title: OpenCV Optical Flow assertion
body: <p>I'm attempting to track landmarks along the contour of facial features obtained via dsift with python 2.7 and openCV 2.4.11. I want to track these features between frames.</p>
  
  <p>However I am receiving the following error. I have checked the input images are 1-channel equal dimensions (and unsigned 8bit type), and likewise with the prev points: </p>
  
  <pre><code>OpenCV Error: Assertion failed ((npoints = prevPtsMat.checkVector(2, CV_32F, tru
  e)) &gt;= 0) in cv::calcOpticalFlowPyrLK, file ..\..\..\modules\video\src\lkpyramid.cpp
  cv2.error: ..\..\..\modules\video\src\lkpyramid.cpp:845: error: (-215) (npoints
  = prevPtsMat.checkVector(2, CV_32F, true)) &gt;= 0 in function cv::calcOpticalFlowP
  yrLK
  </code></pre>
  
  <p>Line causing issue:             </p>
  
  <pre><code>new_pts, ttl, err = cv2.calcOpticalFlowPyrLK(self.old_img, i_img, i_old_pts, None)
  </code></pre>
  
  <p>Does anyone have any idea where I can begin debugging this?</p>