title: Calculating optical flow with opencv
body: <p>I am using the function: </p>
  
  <pre><code>cvCalcOpticalFlowBM
  </code></pre>
  
  <p>To calculate the optical flow.
  The code is running, but the velocity matrices are zero (both x and y), although the input video clearly changes.
  Does anyone have an idea why would the velocity matrices be zero (black)?</p>