title: Finding non-convex (or convac) hull in opencv
body: <p>Does OpenCV contain a non-convex hull function?</p>
  
  <p>By non-convex hull I mean something like:
  <a href="https://i.stack.imgur.com/hwxSW.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/hwxSW.jpg" alt="enter image description here"></a></p>