Matplotlib: animation saved with trace - python-3.x

Question

In the following Python code, I am trying to make an animation of a 2 rotating vectors around the origin. I am using matplotlib 3.2.1 and Python 3.8.2 on Ubuntu 20.04.
import numpy as np
import matplotlib.pyplot as plt
from matplotlib.animation import FuncAnimation
r = 2.0
def circle(phi):
return np.array([r*np.cos(phi), r*np.sin(phi)])
fig, ax = plt.subplots(figsize=(10,6))
ax.axis([-3.5*r,3.5*r,-2.5*r,2.5*r])
ax.get_xaxis().set_visible(False)
ax.get_yaxis().set_visible(False)
# set equal aspect
ax.set_aspect("equal")
point, = ax.plot(0, r, marker="o")
traj = plt.Circle((0,0), r, fill=False, color='black')
ax.add_artist(traj) # draw the circular trajectory
def update(phi):
x, y = circle(phi)
point.set_data([x], [y])
er_vec = np.array([0.5*x, 0.5*y])
eθ_vec = np.array([-0.5*y, 0.5*x])
er_arr = plt.arrow(x, y, dx=er_vec[0], dy=er_vec[1], head_width=0.1, head_length=0.2, color='gray')
eθ_arr = plt.arrow(x, y, dx=eθ_vec[0], dy=eθ_vec[1], head_width=0.1, head_length=0.2, color='grey')
annot_er = plt.text(1.7*x, 1.7*y, r'$\mathbf{e}_r$', fontsize=11)
annot_eθ = plt.text(1.1*(x-0.5*y), 1.1*(y+0.5*x), r'$\mathbf{e}_\theta$', fontsize=11)
ax.add_artist(er_arr)
ax.add_artist(eθ_arr)
ax.add_artist(annot_er)
ax.add_artist(annot_eθ)
return point, er_arr, eθ_arr, annot_er, annot_eθ
anim = FuncAnimation(fig, update, interval=10, blit=True, repeat=False, frames=np.linspace(0, 2.0*np.pi, 360, endpoint=False))
plt.show()
The code above runs smoothly and without any issues.
This is a screenshot of the animation:
However, when I try to save the animation to an mp4 video:
anim.save('anim-issue.mp4', writer='ffmpeg')
the animation in the video appears with traces which, something like this screenshot:
Could someone help me fix that issue with the video animation?
I appreciate your help.
Edit 1: According to this answer this is due to blit=True. But that doesn't solve the issue here, since the arrows have no set_position method.
Edit 2: I found another related question with the same issue I described above but I don't know how to adapt my code to make it work as expected in both cases (plt.show, anim.save).

Answer 1

Updating the position of an arrow would be quite tricky.
The easiest solution is indeed to create new arrows at each frame, but you have to make sure you remove the previous arrows first
full code:
import numpy as np
import matplotlib.pyplot as plt
from matplotlib.animation import FuncAnimation
r = 2.0
def circle(phi):
return np.array([r*np.cos(phi), r*np.sin(phi)])
fig, ax = plt.subplots(figsize=(10,6))
ax.axis([-3.5*r,3.5*r,-2.5*r,2.5*r])
ax.get_xaxis().set_visible(False)
ax.get_yaxis().set_visible(False)
# set equal aspect
ax.set_aspect("equal")
point, = ax.plot(0, r, marker="o")
traj = plt.Circle((0,0), r, fill=False, color='black')
ax.add_artist(traj) # draw the circular trajectory
er_arr = None
eθ_arr = None
annot_er = None
annot_eθ = None
def init():
global er_arr, eθ_arr, annot_er, annot_eθ
x,y = 0,0
er_vec = np.array([0.5*x, 0.5*y])
eθ_vec = np.array([-0.5*y, 0.5*x])
er_arr = plt.arrow(x, y, dx=er_vec[0], dy=er_vec[1], head_width=0.1, head_length=0.2, color='gray')
eθ_arr = plt.arrow(x, y, dx=eθ_vec[0], dy=eθ_vec[1], head_width=0.1, head_length=0.2, color='grey')
annot_er = plt.text(1.7*x, 1.7*y, r'$\mathbf{e}_r$', fontsize=11)
annot_eθ = plt.text(1.1*(x-0.5*y), 1.1*(y+0.5*x), r'$\mathbf{e}_\theta$', fontsize=11)
return er_arr, eθ_arr, annot_er, annot_eθ
def update(phi):
global er_arr, eθ_arr, annot_er, annot_eθ
x, y = circle(phi)
point.set_data([x], [y])
er_vec = np.array([0.5*x, 0.5*y])
eθ_vec = np.array([-0.5*y, 0.5*x])
#remove previous artists
er_arr.remove()
eθ_arr.remove()
er_arr = plt.arrow(x, y, dx=er_vec[0], dy=er_vec[1], head_width=0.1, head_length=0.2, color='gray')
eθ_arr = plt.arrow(x, y, dx=eθ_vec[0], dy=eθ_vec[1], head_width=0.1, head_length=0.2, color='grey')
annot_er.set_position((1.7*x, 1.7*y))
annot_eθ.set_position((1.1*(x-0.5*y), 1.1*(y+0.5*x)))
return point, er_arr, eθ_arr, annot_er, annot_eθ
anim = FuncAnimation(fig, update, init_func=init, interval=10, blit=True, repeat=False, frames=np.linspace(0, 2.0*np.pi, 360, endpoint=False))
plt.show()