I am trying to test a Matplotlib animation example on my jupyter notebook which is listed as following:
from matplotlib import animation
# solve the ode problem of the double compound pendulum again
from scipy.integrate import odeint
from numpy import cos, sin
g = 9.82; L = 0.5; m = 0.1
def dx(x, t):
x1, x2, x3, x4 = x[0], x[1], x[2], x[3]
dx1 = 6.0/(m*L**2) * (2 * x3 - 3 * cos(x1-x2) * x4)/(16 - 9 * cos(x1-x2)**2)
dx2 = 6.0/(m*L**2) * (8 * x4 - 3 * cos(x1-x2) * x3)/(16 - 9 * cos(x1-x2)**2)
dx3 = -0.5 * m * L**2 * ( dx1 * dx2 * sin(x1-x2) + 3 * (g/L) * sin(x1))
dx4 = -0.5 * m * L**2 * (-dx1 * dx2 * sin(x1-x2) + (g/L) * sin(x2))
return [dx1, dx2, dx3, dx4]
x0 = [np.pi/2, np.pi/2, 0, 0] # initial state
t = np.linspace(0, 10, 250) # time coordinates
x = odeint(dx, x0, t) # solve the ODE problem
fig, ax = plt.subplots(figsize=(5,5))
ax.set_ylim([-1.5, 0.5])
ax.set_xlim([1, -1])
pendulum1, = ax.plot([], [], color="red", lw=2)
pendulum2, = ax.plot([], [], color="blue", lw=2)
def init():
pendulum1.set_data([], [])
pendulum2.set_data([], [])
def update(n):
# n = frame counter
# calculate the positions of the pendulums
x1 = + L * sin(x[n, 0])
y1 = - L * cos(x[n, 0])
x2 = x1 + L * sin(x[n, 1])
y2 = y1 - L * cos(x[n, 1])
# update the line data
pendulum1.set_data([0 ,x1], [0 ,y1])
pendulum2.set_data([x1,x2], [y1,y2])
anim = animation.FuncAnimation(fig, update, init_func=init, frames=len(t), blit=True)
# anim.save can be called in a few different ways, some which might or might not work
# on different platforms and with different versions of matplotlib and video encoders
#anim.save('animation.mp4', fps=20, extra_args=['-vcodec', 'libx264'], writer=animation.FFMpegWriter())
#anim.save('animation.mp4', fps=20, extra_args=['-vcodec', 'libx264'])
#anim.save('animation.mp4', fps=20, writer="ffmpeg", codec="libx264")
anim.save('animation.mp4', fps=20, writer="avconv", codec="libx264")
plt.close(fig)
However, when I run the cell in notebook it gives me this error:
RuntimeError: The init_func must return a sequence of Artist objects.
I am not quite sure what is going on.. I don't see anything wrong in the init function.
The link to this example: https://github.com/jrjohansson/scientific-python-lectures/blob/master/Lecture-4-Matplotlib.ipynb
It is at the very bottom of this notebook.
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