title: Flipping an angle horizontally
body: <p>I want to know a way to flip an angle in a horizontal axis, without having to do many operations. Say I have an angle of 0 ("pointing right" in my code's coordinate system), the flipped angle should be 180 (pointing left). If 90 (pointing up), flipped it should still be 90. 89 is 91, and so on.
  I can operate on the X/Y speeds implied by the angle but that would slow things down, and I feel it's not the proper way to go.
  I don't know much math so I might be calling things by the wrong name...Can anyone help?</p>
  
  <p>EDIT: Sorry I took long, I had to be out of the computer for long, OK...
  <a href="http://img215.imageshack.us/img215/8095/screenshot031v.jpg" rel="nofollow noreferrer">http://img215.imageshack.us/img215/8095/screenshot031v.jpg</a></p>
  
  <p>This screenshot might do.The above structure are two satellites and a beam linked to the white dot in the center. The two satellites should inherit the angle of the white dot (it's visible for debug purposes), so if it's aiming at an angle, they will follow. The satellite at the left is mirrored, so I calculated it with 180-angle as suggested, although it was my first try as well. As you can see it is not mirrored but flipped. And when the white dot rotates, it rotates backwards. The other does alright.</p>
  
  <p>This is the angle recalculation for something linked to something else, pid would be the parent, and id the current. pin.ang is the angle offset copied when the object is linked to another, so it keeps position when rotated:</p>
  
  <pre><code>if(object[id].mirror)
      object[id].angle = 180 - (object[id].pin.ang + object[pid].angle);
  else
      object[id].angle = object[id].pin.ang + object[pid].angle;
  </code></pre>
  
  <p>And this is the specific rotation part. OpenGL. the offx/y is for things rotated off-center, like the beam about to come out there, it renders everything else right.</p>
  
  <pre><code>glTranslatef(list[index[i]].x, list[index[i]].y, 0);
  glRotatef(list[index[i]].angle, 0.0, 0.0, 1.0);
  glTranslatef(list[index[i]].offx, -list[index[i]].offy, 0);
  </code></pre>
  
  <p>The rotation also seems to miss when the rotation speed (an integer added every redraw to the current angle, positive for rotating clockwise, like in this next one:
  <a href="http://img216.imageshack.us/img216/7/screenshot032ulr.jpg" rel="nofollow noreferrer">http://img216.imageshack.us/img216/7/screenshot032ulr.jpg</a></p>
  
  <p>So it's definitely not 180-angle, despite how obvious it'd be. The mirroring is done by just reversing the texture coordinates so it doesn't affect angle. I am afraid it might be a quirk on the GL rotation thing.</p>