Difference between data.cvel[1,3:] and mujoco.mj_objectVelocity(model, data, mujoco.mjtObj.mjOBJ_BODY, 1, vel_array, 0) #2602

Longyouyuan · 2025-04-28T14:20:13Z

Longyouyuan
Apr 28, 2025

Intro

Hi!

I am a graduate student at IIT, I use MuJoCo for my research on rope simulation.

My setup

Python/Windows 11/ Mujoco 3.2.2

My question

I want to get the velocity of center of mass of a body with ID 1.

The attribute data.cvel[1,3:] provides the velocity of COM in global frame.

mujoco.mj_objectVelocity(model, data, mujoco.mjtObj.mjOBJ_BODY, 1, vel_array, 0) should also give the linear velocity of COM (inertial frame based on official documentation) in global frame.

However, these two values are different. Do I misunderstand any thing?

Thanks for your help in advance!!!

Minimal model and/or code that explain my question

If you encountered the issue in a complex model, please simplify it as much as possible (while still reproducing the issue).

Model:

minimal XML

<mujoco>
  <worldbody>
    <light pos="0 0 1"/>
    <geom type="sphere" size="1" rgba="1 0 0 1"/>
  </worldbody>
</mujoco>

Code:

import mujoco
import mediapy as media
model = mujoco.MjModel.from_xml_string(xml)
data = mujoco.MjData(model)

with mujoco.Renderer(model) as renderer:
  mujoco.mj_forward(model, data)
  renderer.update_scene(data)

  media.show_image(renderer.render())

Confirmations

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

v-r-a · 2025-04-28T15:18:04Z

v-r-a
Apr 28, 2025

Hi, I am a MuJoCo user. I hope the following helps.

There does not seem to be any discrepancy. Could you run the following and confirm? This is an example of a free-falling ball.
Your minimal XML does not have a moving body.

import mujoco
import numpy as np

xml=""" 
<mujoco>
  <worldbody>
    <light pos="0 0 1"/>
    <body pos="0 0 1">
        <geom type="sphere" size="0.1" rgba="1 0 0 1"/>
        <freejoint/>
    </body>
  </worldbody>
</mujoco>
"""

m = mujoco.MjModel.from_xml_string(xml)
d = mujoco.MjData(m)
vel_array = np.zeros(6)
print(d.cvel)

d.qvel[1]=0.5

for i in range(10):
    mujoco.mj_step(m,d)
    print(d.time)
    print("cvel lin")
    print(d.cvel[1][3:6]) # body 1
    mujoco.mj_objectVelocity(m, d, mujoco.mjtObj.mjOBJ_BODY, 1, vel_array, 0)
    print("objvel lin")
    print(vel_array[3:6])
    i+=1

This checks out only because there is a single body in the model.

4 replies

Longyouyuan Apr 28, 2025
Author

Hi, v-r-a, there isn't any difference in your code, but in my code, they show different values.

Here is my code:

import mujoco
import mujoco_viewer
import numpy as np
import time
import mujoco.viewer as viewer
import time

xml="""
<mujoco model="Cable">
  <include file="scene.xml"/>

  <extension>
    <plugin plugin="mujoco.elasticity.cable"/>
  </extension>

  <statistic center="0 0 5" extent="5"/>  <!--  -->
  <visual>
    <global elevation="-30"/>
  </visual>

  <compiler autolimits="true"/>

  <size memory="200M"/>

  <worldbody>
    <composite type="cable" curve="s" count="41 1 1" size="1" offset="0 0 1.2" initial="free">
      <plugin plugin="mujoco.elasticity.cable">
        <!--Units are in Pa (SI)-->
        <config key="twist" value="0"/>
        <config key="bend" value="0"/>
        <config key="vmax" value="0.05"/>
      </plugin>
      <joint kind="main" damping=".015"/>
      <geom type="capsule" size=".005" rgba=".8 .2 .1 1" condim="1"/>
    </composite>
    <body name="slider" pos="1.0 0 1.2">
      <joint type="slide" axis="1 0 0" damping=".1"/>
      <geom size=".01"/>
    </body>
  </worldbody>

