How to modify each element of a numpy multidimentional array conditionally?

Question

I converted an image into a gray scale numpy array using opencv:

im_g=cv2.imread("smallgray.png",0)
print(im_g)

[[187 158 104 121 143]
 [198 125 255 255 147]
 [209 134 255  97 182]]

I want to darken the image for those values that are higher than 200 for example, being 255 white and 0 black. If I do this I get the correct result:

im_g[im_g>200] = 150
print(im_g)

[[187 158 104 121 143]
 [198 125 150 150 147]
 [150 134 150  97 182]]

But my question is, if I don't want to use a constant (like 150 in the example) and instead perform some calculation on the current element, how do I refer to that element??

Thanks in advance

Answer 1

You can easily vectorize your operation using where:

im_g = np.where(im_g < 150, im_g, np.random.randint(1, 40, size=im_g.shape))