Animations using Matplotlib

Based on its plotting functionality, Matplotlib also provides an interface to generate animations using the animation module. An animation is a sequence of frames where each frame corresponds to a plot on a Figure. This tutorial covers a general guideline on how to create such animations and the different options available.

import matplotlib.pyplot as plt
import matplotlib.animation as animation
import numpy as np

Animation Classes

The animation process in Matplotlib can be thought of in 2 different ways:

• FuncAnimation: Generate data for first frame and then modify this data for each frame to create an animated plot.

• ArtistAnimation: Generate a list (iterable) of artists that will draw in each frame in the animation.

FuncAnimation is more efficient in terms of speed and memory as it draws an artist once and then modifies it. On the other hand ArtistAnimation is flexible as it allows any iterable of artists to be animated in a sequence.

FuncAnimation

The FuncAnimation class allows us to create an animation by passing a function that iteratively modifies the data of a plot. This is achieved by using the setter methods on various Artist (examples: Line2D, PathCollection, etc.). A usual FuncAnimation object takes a Figure that we want to animate and a function func that modifies the data plotted on the figure. It uses the frames parameter to determine the length of the animation. The interval parameter is used to determine time in milliseconds between drawing of two frames. Animating using FuncAnimation would usually follow the following structure:

• Plot the initial figure, including all the required artists. Save all the artists in variables so that they can be updated later on during the animation.

• Create an animation function that updates the data in each artist to generate the new frame at each function call.

• Create a FuncAnimation object with the Figure and the animation function, along with the keyword arguments that determine the animation properties.

• Use animation.Animation.save or pyplot.show to save or show the animation.

The update function uses the set_* function for different artists to modify the data. The following table shows a few plotting methods, the artist types they return and some methods that can be used to update them.

Plotting method	Artist	Set method
Axes.plot	lines.Line2D	set_data
Axes.scatter	collections.PathCollection	set_offsets
Axes.imshow	image.AxesImage	AxesImage.set_data
Axes.annotate	text.Annotation	update_positions
Axes.barh	patches.Rectangle	set_angle, set_bounds, set_height, set_width, set_x, set_y, set_xy
Axes.fill	patches.Polygon	set_xy
Axes.add_patch(patches.Ellipse)	patches.Ellipse	set_angle, set_center, set_height, set_width

Covering the set methods for all types of artists is beyond the scope of this tutorial but can be found in their respective documentations. An example of such update methods in use for Axes.scatter and Axes.plot is as follows.

fig, ax = plt.subplots()
t = np.linspace(0, 3, 40)
g = -9.81
v0 = 12
z = g * t**2 / 2 + v0 * t

v02 = 5
z2 = g * t**2 / 2 + v02 * t

scat = ax.scatter(t[0], z[0], c="b", s=5, label=f'v0 = {v0} m/s')
line2 = ax.plot(t[0], z2[0], label=f'v0 = {v02} m/s')[0]
ax.set(xlim=[0, 3], ylim=[-4, 10], xlabel='Time [s]', ylabel='Z [m]')
ax.legend()


def update(frame):
    # for each frame, update the data stored on each artist.
    x = t[:frame]
    y = z[:frame]
    # update the scatter plot:
    data = np.stack([x, y]).T
    scat.set_offsets(data)
    # update the line plot:
    line2.set_xdata(t[:frame])
    line2.set_ydata(z2[:frame])
    return (scat, line2)


ani = animation.FuncAnimation(fig=fig, func=update, frames=40, interval=30)
plt.show()

Once Loop Reflect

ArtistAnimation

ArtistAnimation can be used to generate animations if there is data stored on various different artists. This list of artists is then converted frame by frame into an animation. For example, when we use Axes.barh to plot a bar-chart, it creates a number of artists for each of the bar and error bars. To update the plot, one would need to update each of the bars from the container individually and redraw them. Instead, animation.ArtistAnimation can be used to plot each frame individually and then stitched together to form an animation. A barchart race is a simple example for this.

fig, ax = plt.subplots()
rng = np.random.default_rng(19680801)
data = np.array([20, 20, 20, 20])
x = np.array([1, 2, 3, 4])

artists = []
colors = ['tab:blue', 'tab:red', 'tab:green', 'tab:purple']
for i in range(20):
    data += rng.integers(low=0, high=10, size=data.shape)
    container = ax.barh(x, data, color=colors)
    artists.append(container)


ani = animation.ArtistAnimation(fig=fig, artists=artists, interval=400)
plt.show()

Once Loop Reflect