title: Contour of 2d point clouds
body: <p>I have scattered point clouds in 2d. My problem is I would like to get the boundary's point to get the contour of the point clouds. </p>
  
  <p>I know about this 'marching square' however this algorithm usually used for pixel in 2d.
  Any one have idea how to get the contour from 2d point clouds or details of 'Marching square' algorithm? In addition, convex hull does not work in my case.</p>
  
  <p>Thanks in advance.</p>