title: convert an image from IPL_DEPTH_8U to IPL_DEPTH_32S
body: <p>How can I convert an image from IPL_DEPTH_8U to IPL_DEPTH_32S?</p>
  
  <p>I need to use the watershed function and I have a marker image in
  IPL_DEPTH_8U. The problem is that the watershed function only accepts
  IPL_DEPTH_32S for markers.</p>
  
  <p>Thanks in advance,</p>