title: OpenCV Optical flow Sample Code Error
body: <pre><code>import numpy as np
  import cv2
  
  cap = cv2.VideoCapture("slow.avi")
  while not cap.isOpened():
      cap = cv2.VideoCapture("slow.avi")
      cv2.waitKey(1000)
      print "Wait for the header"
  
  # params for ShiTomasi corner detection
  feature_params = dict( maxCorners = 100,
                         qualityLevel = 0.3,
                         minDistance = 7,
                         blockSize = 7 )
  
  # Parameters for lucas kanade optical flow
  lk_params = dict( winSize  = (15,15),
                    maxLevel = 2,
                    criteria = (cv2.TERM_CRITERIA_EPS | cv2.TERM_CRITERIA_COUNT, 10, 0.03))
  
  # Create some random colors
  color = np.random.randint(0,255,(100,3))
  
  # Take first frame and find corners in it
  ret, old_frame = cap.read()
  old_gray = cv2.cvtColor(old_frame, cv2.COLOR_BGR2GRAY)
  p0 = cv2.goodFeaturesToTrack(old_gray, mask = None, **feature_params)
  
  #print old_frame
  
  # Create a mask image for drawing purposes
  mask = np.zeros_like(old_frame)
  
  while(1):    
      ret,frame = cap.read()
  
      frame_gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
  
      # calculate optical flow
      p1, st, err = cv2.calcOpticalFlowPyrLK(old_gray, frame_gray, p0, None, **lk_params)
  
      # Select good points
      good_new = p1[st==1]
      good_old = p0[st==1]
  
      # draw the tracks
      for i,(new,old) in enumerate(zip(good_new,good_old)):
          a,b = new.ravel()
          c,d = old.ravel()
          mask = cv2.line(mask, (a,b),(c,d), color[i].tolist(), 2)
          frame = cv2.circle(frame,(a,b),5,color[i].tolist(),-1)
      img = cv2.add(frame,mask)
  
      cv2.imshow('frame',img)
      k = cv2.waitKey(30) &amp; 0xff
      if k == 27:
          break
  
      # Now update the previous frame and previous points
      old_gray = frame_gray.copy()
      p0 = good_new.reshape(-1,1,2)
  
  cv2.destroyAllWindows()
  cap.release()
  </code></pre>
  
  <p>I copied Optical flow sample code from <a href="http://docs.opencv.org/trunk/doc/py_tutorials/py_video/py_lucas_kanade/py_lucas_kanade.html" rel="nofollow">http://docs.opencv.org/trunk/doc/py_tutorials/py_video/py_lucas_kanade/py_lucas_kanade.html</a> but doesn't work and get this error message I tried to fix but could't</p>
  
  <pre><code>line 56, in &lt;module&gt;
  cv2.imshow('frame',img)
  error: ..\..\..\opencv-2.4.8.1\modules\highgui\src\window.cpp:269: 
  error: (-215) size.width&gt;0 &amp;&amp; size.height&gt;0 in function cv::imshow
  </code></pre>