URDF vs MJCF (G1)

[jloganolson]

Hello! I noticed the menagerie uses XML/MJCF universally but Unitree seems to prefer URDF (per the g1_rev MJCFs getting stale in their ROS repo) - I've also had a lot better luck importing the URDFs into Isaac for USD. So three questions:

1. Can you confirm that MJCF is the hero format for Playground?
2. If so, why? (sorry, I've been searching, asking LLMs, etc and still not quite getting a clear answer)
3. Are there better ways to update keep all three of these formats in sync beyond just Gemini-2.5-pro and Cursor?

Any answers/links/doc are greatly appreciated,
Logan

[btaba]

Hi @jloganolson , yes MJCF is the main format for Playground, as it is the main format for MuJoCo. The best way to keep things up-to-date is to PR to MuJoCo Menagerie. Hope that answers the question.

[kevinzakka]

Adding to @btaba, we put a lot of care into the models in Menagerie (and Playground) and our models may significantly diverge from the original source -- we strive to document all such changes in each models' respective README.

Regarding keeping things in sync, I think that's a little difficult but if you choose one source of truth (e.g. URDF), you can probably programmatically generate a corresponding XML.

[jloganolson]

Thanks for the quick responses! Re #2, I'm struggling to find a source (docs/paper/blogpost/etc) that explains why MJCF is preferred along with functional deltas between it and USD and URDF for practitioners. Is there a go-to you recommend to folks?