如何只绘制某些具有某些数组值的单元格，而另一些则不使用matplotlib.pyplot绘制？

Question

I have an image that consists of float values and another one that consist only of ones and zeros. I want to plot the second image over the first one, but I only want to plot the ones from the second image. The zeros shall not be plotted.

Ì have tried the following code and I also changed the alpha of y to 1. The problem is, that either the red windows of y are changed from x (alpha of y = 0.5), or one can not even see the plots of x (alpha of y=1).

import matplotlib.pyplot as plt
import numpy as np

x = np.random.random(size=(20,20))
y = np.random.randint(2, size=(20,20))

fig = plt.figure()
plt.imshow(x, cmap="Greys", alpha = 0.5)
plt.imshow(y, cmap="Reds", alpha = 0.5)
plt.show()

How can I only plot the ones of y?

UPDATE: Thank you for your answers! But this is not want I am looking for. I will explain again:

The result should be something like: x as background and every position, where y is 1, should be colored pure red.

Answer 1

Following the approach in this answer linked by @ImportanceOfBeingEarnest, the exact solution in your case would look like below. Here, np.ma.masked_where will mask your y array at places where it is 0. The resulting array will only contain 1.

EDIT: The problem of overlaying seems to stem from the choice of cmap . If you don't specify the cmap for the y , you can clearly see below that indeed only 1's are plotted and overlaid on the top of x . In order to have a discrete color (red in your case), you can create a custom color map

import matplotlib.pyplot as plt
import numpy as np
from matplotlib import colors

x = np.random.random(size=(20,20))
y = np.random.randint(2, size=(20,20))

y_new =np.ma.masked_where(y==0, y)
cmap = colors.ListedColormap(['red'])

fig = plt.figure()
plt.imshow(x, cmap="Greys", alpha = 0.5)
plt.imshow(y_new, cmap=cmap, alpha=1)
plt.show()

Answer 2

We can inverse "The result should be [..] x as background and every position, where y is 1, should be colored pure red." , namely to just plot x , masked by y and set the background to red.

import matplotlib.pyplot as plt
import numpy as np

y = np.random.randint(2, size=(20,20))
x = np.random.random(size=(20,20))
X = np.ma.array(x, mask=y)

fig = plt.figure()

plt.imshow(X, cmap="Greys")
plt.gca().set_facecolor("red")
plt.show()

There are of course related Q&As like

Matplotlib imshow: how to apply a mask on the matrix or How can I plot NaN values as a special color with imshow in matplotlib?

and there is also an example on the matplotlib page: Image masked