# Circle to Circle collision, checking each circle against all others

I'm currently coding a little circle to circle collision demo but I've got a little stuck. I think I currently have the code to detect collision but I'm not sure how to loop through my list of circles and check them off against one another.

Collision check code:

public static Vector2 GetIntersectionDepth(Circle a, Circle b)
{
float xValue = a.Center.X - b.Center.X;
float yValue = a.Center.Y - b.Center.Y;

Vector2 depth = Vector2.Zero;

float distance = Vector2.Distance(a.Center, b.Center);

{
float result = (a.Radius + b.Radius) - distance;
depth.X = (float)Math.Cos(result);
depth.Y = (float)Math.Sin(result);
}

return depth;
}


Loop through code:

        Vector2 depth = Vector2.Zero;

for (int i = 0; i < bounds.Count; i++)
for (int j = i+1; j < bounds.Count; j++)
{
depth = CircleToCircleIntersection.GetIntersectionDepth(bounds[i], bounds[j]);
}


Clearly I'm a complete newbie, wondering if anyone can give any suggestions or point out my errors, thanks in advance. :)

• You have neglected to state what your actual problem is. Also, instead of comparing radii to distance, compare a.Radius*a.Radius + b.Radius*b.Radius > xValue*xValue + yValue*yValue. It is less work, both for you and the computer. – Superbest Mar 29 '12 at 22:28
• Superbeast is right. You could further optimize this by only using a sqrt operation when your circles actually overlap (to calculate the depth of intersection). Vector2.Distance uses sqrt which is computationally expensive. It would make a difference if you're checking a lot of circles for intersection. – bummzack Mar 30 '12 at 6:48

The functions look correct assuming bounds is a Circle. The loop through code is also seems correct, as you are avoiding checking the same 2 circles again which is good.

For the collision detection bit, from the source your provided, the function GetIntersectionDepth(Circle a, Circle b) will return a zero vector IF the two circles are not colliding, so all you have to do is change your code to this:

Vector2 depth = Vector2.Zero;

for (int i = 0; i < bounds.Count; i++)
for (int j = i+1; j < bounds.Count; j++)
{
depth = CircleToCircleIntersection.GetIntersectionDepth(bounds[i], bounds[j]);
if(depth == Vector2.Zero){
//this means that did not collide
}else{
//this means they collided
}
}

• Nice work. Welcome to GameDev SE! – MichaelHouse Mar 30 '12 at 14:40