title: "The function/feature is not implemented" error when running opencv example
body: <p>I get the error below when running the following code:</p>
  <pre><code>import cv2, sys, numpy, os 
  haar_file = 'haarcascade_frontalface_default.xml'
  datasets = 'datasets'
  print('Recognizing Face Please Be in sufficient Lights...') 
  (images, lables, names, id) = ([], [], {}, 0) 
  for (subdirs, dirs, files) in os.walk(datasets): 
      for subdir in dirs: 
          names[id] = subdir 
          subjectpath = os.path.join(datasets, subdir) 
          for filename in os.listdir(subjectpath): 
              path = subjectpath + '/' + filename 
              lable = id
              images.append(cv2.imread(path)) 
              lables.append(int(lable)) 
          id += 1
  (width, height) = (130, 100) 
  (images, lables) = [numpy.array(lis) for lis in [images, lables]] 
  model = cv2.face.LBPHFaceRecognizer_create() 
  
  model.train(images, lables) # error comes here
  
  face_cascade = cv2.CascadeClassifier(haar_file) 
  webcam = cv2.VideoCapture(0) 
  while True: 
      (_, im) = webcam.read() 
      gray = cv2.cvtColor(im, cv2.COLOR_BGR2GRAY) 
      faces = face_cascade.detectMultiScale(gray, 1.3, 5) 
      for (x, y, w, h) in faces: 
          cv2.rectangle(im, (x, y), (x + w, y + h), (255, 0, 0), 2) 
          face = gray[y:y + h, x:x + w] 
          face_resize = cv2.resize(face, (width, height)) 
          prediction = model.predict(face_resize) 
          cv2.rectangle(im, (x, y), (x + w, y + h), (0, 255, 0), 3) 
          if prediction[1]&lt;500: 
               cv2.putText(im, '% s' % 
                           (names[prediction[0]]), (x-10, y-10),  
                           cv2.FONT_HERSHEY_PLAIN, 1, (0, 255, 0)) 
          else: 
              cv2.putText(im, 'not recognized',  
                          (x-10, y-10), cv2.FONT_HERSHEY_PLAIN, 1, (0, 255, 0)) 
              cv2.imshow('OpenCV', im) 
              key = cv2.waitKey(10) 
              if key == 27: 
                  break
  
  cv2.destroyAllWindows()
  </code></pre>
  <p>the error that pops up is:</p>
  <pre><code>Traceback (most recent call last):
    File &quot;Y:\vigyantram\AI-20200807T104521Z-001\AI\img processing1\face_recognize.py&quot;, line 19, in &lt;module&gt;
      model.train(images, lables)
  cv2.error: OpenCV(4.3.0) C:\projects\opencv-python\opencv_contrib\modules\face\src\lbph_faces.cpp:265: error: (-213:The function/feature is not implemented) Using Original Local Binary Patterns for feature extraction only works on single-channel images (given 16). Please pass the image data as a grayscale image! in function 'cv::face::elbp'?
  </code></pre>
  <p>Thank you in advance.</p>