How to swap blue and red channel in an image using OpenCV

Question

I am facing a little bit of problem in swapping the channels (specifically red and blue) of an image. I am using Opencv 3.0.0 and Python 2.7.12. Following is my code for swapping the channels

import cv2

img = cv2.imread("input/car1.jpg")

#The obvious approach
Cimg = cv2.cvtColor(img, cv2.COLOR_BGR2RGB)

#Manual Approach
red = img[:,:,2]
blue = img[:,:,0]

img[:,:,0] = red
img[:,:,2] = blue

cv2.imshow("frame",Cimg)
cv2.imshow("frame2", img)
cv2.waitKey(0)
cv2.destroyAllWindows()

I am unable to figure out why the same image undergoing through the same(probably) operation is giving two different outputs. Can someone throw some light on what's going wrong?

Original Image

Manual Operation

COLOR_BGR2RGB

Answer 1

red and blue are just views of your image. When you do img[:,:,0] = red this changes img but also blue which is just a view (basically just a reference to the sub-array img[:,:,0]) not a copy, so you loose the original blue channel values. Basically what you assume is a temp copy just is not. Add .copy() and it will work.

img = np.arange(27).reshape((3,3,3))

red = img[:,:,2].copy()
blue = img[:,:,0].copy()

img[:,:,0] = red
img[:,:,2] = blue

print("with copy:\n", img)

img = np.arange(27).reshape((3,3,3))

red = img[:,:,2]
blue = img[:,:,0]

img[:,:,0] = red
img[:,:,2] = blue

print("without copy:\n",img)

results:

with copy:

 [[[ 2  1  0]
  [ 5  4  3]
  [ 8  7  6]]

 [[11 10  9]
  [14 13 12]
  [17 16 15]]

 [[20 19 18]
  [23 22 21]
  [26 25 24]]]

without copy:

 [[[ 2  1  2]
  [ 5  4  5]
  [ 8  7  8]]

 [[11 10 11]
  [14 13 14]
  [17 16 17]]

 [[20 19 20]
  [23 22 23]
  [26 25 26]]]

Note: you actually only need 1 temp copy of 1 channel. Or you could also simply do img[:,:,::-1] this will create a view again but with swapped channels, img will stay unchanged, unless you reassign it:

img = np.arange(27).reshape((3,3,3))

print(img[:,:,::-1])
print(img)
img = img[:,:,::-1]
print(img)

results:

[[[ 2  1  0]
  [ 5  4  3]
  [ 8  7  6]]

 [[11 10  9]
  [14 13 12]
  [17 16 15]]

 [[20 19 18]
  [23 22 21]
  [26 25 24]]]


[[[ 0  1  2]
  [ 3  4  5]
  [ 6  7  8]]

 [[ 9 10 11]
  [12 13 14]
  [15 16 17]]

 [[18 19 20]
  [21 22 23]
  [24 25 26]]]


[[[ 2  1  0]
  [ 5  4  3]
  [ 8  7  6]]

 [[11 10  9]
  [14 13 12]
  [17 16 15]]

 [[20 19 18]
  [23 22 21]
  [26 25 24]]]