title: Calibrating Webcam using Python and OpenCV Error
body: <p>Pretty new to all this and I'm trying to do the calibration for a Webcam following <a href="http://opencv-python-tutroals.readthedocs.org/en/latest/py_tutorials/py_calib3d/py_calibration/py_calibration.html#calibration" rel="noreferrer">this</a> guide and using the code below. I get the following error ..</p>
  
  <blockquote>
    <p>OpenCV Error: Assertion failed (ni > 0 &amp;&amp; ni == ni1) in collectCalibrationData, file /build/buildd/opencv-2.4.8+dfsg1/modules/calib3d/src/calibration.cpp, line 3193</p>
    
    <p>cv2.error: /build/buildd/opencv-2.4.8+dfsg1/modules/calib3d/src/calibration.cpp:3193: error: (-215) ni > 0 &amp;&amp; ni == ni1 in function collectCalibrationData</p>
  </blockquote>
  
  <p>Can someone explain what this error is and how to fix it?</p>
  
  <p>(Full error at the bottom)</p>
  
  <pre><code>import numpy as np
  import cv2
  import glob
  
  
  criteria = (cv2.TERM_CRITERIA_EPS + cv2.TERM_CRITERIA_MAX_ITER, 30, 0.001)
  # Arrays to store object points and image points from all the images.
  objpoints = [] # 3d point in real world 
  imgpoints = [] # 2d points in image plane.
  images = glob.glob('*.png')
  
  
  objp = np.zeros((6*7,3), np.float32)
  objp[:,:2] = np.mgrid[0:7,0:6].T.reshape(-1,2)
  objp = objp * 22
  
  
  for fname in images:
  
      img = cv2.imread(fname)
      gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
      ret = False
      # Find the chess board corners
      ret, corners = cv2.findChessboardCorners(gray, (6,9))
      # If found, add object points, image points (after refining them)
      if ret == True:
          objpoints.append(objp)
          cv2.cornerSubPix(gray, corners, (11,11), (-1,-1), criteria)
  
          imgpoints.append(corners)
          # Draw and display the corners 
          cv2.drawChessboardCorners(img, (6,9), corners, ret)
          cv2.imshow('img',img)
          cv2.waitKey(0)
  
  cv2.waitKey(0)
  for i in range (1,5):
      cv2.waitKey(1)
      cv2.destroyAllWindows()
      cv2.waitKey(1)
  
  
  ret, mtx, dist, rvecs, tvecs = cv2.calibrateCamera(objpoints, imgpoints, gray.shape[::-1],None,None)
  </code></pre>
  
  <blockquote>
    <p>OpenCV Error: Assertion failed (ni > 0 &amp;&amp; ni == ni1) in collectCalibrationData, file /build/buildd/opencv-2.4.8+dfsg1/modules/calib3d/src/calibration.cpp, line 3193
    Traceback (most recent call last):
      File "", line 1, in 
      File "/usr/lib/python2.7/dist-packages/spyderlib/widgets/externalshell/sitecustomize.py", line 540, in runfile
        execfile(filename, namespace)
      File "/home/students/Test/test.py", line 49, in 
        ret, mtx, dist, rvecs, tvecs = cv2.calibrateCamera(objpoints, imgpoints, gray.shape[::-1],None,None)
    cv2.error: /build/buildd/opencv-2.4.8+dfsg1/modules/calib3d/src/calibration.cpp:3193: error: (-215) ni > 0 &amp;&amp; ni == ni1 in function collectCalibrationData</p>
  </blockquote>