title: Generate 2D cross-section polygon from 3D mesh
body: <p>I'm writing a game which uses 3D models to draw a scene (top-down orthographic projection), but a 2D physics engine to calculate response to collisions, etc. I have a few 3D assets for which I'd like to be able to automatically generate a hitbox by 'slicing' the 3D mesh with the X-Y plane and creating a polygon from the resultant edges.</p>
  
  <p>Google is failing me on this one (and not much helpful material on SO either). Suggestions?</p>
  
  <p>The meshes I'm dealing with will be simplified versions of the displayed models, which are connected, closed, non-convex and have zero genus.</p>