title: circle-circle collision
body: <p>I am going to develop a 2-d ball game where two balls (circles) collide.  Now I have the problem with determining the colliding point (in fact, determining whether they are colliding in x-axis/y-axis).  I have an idea that when the difference between the y coordinate of 2 balls is greater than the x coordinate difference then they collide in their y axis, otherwise, they collide in their x axis.  Is my idea correct?  I implemented this thing in my games.  Normally it works well, but sometimes, it fails.  Can anyone tell me whether my idea is right?  If not, then why, and is any better way?</p>
  
  <p>By collision in the x axis, I mean the circle's 1st, 4th, 5th, or 8th octant, y axis means the circle's 2nd, 3rd, 6th, or 7th octant.</p>
  
  <p>Thanks in advance!</p>