml')
mx = mjx.put_model(m)

---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
Cell In[5], line 1
----> 1 mx = mjx.put_model(m)

File ~/mujoco/mjx/mujoco/mjx/_src/io.py:91, in put_model(m, device)
     88   raise NotImplementedError('tendons are not supported')
     90 if (m.geom_condim != 3).any() or (m.pair_dim != 3).any():
---> 91   raise NotImplementedError('only condim=3 is supported')
     93 # check collision geom types
     94 for (g1, g2, *_), c in collision_driver.collision_candidates(m).items():

NotImplementedError: only condim=3 is supported

Now regarding the brax issue, brax.io.mjcf.loads does a "fuse bodies" step before loading, which is annoying. @erikfrey

You can do this in brax instead:

import mujoco
from brax.io import mjcf
m = mujoco.MjModel.from_xml_path('agility_cassie/scene.xml')
mjcf.load_model(m)

[Icepomel0]

Hi @btaba

Thanks for the help and declearation. Is there any pointer for how to edit the model to make it compatIble to MJX?

[kevinzakka]

Hi @Icepomel0, you can take a look at the MJX page in the docs and an example of a MuJoCo model vs its MJX counterpart in the Google Barkour vB description of this repo. In particular, look at the MJX section of the README to understand what changes were necessary to make the model work with MJX.

[Icepomel0]

Hi @kevinzakka

Thanks for your help. Will see and try to make it compatible and contribute it back.