title: Calculate bounding polygon of alpha shape from the Delaunay triangulation
body: <p>Given a set of points in the plane, a notion of alpha-shape, for a given positive number alpha, is defined by finding the Delaunay triangulation and deleting any triangles for which at least one edge exceeds alpha in length. Here's an example using d3:</p>
  
  <p><a href="http://bl.ocks.org/gka/1552725" rel="noreferrer">http://bl.ocks.org/gka/1552725</a></p>
  
  <p>The problem is that, when there are thousands of points, simply drawing all the interior triangles is too slow for an interactive visualization, so I'd like to just find the bounding polygons. This isn't so simple, because as you can see from that example sometimes there might be two such polygons.</p>
  
  <p>As a simplification, suppose some clustering has been performed so that there's guaranteed to be a unique bounding polygon for each triangulation. What's the best way to find this bounding polygon? In particular, the edges have to be ordered consistently and it must support the possibility of "holes" (think a torus or donut shape--this is expressible in GeoJSON).</p>