Python Opencv - Cannot change pixel value of a picture

Question

Need to change the white pixels to black and black pixels to white of the picture given below

    import cv2

    img=cv2.imread("cvlogo.png")

A basic opencv logo with white background and resized the picture to a fixed known size

    img=cv2.resize(img, (300,300))#(width,height)


    row,col=0,0
    i=0

Now checking each pixel by its row and column positions with for loop

If pixel is white, then change it to black or if pixel is black,change it to white.

    for row in range(0,300,1):
        print(row)
        for col in range(0,300,1):
            print(col)
            if img[row,col] is [255,255,255] : #I have used == instead of 'is'..but there is no change 
                img[row,col]=[0,0,0]
            elif img[row,col] is [0,0,0]:
                img[row,col]=[255,255,255]

There is no error in execution but it is not changing the pixel values to black or white respectively. More over if statement is also not executing..Too much of confusion..

    cv2.imshow('img',img)
    cv2.waitKey(0)
    cv2.destroyAllWindows()

Answer 1

I am not very experienced, but I would do it using numpy.where(), which is faster than the loops.

import cv2
import numpy as np
import matplotlib.pyplot as plt

# Read the image
original_image=cv2.imread("cvlogo.png")
# Not necessary. Make a copy to plot later
img=np.copy(original_image)

#Isolate the areas where the color is black(every channel=0) and white (every channel=255)
black=np.where((img[:,:,0]==0) & (img[:,:,1]==0) & (img[:,:,2]==0))
white=np.where((img[:,:,0]==255) & (img[:,:,1]==255) & (img[:,:,2]==255))

#Turn black pixels to white and vice versa
img[black]=(255,255,255)
img[white]=(0,0,0)

# Plot the images
fig=plt.figure()
ax1 = fig.add_subplot(1,2,1)
ax1.imshow(original_image)
ax1.set_title('Original Image')
ax2 = fig.add_subplot(1,2,2)
ax2.imshow(img)
ax2.set_title('Modified Image')
plt.show()

Answer 2

I think this should work. :) (I used numpy just to get width and height values - you dont need this)

import cv2

img=cv2.imread("cvlogo.png")
img=cv2.resize(img, (300,300))
height, width, channels = img.shape

white = [255,255,255]
black = [0,0,0]

for x in range(0,width):
    for y in range(0,height):
        channels_xy = img[y,x]
        if all(channels_xy == white):    
            img[y,x] = black

        elif all(channels_xy == black):
            img[y,x] = white

cv2.imshow('img',img)
cv2.waitKey(0)
cv2.destroyAllWindows()