title: How to reflect an angle across the Y axis
body: <p>I am no mathematician, but I somehow got into game development as a hobby.</p>
  
  <p>Having never studied anything beyond basic math, I have a lot of trouble figuring out how to reverse the angle of something, facing to the opposite direction, along the X axis &amp; across the Y axis.</p>
  
  <p>One image says more than 1000 words though (specially uneducated words):
  <a href="http://img156.imageshack.us/i/wihwin.png/" rel="noreferrer">http://img156.imageshack.us/i/wihwin.png/</a></p>
  
  <p>I basically want to reverse the direction of cannon objects adhered to a robot. When the robot changes from facing right to facing left, I do <code>(180 - angle)</code> as everyone suggested me, but it literally reverses the angle, making the cannons aim up when they are aiming down. So, I need to do something else, but it escapes my knowledge.</p>
  
  <p>To put it in other words, I work in 2D, so I want an angle that is facing right to face left.  My angles are defined:</p>
  
  <ul>
  <li>0 being "totally to the right"</li>
  <li>180 "left"</li>
  <li>90 "up" and</li>
  <li>270 "down"</li>
  </ul>
  
  <p>I want something that is aiming with an angle of 91 to turn into 89 when reversed.  There's no Z axis present.  Anyone would be so kind to help me with this?</p>