title: OpenCV error: Assertion faild in a simple face detection beginers code
body: <p>I'm starting to learn how to use OpenCV and face detection. I'm following code example from: <a href="https://opencv-python-tutroals.readthedocs.io/en/latest/py_tutorials/py_objdetect/py_face_detection/py_face_detection.html#face-detection" rel="nofollow noreferrer">https://opencv-python-tutroals.readthedocs.io/en/latest/py_tutorials/py_objdetect/py_face_detection/py_face_detection.html#face-detection</a> . </p>
  
  <pre class="lang-py prettyprint-override"><code>import cv2
  face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml')
  eye_cascade = cv2.CascadeClassifier('haarcascade_eye.xml')
  
  img = cv2.imread('face.jpg')
  gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
  
  faces = face_cascade.detectMultiScale(gray, 1.3, 5)
  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>I keep getting this: </p>
  
  <pre><code>Traceback (most recent call last):
    File "D:/Korisnici/Josipa/Desktop/CV/face_detection.py", line 10, in &lt;module&gt;
      faces = face_cascade.detectMultiScale(gray, 1.3, 5)
  error: OpenCV(4.1.1) C:\projects\opencv-python\opencv\modules\objdetect\src\cascadedetect.cpp:1689: error: (-215:Assertion failed) !empty() in function 'cv::CascadeClassifier::detectMultiScale' 
  </code></pre>
  
  <p>and I don't know what I'm doing wrong.</p>