MJX: Mocap Position Control | Direct Force Control

[NadeenElh]

Intro

Hi!

I am a graduate student and I use MuJoCo for my research. Recently, I've been trying to transition to MJX for parallelization and large-scale domain randomization.

My setup

MuJoCo 3.4.0

My question

In classic MuJoCo, a mocap body can be used for position control where the mocap pos/quat assigning function generates a force that works on reducing the distance function between current mocap pos/quat and desired ones. Due to this generation of force, physics can also measure resulting contact and forces. Is this directly transferred to MJX, with MJX's limited contact behavior?

In classic MuJoCo, direct force control can work on an object by directly applying an external force to a specified point. Same reaction force physics appear because as a result. Is this also viable in MJX?

In both control systems, is "mjx.rne_postconstraint(mjx_model, mjx_data)" also required after stepping?

Thank you in advance.

Minimal model and/or code that explain my question

No response

Confirmations

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

[rueyday]

Updating mocap_pos still generates the virtual forces needed to satisfy the kinematic constraint. Despite the optimized contact pipeline, the contact forces between the mocap body and other objects are still calculated using the standard constraint solver.

You can apply forces directly via mjx_data.xfrc_applied. MJX will resolve the resulting physics and reaction forces just like classic MuJoCo.

I don't think mjx.rne_postconstraint is required because mjx.step already handles the core physics loop.

[NadeenElh]

Thank you for the clarifications!