title: How to calculate the normal vector?
body: <p>I'm making a game with lasers. I need to reflect the lasers against a rotated object, but I can't find the correct normal vector.</p>
  
  <p>Example:</p>
  
  <ul>
  <li>Point A (0, 20) goes to point B (15, 5).</li>
  <li>My velocity is (1, -1) * Speed. </li>
  <li>A wall from point C (15, 0) to point D (15, 50).</li>
  <li>Wall standing upright. </li>
  </ul>
  
  <p>Now I need to calculate the normal and normalize it (I know how to normalize). The normalize vector should be (-1, -1), but how do I calculate this? Do I need to use dotproduct, the angle or something else? </p>