title: solvePnPRansac returns zero value for rvecs and tvecs
body: <p>I want to find out the rotation and translation vectors for my camera.</p>
  
  <p>However, the solvePnPRansac method from the documentation is giving zero matrix for each; it returns values for corners as output.</p>
  
  <pre><code> print corners
  [[[ 372.48184204   80.71842194]]
   [[ 374.40280151  103.50676727]]
   [[ 377.49230957  128.53459167]]
        ... so on till ..
   [[ 204.40803528  168.18978882]]
   [[ 203.94168091  193.23649597]]
   [[ 204.39024353  220.48114014]]
   [[ 204.54725647  248.10583496]]]
  </code></pre>
  
  <p>I am using the code from the documentation and modified it according to my chessboard.</p>
  
  <pre><code>criteria = (cv2.TERM_CRITERIA_EPS + cv2.TERM_CRITERIA_MAX_ITER, 30, 0.001)
  objp = np.zeros((7*7,3), np.float32)
  objp[:,:2] = np.mgrid[0:7,0:7].T.reshape(-1,2)
  axis = np.float32([[3,0,0], [0,3,0], [0,0,-3]]).reshape(-1,3)
  
  for fname in glob.glob('../Right*.jpg'):
        img = cv2.imread(fname)
        gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
        ret, corners = cv2.findChessboardCorners(gray, (7,7),None)
        print ret,corners #ret is true and corners returns an array
        if ret == True:
          # Find the rotation and translation vectors.
           rvecs, tvecs , inliers = cv2.solvePnPRansac(objp, corners, mtx, dist)
  </code></pre>
  
  <p><code>rvecs</code> and <code>tvecs</code> are coming out to be <code>[0,0,0]</code> and <code>[0,0,0]</code>, respectively.</p>