How to attach MjSpec in a running passive viewer #2835

TDC-TangChaoBanLi · 2025-09-05T04:05:18Z

TDC-TangChaoBanLi
Sep 5, 2025

Intro

Hi!

I am a student, I use MuJoCo for my research on multi object assembly.

My setup

mujoco-python-3.3.5 WSL-ubuntu22.04

My question

I hope to add objects with physical collision during the simulation and view them in real-time with a passive viewer. Therefore, I used the attach function to add them and recompiled. The model after recompilation is correct, but it is not updated properly in the viewer. I would like to know how to update the model in the viewer or if there are any other methods to achieve the above functionality?

Minimal model and/or code that explain my question

In the following example, I used a sphere to replace my mesh. When the example runs and the terminal displays "add a ball," there is no change in the viewer.When the second viewer opens, the models of both balls are loaded correctly.

Code:

import mujoco as mj
import mujoco.viewer

import time

ball = """
<mujoco>

  <default>
  <default class="robot">

    <default class="visual">
    <geom material="visual_material" contype="0" conaffinity="0" group="2" />
    </default>

    <default class="collision">
    <geom  material="collision_material" group="3"/>
    </default>

  </default>
  </default>

  <asset>
    <material name="visual_material" rgba="0.1 0.3 1.0 1.0" />
    <material name="collision_material" rgba="0.0 0.0 0.5 1.0" />
  </asset>


  <worldbody>
  <body name="ball" pos="0 0 0" quat="1 0 0 0" >
    <geom pos="0 0 0" quat="1.0 0.0 0.0 0.0" type="sphere" size="0.0325" class="visual" />
    <geom pos="0 0 0" quat="1.0 0.0 0.0 0.0" type="sphere" size="0.0325" class="collision" />
  </body>
  </worldbody>
</mujoco>
"""

truss = mj.MjSpec()
truss.worldbody.add_body()

frame_0 = truss.worldbody.add_frame(pos=[0,0,0], quat=[1,0,0,0])
ball_0 = mj.MjSpec.from_string(ball)
truss.attach(ball_0, frame=frame_0, prefix='ball0-')

model = truss.compile()
data = mj.MjData(model)
# mj.viewer.launch(model, data)

with mj.viewer.launch_passive(model, data) as viewer:
    step = 0
    while viewer.is_running() and step < 5000:
        mj.mj_step(model, data)

        if step == 1000:
            with viewer.lock():
                frame_1 = truss.worldbody.add_frame(pos=[0,0.1,0], quat=[1,0,0,0])
                ball_1 = mj.MjSpec.from_string(ball)
                truss.attach(ball_1, frame=frame_1, prefix='ball1-')
                model, data = truss.recompile(model, data)
                print("add a ball")

        step += 1
        time.sleep(0.001)
        viewer.sync()
print("second launch")
mj.viewer.launch(model, data)

Confirmations

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

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

How to attach MjSpec in a running passive viewer #2835

Uh oh!

{{title}}

Uh oh!

Uh oh!

{{editor}}'s edit

{{editor}}'s edit

Uh oh!

Replies: 0 comments

Select a reply

Uh oh!

How to attach MjSpec in a running passive viewer #2835

Uh oh!

Uh oh!

TDC-TangChaoBanLi Sep 5, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 0 comments

TDC-TangChaoBanLi
Sep 5, 2025