title: error: (-215) nimages > 0 in function calibrateCamera using Python and OpenCV
body: <p>I'm trying to calibrate my web-cam based on the example given in the opencv samples but when i run the code provided here:</p>
  
  <pre><code>def caliLeftCam():    
      args, img_mask = getopt.getopt(sys.argv[1:], '', ['save=', 'debug=', 'square_size='])
      args = dict(args)
      try: img_mask = img_mask[0]
      except: img_mask = '../cpp/img*.jpg'
      img_names = glob(img_mask)
      debug_dir = args.get('--debug')
      square_size = float(args.get('--square_size', 1.0))
  
      pattern_size = (7, 5)
      pattern_points = np.zeros( (np.prod(pattern_size), 3), np.float32 )
      pattern_points[:,:2] = np.indices(pattern_size).T.reshape(-1, 2)
      pattern_points *= square_size
  
      obj_points = []
      img_pointsL = []
      h, w = 0, 0
      for fn in img_names:
          print "processing %s..." % fn,
          imgL = cv2.imread(fn, 0)
          h, w = imgL.shape[:2]
          found, corners = cv2.findChessboardCorners(imgL, pattern_size)
          if found:
              term = ( cv2.TERM_CRITERIA_EPS + cv2.TERM_CRITERIA_COUNT, 30, 0.1 )
              cv2.cornerSubPix(imgL, corners, (5, 5), (-1, -1), term)
          if debug_dir:
              vis = cv2.cvtColor(imgL, cv2.COLOR_GRAY2BGR)
              cv2.drawChessboardCorners(vis, pattern_size, corners, found)
              path, name, ext = splitfn(fn)
              cv2.imwrite('%s/%s_chess.bmp' % (debug_dir, name), vis)
          if not found:
              print "chessboard not found"
              continue
          img_pointsL.append(corners.reshape(-1, 2))
          obj_points.append(pattern_points)
  
          print 'ok'
  
      rmsL, cameraL_matrix, dist_coefsL, rvecsL, tvecsL = cv2.calibrateCamera(obj_points, img_pointsL, (w, h))
  </code></pre>
  
  <p>i got this error:</p>
  
  <pre><code>Traceback (most recent call last):
  File "/home/sabrine/Downloads/opencv-2.4.9/samples/python2/Memo.py", line 293, in &lt;module&gt;
  Img_pointsL, Cam_MatL, DisL = caliLeftCam()
  File "/home/sabrine/Downloads/opencv-2.4.9/samples/python2/Memo.py", line 124, in caliLeftCam
  rmsL, cameraL_matrix, dist_coefsL, rvecsL, tvecsL = cv2.calibrateCamera(obj_points, img_pointsL, (w, h))
  error: /build/buildd/opencv-2.4.8+dfsg1/modules/calib3d/src/calibration.cpp:3415: 
  error: (-215) nimages &gt; 0 in function calibrateCamera
  </code></pre>
  
  <p>what does this error mean?
  and how can i solve it?</p>