title: Compare cropped images python opencv
body: <p>I want to compare two images to check if they are equal or not, but for that i need to compare a specific region (ROI) of both images.
  I've cropped the areas i want to compare, but now i would like to know how can i do that process, because i can't directly compare the cropped images.
  How can i for example get the average pixels values of both cropped images and compare them?</p>
  
  <p>Update: I've solved the situation.
  Current code:</p>
  
  <pre><code>import cv2
  import numpy as np
  from skimage.measure import compare_ssim as ssim
  
  
  def mse(imageA, imageB):
      # the 'Mean Squared Error' between the two images is the sum of the squared difference between the two images;
      err = np.sum((imageA.astype("float") - imageB.astype("float")) ** 2) 
      err /= float(imageA.shape[0] * imageA.shape[1])
  
  
  polarity_ok = cv2.resize(cv2.imread("polarity_OK_edited.jpg"),None,fx=0.2, fy=0.2) #resize the image to be smaller
  polarity_nok = cv2.resize(cv2.imread("Polarity_NOK1.JPG"), None,fx=0.2, fy=0.2) #resize the image to be smaller
  
  polarity_ok_cropped = polarity_ok[350:408, 97:111]
  polarity_nok_cropped = polarity_nok[350:408, 97:111]
  
  polarity_ok_cropped1 = polarity_ok[359:409, 232:240]
  polarity_nok_cropped1 = polarity_nok[359:409, 232:240]
  
  polarity_ok_cropped2 = polarity_ok[118:153, 44:69]
  polarity_nok_cropped2 = polarity_nok[118:153, 44:69]
  
  polarity_ok_cropped3 = polarity_ok[94:142, 192:197]
  polarity_nok_cropped3 = polarity_nok[94:142, 192:197]
  
  m = mse(polarity_ok_cropped, polarity_nok_cropped)
  s = ssim(polarity_ok_cropped, polarity_nok_cropped, multichannel=True)
  
  diff = cv2.subtract(polarity_ok_cropped, polarity_nok_cropped)
  result = not np.any(diff)
  
  m1 = mse(polarity_ok_cropped1, polarity_nok_cropped1)
  s1 = ssim(polarity_ok_cropped1, polarity_nok_cropped1, multichannel=True)
  
  diff1 = cv2.subtract(polarity_ok_cropped1, polarity_nok_cropped1)
  result1 = not np.any(diff1)
  
  m2 = mse(polarity_ok_cropped2, polarity_nok_cropped2)
  s2 = ssim(polarity_ok_cropped2, polarity_nok_cropped2, multichannel=True)
  
  diff2 = cv2.subtract(polarity_ok_cropped2, polarity_nok_cropped2)
  result2 = not np.any(diff2)
  
  m3 = mse(polarity_ok_cropped2, polarity_nok_cropped2)
  s3 = ssim(polarity_ok_cropped2, polarity_nok_cropped2, multichannel=True)
  
  diff3 = cv2.subtract(polarity_ok_cropped3, polarity_nok_cropped3)
  result3 = not np.any(diff3)
  
  
  if (result and result1 and result2 and result3):
      print ("The polarity is correct. Awesome :)")
  else:
      print ("Nice try, but the polarity is incorrect. Take another chance!")
  </code></pre>