title: Converting a direction vector to a quaternion rotation
body: <p>I can find a ton of questions about turning a quaternion into a direction vector but none for the other way around which makes me think I'm doing something wrong, but bear with me.</p>
  
  <p>What I'm trying to do is simply display the direction of a directional light using an arrow model.</p>
  
  <p>The directional light's direction is a unit vector but models are rotated using quaternions.</p>
  
  <p>So.. How do I rotate this model to match the direction of the light? </p>
  
  <p>Or am I crazy and I can't really do that, given that the light has no position but the model does?</p>