How do I animate a rotating link in matplotlib plot?

I will keep the problem simple.

Let’s say I have a link of length 15 units and I want to make it animate in the matplotlib plot as the value of theta(angle between the link and x axis) varies from 0 to 90 degrees. The link should rotate about the (0,0) coordinates, i.e. the link is fixed at the (0,0) coordinates.

Obviously trigonometry rules will be applied to find the coordinates of the other end of the link while one end is fixed at (0,0) coordinates.

I just want to use purely matplotlib and numpy modules.

Answer

As you might know already, matplotlib provides built-in support for animations. The FuncAnimation class is the simplest interface for native matplotlib animations.

%matplotlib widget
from matplotlib import pyplot as plt
from matplotlib.animation import FuncAnimation, PillowWriter
import numpy as np

fig = plt.figure()
ax = plt.subplot(111, projection='polar')

class LinkAnimator:
    # Also check this example from the official documentation for this pattern:
    # https://matplotlib.org/3.3.3/gallery/animation/bayes_update.html

    def __init__(self, ax, link_size=15):
        self._ax = ax
        self._link_size = link_size
        self._link = self._ax.plot([0, 0], [0, 15], lw=1.5)[0]
        
    def __call__(self, theta):
        self._link.set_data([theta] * 2, [0, 15])
        return self._link
    
animator = LinkAnimator(ax, link_size=15)
theta = np.linspace(0, np.pi / 2, 91)
anim = FuncAnimation(fig, animator, frames=theta, blit=True, repeat=False)

# if you want to export the animation as a gif
writer = PillowWriter(fps=25)
anim.save('/tmp/link-anim.gif', writer=writer)

# this shall display your animation in the notebook
plt.show()

I have taken the liberty of using polar coordinates. If you are not familiar with this, do check this example from the official documentation.

USER GUIDE: https://matplotlib.org/3.3.3/api/animation_api.html#funcanimation

Here’s what the above code generates: