Animating Particle's Path with "Disappearing Tail" in Python

Question

I'm trying to animate a 2d path, and I would like it to have a sort of "Disappearing Tail", where at any given time, it shows only the last 5 (for example) particles.

What I currently have is quite far from this:

import numpy as np
import matplotlib.pyplot as plt
from matplotlib import animation, rc
from IPython.display import HTML

sample_path = np.random.uniform(size=(100,2))

fig, ax = plt.subplots()

x = np.arange(-1, 1, 0.01)        # x-array
line, = ax.plot(sample_path[0,0], sample_path[0,1])

def connect(i):
    (x0,y0) = sample_path[i-1,:]
    (x1,y1) = sample_path[i,:]
    plt.plot([x0,x1],[y0,y1],'ro-')
    return line,

def init():
    line.set_ydata(np.ma.array(x, mask=True))
    return line,

ani = animation.FuncAnimation(fig, connect, np.arange(1, 100), init_func=init,
interval=200, blit=True)
HTML(ani.to_html5_video())

This retains a 'full tail', i.e. after k steps, it shows all of the first k locations.

Is there a way to adapt what I've got so that the animation only shows the most recent history of the particle?

Answer 1

You would probably want to update the line instead of adding a lot of new points to the plot. Selecting the 5 most recent points can be done via indexing, e.g.

sample_path[i-5:i, 0]

Complete example, where we take care not to have a negative index and also don't use blit (which does not make sense if saving the animation).

import numpy as np
import matplotlib.pyplot as plt
from matplotlib import animation, rc

r = np.sin(np.linspace(0,3.14,100))
t = np.linspace(0, 10, 100)
sample_path = np.c_[r*(np.sin(t)+np.cos(t)), r*(np.cos(t)-np.sin(t))]/1.5

fig, ax = plt.subplots()

line, = ax.plot(sample_path[0,0], sample_path[0,1], "ro-")

def connect(i):
    start=max((i-5,0))
    line.set_data(sample_path[start:i,0],sample_path[start:i,1])
    return line,

ax.set_xlim(-1,1)
ax.set_ylim(-1,1)
ani = animation.FuncAnimation(fig, connect, np.arange(1, 100), interval=200)
plt.show()