  <equality>
<!--    <connect name="right_boundary" body1="B_last" body2="slider" anchor=".025 0 0"/>-->
    <connect name="right_boundary" body1="B_last" body2="slider" anchor=".025 0 0" solref="0.001 1" solimp="0.9 0.95 0.001"/>
  </equality>

  <contact>
    <exclude body1="B_last" body2="slider"/>
  </contact>

  <sensor>
    <framelinvel objname="B_first" objtype="xbody"/>
    <framelinvel objname="B_1" objtype="xbody"/>
    <framelinvel objname="B_last" objtype="xbody"/>
  </sensor>

  <actuator>
    <motor site="S_last" gear="0 0 0 1 0 0" ctrlrange="-.03 .03"/>
  </actuator>
</mujoco>
"""

# load model
# model = mujoco.MjModel.from_xml_path('cable.xml')
model = mujoco.MjModel.from_xml_string(xml)
data = mujoco.MjData(model)

# get name of bodies
body_names = [mujoco.mj_id2name(model, mujoco.mjtObj.mjOBJ_BODY, i) for i in range(model.nbody)]

# get bodies' name of the rope
cable_body_indices = [i for i, name in enumerate(body_names) if name and name.startswith('B_')]

step = 0
with viewer.launch_passive(model, data) as viewer:
    # launch_passive means all the simulation should be done by the user

    start = time.time()
    while viewer.is_running() and time.time() - start < float('inf'):
        step_start = time.time()

        data.ctrl = 0
        mujoco.mj_step(model, data)

        # let viewer show updated info
        viewer.sync()

        # #  make sure the while loop is called every sampling period
        # computation inside the loop may take some nontrivial time.
        time_until_next_step = model.opt.timestep - (time.time() - step_start)
        if time_until_next_step > 0:
            time.sleep(time_until_next_step)

        step += 1
        if step % 100 == 0:
            aaa = 1
            mujoco.mj_forward(model, data)
            vel_array = np.zeros(6)
            mujoco.mj_objectVelocity(model, data, mujoco.mjtObj.mjOBJ_XBODY, 40, vel_array, 0)
            print("------------------")
            print(vel_array[3:])
            print(data.cvel[40, 3:])

I really appreciate it if you can help me check what is the problem!

v-r-a Apr 28, 2025

mj_objectVelocity would give the velocity of that particular object, i.e., body with ID 40, in this case. C-frame variables should give the quantities with respect to the COM of the whole subtree.

Pardon me, I should have mentioned it earlier. Values matched because there was only one body.

v-r-a Apr 28, 2025

Issue #2210 has some discussion on this, but I am not able to figure out the exact relation between d.subtree_linvel, d.cvel, and object velocity (mjOBJ_BODY)

Longyouyuan Apr 29, 2025
Author

Thank you for you replying! I may just ignore the cvel.

Difference between data.cvel[1,3:] and mujoco.mj_objectVelocity(model, data, mujoco.mjtObj.mjOBJ_BODY, 1, vel_array, 0) #2602

Uh oh!

Longyouyuan Apr 28, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 1 comment · 4 replies

Uh oh!

Uh oh!

v-r-a Apr 28, 2025

Uh oh!

Uh oh!

Longyouyuan Apr 28, 2025 Author

Uh oh!

Uh oh!

v-r-a Apr 28, 2025

Uh oh!

Uh oh!

v-r-a Apr 28, 2025

Uh oh!

Longyouyuan Apr 29, 2025 Author

Longyouyuan
Apr 28, 2025

Replies: 1 comment 4 replies

v-r-a
Apr 28, 2025

Longyouyuan Apr 28, 2025
Author

Longyouyuan Apr 29, 2025
Author