How I can realize Swipe animation effect between two images in Python?

Question

I have two images, and I want to animate a "swipe down" effect where each row of image 2 replaces each row of image 1. I think this involves substituting each pixel of 1st row of the 1st image by the 2nd one from top to bottom.

Here's a simple example that shows two grayscale images:

import matplotlib.pyplot as plt
import numpy as np

image2 = np.array([[200, 200, 200], [200, 200, 200], [200, 200, 200]])
image1 = np.array([[100, 100, 100], [100, 100, 100], [100, 100, 100]])

fig, ax = plt.subplots(2, 1)
ax[0].imshow(image2, cmap='gray', vmin=0, vmax=255)
ax[1].imshow(image1, cmap='gray', vmin=0, vmax=255)
plt.show()

But then I'm stuck. I'd like to animate the steps but I'm not sure how. Is it possible to do:

image2[row,column] = image1[row,column]

Answer 1

Assuming that image1 and image2 are the same size, we can stack them on top of each other with np.vstack, roll the matrix, and plot the last N rows. Here's the basic idea:

N = image2.shape[0]                 # How many rows to plot?
im = np.vstack([image2, image1])    # Stack matrices
image = ax.imshow(im[N:])           # imshow the last N rows

# Repeat:
im = np.roll(im, shift=-1, axis=0)  # -1 for up, 1 for down

Original ronin.png	Swipe Up (-1)	Swipe Down (+1)

Full example using the trick to turn whitespace off in matplotlib figures.

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

image2 = image.imread("ronin.png")
image1 = image.imread("ronin.png")

# Trick to turn borders off in matplotlib
# https://stackoverflow.com/questions/37809697/remove-white-border-when-using-subplot-and-imshow-in-python-matplotlib
fig = plt.figure()
fig.set_size_inches(image1.shape[1] / image1.shape[0], 1, forward=False)
ax = plt.Axes(fig, [0., 0., 1., 1.])
ax.set_axis_off()
fig.add_axes(ax)

N = image2.shape[0]                 # How many rows to plot?
im = np.vstack([image2, image1])    # Stack matrices
image = ax.imshow(im[N:])           # imshow the last N rows

def animate(i):
    global im
    im = np.roll(im, shift=-1, axis=0)
    image.set_array(im[N:])
    return [image]

ani = animation.FuncAnimation(fig, animate, N, repeat=False)
ani.save("my_animation.gif", fps=60)