Looping over pixels in an image

Question

I have the following code which iterates over all pixels of an image and does some manipulations on two images of the same size. I would like to speed it up and to avoid iterating over the positions in a for loop:

import numpy as np
import cv2

# Two images of same size 
image_in = cv2.imread('my_image.png')
image_in2 = cv2.imread('my_image2.png')
image_new = np.ones(image_in.shape[:2], dtype="uint8") * 255

counter = 0
counter2 = 0

for i in range(image_in.shape[0]):
    for j in range(image_in.shape[1]):
        if image_in[i, j] < 255:
            counter += 1
            if image_in2[i, j] == 0:
                image_new[i, j] = 0
            else:
                image_new[i, j] = 255
                counter2 += 1

How can I improve my code?

Answer 1

I think the trick is trying to vectorise this as much as possible:

By the look of it, the code is trying to threshold at 0 and count pixels under 255.

We can change the first part of the loop to:

counter = np.sum(image_in < 255) # Sums work on binary values
counter2 = np.sum(np.bitwise_and(image_in < 255, image_in2 != 0)) 

And the second to:

# This is 0 or 1 depending on whether it is == 0
image_new[:,:] = (image_in2 != 0) # image_new[i,j] = (image_in2[i,j] != 0)

# So scale the values up with a simple multiplcation
image_new = image_new*255 # image_new[i,j] = image_new[i,j]*255