如何正确设置OpenCV python中的二进制掩码？

Question

I have an image img. I also have a mask with value 255 at all the places where I want to retain the pixel values of img are 0 at all other places.

I want to use these two images viz. the mask and img such that I create a matrix with original img values at places where the mask is 255, and the value -1 at all places where mask is 0.

So, far, I have written this:

maskedImg = cv2.bitwise_and(img, mask)

but the maskedImg has 0 at all the places where the mask has 0. How can I get the value -1 instead of 0 at all the other places using a fast bitwise operation?

Answer 1

I don't know what is your image's dtype. Default is np.uint8, so you cann't set -1 on the result, it underflows to -1 + 256 = 255 . That is to say, if the dtype is np.uint8 , you cann't set it to negative value.

If you want to set to -1, you should change the dtype .

#masked = cv2.bitwise_and(img, mask).astype(np.int32)
masked = np.int32(img)
masked[mask==0] = -1