Animated matplotlib imshow

Question

Let me first clarify that I'm not trying to generate random walk lines like in this and many other questions. I'm trying to make a random walk heat map that changes color as points are revisted, like this.

I've been able to create still-lifes like this: but I want to see the process.

I can get the figure to show up, and if I print the array at each step I can see that the walk is working. But the figure itself doesn't animate. My code:

import matplotlib as mpl
from matplotlib import pyplot as plt
from matplotlib import animation as anim
import numpy as np
import sys
import random

length = int(sys.argv[1])

fig = plt.figure()
ax = plt.axes(xlim=(0, length-1), ylim=(0, length-1))
arr = np.zeros((length, length), dtype = int)

cmap = mpl.colors.LinearSegmentedColormap.from_list('my_colormap',
                                                    ['black','green','white'],
                                                    256)
bounds=[0,0,10,10]
norm = mpl.colors.BoundaryNorm(bounds, cmap.N)

im=plt.imshow(arr, interpolation='nearest',
              cmap = cmap,
              origin='lower')

x = int(np.random.random_sample() * length)
y = int(np.random.random_sample() * length)

def walk():
    global x, y
        rand = np.random.random_sample()
        if rand < 0.25 :
            if x == length - 1:
                x = 0
            else: x = x + 1
        elif rand < 0.5 :
            if x == 0:
                x = length - 1
            else: x = x - 1
        elif rand < 0.75 :
            if y == length - 1:
                y = 0
            else: y = y + 1
        else:
            if y == 0:
                y = length - 1
            else: y = y - 1
    return

def stand(arr):
    global x,y
    arr[x][y] = arr[x][y] + 1
    return arr

def animate(i):
    arr=im.get_array()
    walk()
    #print(a)
    arr = stand(arr)
    im.set_array(arr)
    return [im]

anim = anim.FuncAnimation(fig, animate, frames=200, interval=20, blit=True)
plt.show()

Running Python 3.6, as you can see by the print.

There are so many videos with these animated grids and I can't find any answers! Somebody must know how to do it. Thanks!

Answer 1

I added animated=True and vmin=0, vmax=255, in the imshow() function below. I also changed the stand() line to arr[x][y] = arr[x][y] + 10.

#!/usr/bin/env python3
import matplotlib as mpl
from matplotlib import pyplot as plt
from matplotlib import animation as anim
import numpy as np
import sys
import random

length = int(sys.argv[1])

fig = plt.figure()
ax = plt.axes(xlim=(0, length-1), ylim=(0, length-1))
arr = np.zeros((length, length), dtype = int)

cmap = mpl.colors.LinearSegmentedColormap.from_list('my_colormap',
                                                    ['black','green','white'],
                                                    256)
bounds=[0,0,10,10]
norm = mpl.colors.BoundaryNorm(bounds, cmap.N)

im=plt.imshow(arr, interpolation='nearest',
        cmap = cmap, vmin=0, vmax=255,
              origin='lower', animated=True) # small changes here

x = int(np.random.random_sample() * length)
y = int(np.random.random_sample() * length)

def walk():
    global x, y
    rand = np.random.random_sample()
    if rand < 0.25 :
        if x == length - 1:
            x = 0
        else: x = x + 1
    elif rand < 0.5 :
        if x == 0:
            x = length - 1
        else: x = x - 1
    elif rand < 0.75 :
        if y == length - 1:
            y = 0
        else: y = y + 1
    else:
        if y == 0:
            y = length - 1
        else: y = y - 1
    return

def stand(arr):
    global x,y
    arr[x][y] = arr[x][y] + 1000
    return arr

def animate(i):
    global x,y
    arr=im.get_array()
    walk()
    #print(a)
    arr = stand(arr)
    im.set_array(arr)
    return [im]

anim = anim.FuncAnimation(fig, animate, frames=200, interval=20, blit=True)
plt.show()

And I ran it with length = 50 and I get an animation. See it here. So you may have to play around with your color choices a bit.