在opencv python中无需更改即可更改图像的像素值

Question

I tried to detect edges of an image. For that, I wrote the following code:

import numpy as np
import cv2
import math
img = cv2.imread('download.jpg',0)
img1 = img
k = img.shape
i=1
x=k[0]
y=k[1]
print x,y
while(i<(x-2)):
    j=1
    while(j<(y-2)):
        a = (int(img[i,j+1])-int(img[i,j]))
        b = (int(img[i+1,j])-int(img[i,j]))
        c = (a**2) + (b**2)
        img1[i,j] = math.sqrt(c)
        #img1[i,j] = a
        j+=1
    i+=1
i=1
print "img"
print img
print "img1"
print img1
print i,j
cv2.imshow("image1",img1)
cv2.imshow("image",img)
cv2.waitKey(0)
cv2.destroyAllWindows()

No where in the code img is modified. Yet, at the end of the code, the pixel values of img are changed(which are same as img1). Can anybody explain me what Iam missing?

Answer 1

Reason is, assignment statements in Python do not copy objects, after you did img1 = img , img and img1 still point to the same Image object, you need to make a copy if you need to mutate the objects independently

You can also simply instantiate two Image objects

img = cv2.imread('download.jpg', 0)
img1 = cv2.imread('download.jpg', 0)

Otherwise you'll need to copy the python object img , eg using https://docs.python.org/2/library/copy.html