title: Is it possible to classify two lines as acute or obtuse with this limited information?
body: <p>I'm trying to determine if two arbitrary lines (in 3D space) form the inside or outside of a corner.</p>
  <p>Going in, the data I have is the two vertices that form the join, the endpoints, and the normal.</p>
  <p><a href="https://i.stack.imgur.com/K72eU.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/K72eU.jpg" alt="Illustration of the data going in" /></a></p>
  <p>The data I have to work with is the position of the purple dot (which is actually two discrete vertices) the orange dots, and the direction of the two face normals (the green arrows).  With just this info, can I determine if the two lines comprise an acute angle or an obtuse angle?</p>
  <p>(I do have a brute-force solution for this, but the routine in question needs to be as fast as possible)</p>