title: How to get right Up vector for LookAt transformation?
body: <p>I do understand how glm::LookAt works and what parameters it requires.
  I can't understand how to provide right Up vector.</p>
  
  <p>For example, this:</p>
  
  <pre><code>glm::mat4x4 transformation = glm::lookAt(cameraPosition, targetPosition, glm::vec3(0.f, 1.f, 0.f));
  </code></pre>
  
  <p>If I want to make camera watch straight down:</p>
  
  <pre><code> mat4x4 transformation = glm::lookAt(vec3(0.f, 1.f, 0.f), vec3(0.f, 0.f, 0.f), vec3(0.f, 1.f, 0.f));
  </code></pre>
  
  <p>It won't work. Matrix will contain <em>NaNs</em> (Not a Number) values!
  So how to get right value for Up vector?</p>