title: Direction of two points
body: <p>Some high school math concept has been forgotten, so I ask here. </p>
  
  <p>If I have two points <code>p1(x1,y1)</code>, <code>p2(x2,y2)</code>, the direction is <code>P1--&gt;p2</code>, that's <code>p1</code> points to <code>p2</code>. To represent this direction by vector, is it <code>Vector(x2-x1,y2-y1)</code> or <code>Vector(x1-x2, y1-y2)</code>?</p>
  
  <p>By the way, what is the purpose to normalize a vector?</p>