title: What does the normals (vn) in wavefront.obj files represent?
body: <p>I have a 3D mesh in an obj file that was the output of a 3D face-reconstruction algorithm. Problem is that the algorithm mirrors the face along the 2nd axis, i.e. every vertex <code>v x y z</code> is in "reality" <code>v -x y z</code>.</p>
  
  <p>I tried the obvious thing to invert all the x's, it works, but now the mesh has inverted face orientation. I was able to fix that by applying <code>Filters -&gt; Normals, Curvatures and Orientation -&gt; Invert Faces Orientation</code> in MeshLab. </p>
  
  <p>Problem is I can't understand what that option does or why my original fix didn't work, Can you explain? Also, can you explain what do the normals in the wavefront.obj format represent geometrically? </p>