Extract specific region in the image

I have an image in which I am interested in a specific region in the image.. for example I want to extract region 5-same pixel value everywhere (and background 0). Meaning region 3 and 4 should not be present in the output image(should be 0). Here is the image looks like.

I can do it with a for loop but since the image is large it takes time.. because I have a 3D stack. Any simpler approach would be appreciated.

>Solution :

First let’s generate some image we can work on.

import numpy as np
import matplotlib.pyplot as plt 

img = np.zeros((50,50), dtype=int)
img += np.arange(-24, 26, 1)**2
img = img.T + np.arange(-24, 26, 1)**2
img += np.arange(1, 51, 1)**2

img = (img/(np.max(img)/4.5)).astype(int)

plt.imshow(img)

Now we can mask it using np.where:

bg_value = 0 # Background value
want_value = 2 # Value that we are interested in

masked = np.where(img==want_value, img, bg_value)

plt.imshow(masked)

Note that the color-scale changed between the images, the original pixel value is still in the masked array:

print(np.max(img))
# 4
print(np.max(masked))
# 2