python opencv, example code for image pixel access

The code is example to access every pixel in opencv python.
More detail, just see below code.
Thank you.

import cv2

def OpenCV_Access_RGBValue(inMat):

#input is supposed as color

# grab the image dimensions

h = inMat.shape[0]

w = inMat.shape[1]

# loop over the image

for y in range(0, h):

for x in range(0, w):

# threshold the pixel

b = inMat[y, x, 0]

g = inMat[y, x, 1]

r = inMat[y, x, 2]

b = 255 - b

g = 255 - g

r = 255 - r

inMat[y, x, 0] = b

inMat[y, x, 1] = g

inMat[y, x, 2] = r

#image[y, x] = 255 if image[y, x] >= 128T else 0

# return the thresholded image

return inMat

#read image

inMat = cv2.imread('iu.jpg')

#test value access

OpenCV_Access_RGBValue(inMat)

#display

cv2.namedWindow('test',0)

cv2.imwrite('output.jpg', inMat)

cv2.imshow('test',inMat)

cv2.waitKey(0)