Peak Signal to Noise Ratio (PSNR) in Python for an Image
The steps for calculation of PSNR value of two images:

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import numpy 
import math
import cv2
original = cv2.imread("original.png")
contrast = cv2.imread("photoshopped.png",1)
def psnr(img1, img2):
    mse = numpy.mean( (img1 - img2) ** 2 )
    if mse == 0:
    return 100
    PIXEL_MAX = 255.0
    return 20 * math.log10(PIXEL_MAX / math.sqrt(mse))

d=psnr(original,contrast)
print(d)