title: Detect motion within a certain region of a video
body: <p>I am a biologist, and for my experimental work I want to develop a software which can detect any movement on a glass slide.</p>
  
  <p>I want to modify the following code so that it can detect motion within a desired radius circle around a particular xy coordinate rather than in the whole frame. Can you advise what changes are necessary?</p>
  
  <pre><code>from SimpleCV import *
  
  from SimpleCV import VirtualCamera
  #from time import *
  vir = VirtualCamera("video.mpg", "video")
  vir.getImage().show()
  cam = Camera()
  threshold = 0.2 # if mean exceeds this amount do something
  
  while True:
          previous = vir.getImage() #grab a frame
          #time.sleep(0.5) #wait for half a second
          current = vir.getImage() #grab another frame
          diff = current - previous
          lines = diff.findLines(threshold=1, minlinelength=1)
          lines.draw(width=2)
          current.addDrawingLayer(diff.dl())  
          matrix = diff.getNumpy()
          mean = matrix.mean()
          current.show()
  
          if mean &gt;= threshold:
                  print "Motion Detected"
                  print mean
  </code></pre>