superimpose matplotlib quiver on image

Question

I'm trying to superimpose quiver gradient arrows on an image, but since the origin locations are different, they don't look right. How would I fix this?

This is an example. The image to the left are the gradients I expect, but as soon as I draw them on top of an image, they point in the wrong directions because of the change in origin location.

import matplotlib.pyplot as plt
import numpy as np

test_array = np.array([[0, 0, 0, 0, 0],
                       [0, 64, 128, 64, 0],
                       [0, 127, 255, 127, 0],
                       [0, 64, 127, 64, 0],
                       [0, 0, 0, 0, 0]]).astype("float")

dy, dx = np.gradient(test_array)
plt.imshow(test_array)
plt.quiver(dx, dy)
plt.show()

Answer 1

If you want to have the same plot orientation that you get with plotting the arrows alone, you can change the origin of the image by adding origin='lower' to the imshow call:

plt.imshow(test_array, origin='lower')

If you want to keep the image origin in the upper left corner, you could just change the direction of the arrows when you call plt.quiver:

dy, dx = np.gradient(test_array)
plt.imshow(test_array)
plt.quiver(dx, -dy)
plt.show()