mujoco Tutorial code bug report

[KindChuankai]

Intro

Hi!

I am a graduate student at university of Leeds, I recently try to learn how to use mujoco.

My setup

All is up to date and normal.

What's happening? What did you expect?

In the section of 'Analysis of contact forces' and in the line of code,
ax[0,0].legend(iter(lines), ('normal z', 'friction x', 'friction y'));
the iter() function should not be used because ‘lines’ is already an iterable, and the len() method cannot be applied to it. It reports error like "TypeError: object of type 'list_iterator' has no len()".

Semicolon is also redundant.

The correct code is
ax[0,0].legend(lines, ('normal z', 'friction x', 'friction y'))

Steps for reproduction

1. Load the model below and run the code below.

2. Report error like:
   TypeError: object of type 'list_iterator' has no len()

Minimal model for reproduction

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

minimal XML

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

Code required for reproduction

import matplotlib.pyplot as plt

import mujoco
import numpy as np
import cv2
import time

def show_video(frames, fps=30, window_size=(3840, 2160)):
delay = int(1000 / fps)

cv2.namedWindow("Video", cv2.WINDOW_NORMAL)
cv2.resizeWindow("Video", window_size[0], window_size[1])

for frame in frames:
frame = cv2.cvtColor(frame, cv2.COLOR_RGB2BGR)
frame = cv2.resize(frame, window_size)
cv2.imshow("Video", frame)
if cv2.waitKey(delay) & 0xFF == ord('q'):
break

cv2.destroyAllWindows()

free_body_MJCF = """

"""

model = mujoco.MjModel.from_xml_string(free_body_MJCF)
data = mujoco.MjData(model)

n_steps = 499

allocate

sim_time = np.zeros(n_steps)
ncon = np.zeros(n_steps)
force = np.zeros((n_steps,3))
velocity = np.zeros((n_steps, model.nv))
penetration = np.zeros(n_steps)
acceleration = np.zeros((n_steps, model.nv))
forcetorque = np.zeros(6)

random initial rotational velocity:

mujoco.mj_resetData(model, data)
data.qvel[3:6] = 2*np.random.randn(3)

simulate and save data

for i in range(n_steps):
mujoco.mj_step(model, data)
sim_time[i] = data.time
ncon[i] = data.ncon
velocity[i] = data.qvel[:]
acceleration[i] = data.qacc[:]

iterate over active contacts, save force and distance

for j,c in enumerate(data.contact):
mujoco.mj_contactForce(model, data, j, forcetorque)
force[i] += forcetorque[0:3]
penetration[i] = min(penetration[i], c.dist)

we could also do

force[i] += data.qfrc_constraint[0:3]

do you see why?

plot

_, ax = plt.subplots(3, 2, sharex=True, figsize=(10, 10))

lines = ax[0,0].plot(sim_time, force)
ax[0,0].set_title('contact force')
ax[0,0].set_ylabel('Newton')
ax[0,0].legend(iter(lines), ('normal z', 'friction x', 'friction y'));

ax[0,0].legend(list(iter(lines)), ('normal z', 'friction x', 'friction y'))

ax[1,0].plot(sim_time, acceleration)
ax[1,0].set_title('acceleration')
ax[1,0].set_ylabel('(meter,radian)/s/s')

ax[2,0].plot(sim_time, velocity)
ax[2,0].set_title('velocity')
ax[2,0].set_ylabel('(meter,radian)/s')
ax[2,0].set_xlabel('second')

ax[0,1].plot(sim_time, ncon)
ax[0,1].set_title('number of contacts')
ax[0,1].set_yticks(range(6))

ax[1,1].plot(sim_time, force[:,0])
ax[1,1].set_yscale('log')
ax[1,1].set_title('normal (z) force - log scale')
ax[1,1].set_ylabel('Newton')
z_gravity = -model.opt.gravity[2]
mg = model.body("box_and_sphere").mass[0] * z_gravity
mg_line = ax[1,1].plot(sim_time, np.ones(n_steps)mg, label='mg', linewidth=1)
ax[1,1].legend()

ax[2,1].plot(sim_time, 1000*penetration)
ax[2,1].set_title('penetration depth')
ax[2,1].set_ylabel('millimeter')
ax[2,1].set_xlabel('second')

plt.tight_layout()
plt.show()

Confirmations

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