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

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

[yuvaltassa]

Unfortunately because callbacks are global there is no way to disable then during compilation. So the solution is indeed what you say, install the callback after compilation. We should document this. I guess here?

[v-r-a]

I guess here?

Sure.

For the sake of completeness of the post:
If someone is recompiling the spec after making changes to it in an optimisation loop or so, remove the controller callback before compiling:
mj.set_mjcb_control(None)
Edit spec
model = spec.compile()
then set the callback.