I'm wondering if it's possible to add objects in real time during runtime.

[mg638]

Intro

Hi!

I am a researche. I use MuJoCo for my research on

My setup

Mujoco : 3.3.0

API : Python, Unity

OS : Linux, Windows

My question

I'm wondering if it's possible to create objects and add them to the path while the mujoco simulation is running, or if I have to add them when I initially load the model for it to be usable.

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

[quagla]

You can modify the mjSpec and call spec.recompile() to get new mjModel and mjData, where mjData will contain the state mapped from the current state.

[mg638]

Thanks for the answer. In the MuJoco Unity documentation, it says:
"MuJoCo has functionality for dynamic scene editing, however, this is not yet supported in the Unity plugin. Therefore, adding and removing MuJoCo components causes complete scene recreation. This can be expensive for large models or if it happens frequently. We intend to lift this performance limitation to be in a future versions of the plugin."
So does this mean it is not yet available in Unity?

[Balint-H]

In python you can use mjSpec like Alessio says, but in Unity it is not yet supported, and instead will redo the entire scene compilation from scratch ( whereas recompile is more efficient)

[Balint-H]

Hello! You can add new objects in the Unity scene even during runtime, but it will cause a scene recompile which is a moderately expensive operation. If you don't do it every frame in most cases it would not be noticeable, especially if you are not running much faster than real time (i.e. not setting timescale of Unity to a high value)

[mg638]

Thank you. Then, is it possible to import a model from Unity as mjcf and save it as mjcf after adding new objects or changing the size of the model? The existing export function does not seem to work well. Is there something I need to set separately?

[Balint-H]

Yes that is expected to work, albeit the exported MJCF will be explicit/verbose, meaning it will add list all default values dorrctly in each element instead of letting the scene compilation figure it out.