title: Angle between two lines is wrong
body: <p>I want to get angles between two line.
  So I used this code.</p>
  
  <hr>
  
  <pre><code>int posX = (ScreenWidth) &gt;&gt; 1;
  
  int posY = (ScreenHeight) &gt;&gt; 1;
  
  double radians, degrees;
  
  radians = atan2f( y - posY , x - posX);
  
  degrees = -CC_RADIANS_TO_DEGREES(radians);
  
  NSLog(@"%f %f",degrees,radians);
  </code></pre>
  
  <hr>
  
  <p>But it doesn't work .
  The Log is that: 146.309935 -2.553590</p>
  
  <p>What's the matter?
  I can't know the reason.
  Please help me.</p>
  
  <p><img src="https://i.stack.imgur.com/sfKuB.png" alt="enter image description here"></p>