title: Face Detection using Haar - OpenCV python
body: <p>I am trying this python code snippet:</p>
  
  <pre><code>import numpy as np
  import cv2
  
  face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml')
  eye_cascade = cv2.CascadeClassifier('haarcascade_eye.xml')
  
  img = cv2.imread('img.jpg')
  gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
  
  faces = face_cascade.detectMultiScale(gray, 1.3, 3)
  for (x,y,w,h) in faces:
      img = cv2.rectangle(img,(x,y),(x+w,y+h),(255,0,0),2)
      roi_gray = gray[y:y+h, x:x+w]
      roi_color = img[y:y+h, x:x+w]
      eyes = eye_cascade.detectMultiScale(roi_gray)
      for (ex,ey,ew,eh) in eyes:
          cv2.rectangle(roi_color,(ex,ey),(ex+ew,ey+eh),(0,255,0),2)
  
  cv2.imshow('img',img)
  cv2.waitKey(0)
  cv2.destroyAllWindows()
  </code></pre>
  
  <p>but I get this error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "test.py", line 14, in &lt;module&gt;
      roi_color = img[y:y+h, x:x+w]
  TypeError: 'NoneType' object has no attribute '__getitem__'
  </code></pre>
  
  <p>I found this program <a href="http://opencv-python-tutroals.readthedocs.org/en/latest/py_tutorials/py_objdetect/py_face_detection/py_face_detection.html#face-detection" rel="nofollow">here</a>.</p>