mjSpec and mjcb_control callback

[v-r-a]

Intro

Hi!

I am a graduate student, I am using mjSpec for a legged robot's system identification task.

My setup

Currently using MuJoCo Python API.

My question

I am new to using MjSpec. I am using mjSpec to tweak the robot model in an optimization cost function. I realised that we get ValueError: Error: engine error: Python exception raised if we set the controller callback before model = spec.compile().

I wish this were illustrated in an example in the mjSpec Colab tutorial. All the examples include passive simulations. Politely tagging @quagla, since he seems to answer all the spec related queries!

Minimal model and/or code that explain my question

MRE:

import numpy as np
import mujoco as mj

def ctrl(m, d):
  print("hi")

def main():
    # Doesn't work
    mj.set_mjcb_control(ctrl)

    spec = mj.MjSpec()
    spec.worldbody.add_light(name="top", pos=[0, 0, 1])
    body = spec.worldbody.add_body(name="box_and_sphere",
                                euler=[0, 0, -30])
    body.add_joint(name="swing", type=mj.mjtJoint.mjJNT_HINGE,
                axis=[1, -1, 0], pos=[-.2, -.2, -.2])
    body.add_geom(name="red_box", type=mj.mjtGeom.mjGEOM_BOX,
                size=[.2, .2, .2], rgba=[1, 0, 0, 1])
    body.add_geom(name="green_sphere", pos=[.2, .2, .2],
                size=[.1, 0, 0], rgba=[0, 1, 0, 1])
    model = spec.compile()

    duration = 0.01    # (seconds)

    # Works fine
    # mj.set_mjcb_control(ctrl)

    data = mj.MjData(model)
    mj.mj_resetData(model, data)
    while data.time < duration:
        mj.mj_step(model, data)

if __name__ == "__main__":
    main()

Confirmations

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• I searched previous Issues and Discussions, I am certain this has not been raised before.