No, the origin is in the center. I'm using OpenGL. The big circle is at the center.

this is the function i use to detect collision beetween spheres
this function uses the concept of midrange osculation plane
( i don't know how else to call it ).
Note that this function gives correct result even
with high speed for both spehres.


int C2DPhysicEngine::SphereSphereCollTest( CParticle *A,
CParticle *B )
{

Vector P,Q,Dir,MidPoint;

double eps = 0.000001f;

Dir = A->Position-B->Position;

Dir.Normalise();

P=B->Position+B->GetRadius()*Dir;

Q=A->Position-A->GetRadius()*Dir;

MidPoint = ( Q + P ) *0.5f;

if ( ( MidPoint-P ) * Dir <= eps )
{

// length of vector

Vector W=Q-P;

// relative penetration depth

Vector RelDist=W.Length()*Dir;

// sphere collided , handle
// repositioning and collision response

return 1;
}

return 0;
}