title: How to calculate Optical Flow magnitude in OpenCV
body: <p>I'm trying to see how big different two given video frames are. My goal is to calculate a single value showing how fast objects inside those frames are moving. </p>
  
  <p>I can calculate Optical Flow matrix below, both the HSV and magnitude matrices. But I don't know how to calculate a <strong>average</strong> total movement magnitude. How can I calculate it from those matrices?</p>
  
  <pre><code>def optical_flow(one, two):
      one_g = cv2.cvtColor(one, cv2.COLOR_RGB2GRAY)
      two_g = cv2.cvtColor(two, cv2.COLOR_RGB2GRAY)
      hsv = np.zeros((120, 320, 3))
      # set saturation
      hsv[:,:,1] = cv2.cvtColor(two, cv2.COLOR_RGB2HSV)[:,:,1]
      # obtain dense optical flow paramters
      flow = cv2.calcOpticalFlowFarneback(one_g, two_g, flow=None,
                                          pyr_scale=0.5, levels=1, winsize=15,
                                          iterations=2,
                                          poly_n=5, poly_sigma=1.1, flags=0)
      # convert from cartesian to polar
      mag, ang = cv2.cartToPolar(flow[..., 0], flow[..., 1])
      # hue corresponds to direction
      hsv[:,:,0] = ang * (180/ np.pi / 2)
      # value corresponds to magnitude
      hsv[:,:,2] = cv2.normalize(mag,None,0,255,cv2.NORM_MINMAX)
      # convert HSV to int32's
      hsv = np.asarray(hsv, dtype= np.float32)
      rgb_flow = cv2.cvtColor(hsv,cv2.COLOR_HSV2RGB)
      return rgb_flow 
  </code></pre>
  
  <p>The <code>rgb_flow</code> is a 3D array looks like this:</p>
  
  <pre><code>[[[0 0 0]
    [0 0 0]
    [0 0 0]
    ...
    [0 0 0]
    [0 0 0]
    [0 0 0]]
  
   [[0 0 0]
    [0 0 0]
    [0 0 0]
    ...
    [0 0 0]
    [0 0 0]
    [0 0 0]]
  
  
   ...
  
  
   [[0 0 0]
    [0 0 0]
    [0 0 0]
    ...
    [0 0 0]
    [0 0 0]
    [0 0 0]]]
  </code></pre>
  
  <p>And the <code>mag</code> matrix is 2D array like this:</p>
  
  <pre><code>[[3.2825139e-03 3.9561605e-03 4.8938910e-03 ... 3.7310597e-02
    3.2986153e-02 2.5520157e-02]
   [4.9569397e-03 6.3276174e-03 7.7017904e-03 ... 3.9564677e-02
    3.2582227e-02 2.6329078e-02]
   ...
  
   [6.9548332e-06 8.3683852e-05 6.0906638e-03 ... 8.3484064e-04
    6.4721738e-04 2.9505073e-04]]
  </code></pre>