title: OpenCV 2.4.9 for Python, cannot find chessboard (camera calibration tutorial)
body: <p>I am trying to calibrate camera using OpenCV tools according to the following <a href="http://opencv-python-tutroals.readthedocs.org/en/latest/py_tutorials/py_calib3d/py_calibration/py_calibration.html#calibration" rel="nofollow noreferrer">this guide</a>.</p>
  
  <p>The problem is that function <code>findChessboardCorners</code> cannot find any chessboard on images I tried. I used a lot of them - even just plain chessboard pattern. In any case, nothing was detected. </p>
  
  <p>Here is the code (almost the same as from link above):</p>
  
  <pre><code>import numpy as np
  import cv2
  import glob
  
  # termination criteria
  criteria = (cv2.TERM_CRITERIA_EPS + cv2.TERM_CRITERIA_MAX_ITER, 30, 0.001)
  
  # prepare object points, like (0,0,0), (1,0,0), (2,0,0) ....,(6,5,0)
  objp = np.zeros((6*7,3), np.float32)
  objp[:,:2] = np.mgrid[0:7,0:6].T.reshape(-1,2)
  
  # Arrays to store object points and image points from all the images.
  objpoints = [] # 3d point in real world space
  imgpoints = [] # 2d points in image plane.
  
  images = glob.glob('*.png')
  
  for fname in images:
      img = cv2.imread(fname)
      gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
  
      # Find the chess board corners
      ret, corners = cv2.findChessboardCorners(gray, (7,6),None)
  
      # If found, add object points, image points (after refining them)
      if ret == True:
          objpoints.append(objp)
  
          corners2 = cv2.cornerSubPix(gray,corners,(11,11),(-1,-1),criteria)
          imgpoints.append(corners2)
  
          # Draw and display the corners
          img = cv2.drawChessboardCorners(img, (7,6), corners2,ret)
          cv2.imshow('img',img)
          cv2.waitKey(500)
  
  cv2.destroyAllWindows()
  </code></pre>
  
  <p>The only change I made is that i switched from .jpg files to .png files - for some reason, function imread cannot read jpg images (that's another strange problem for other topic).</p>
  
  <p>Thank you in advance for advices!</p>
  
  <p>Image ref:</p>
  
  <p><img src="https://i.stack.imgur.com/fP0Im.jpg" alt="Sample Image"></p>