private static Vector2 PenetrationVector(CircleF circ, RectangleF rect)
        {
            var collisionPoint = rect.ClosestPointTo(circ.Center);
            var cToCollPoint   = collisionPoint - circ.Center;

            if (rect.Contains(circ.Center) || cToCollPoint.Equals(Vector2.Zero))
            {
                var displacement = PointF.Displacement(circ.Center, rect.Center);

                Vector2 desiredDisplacement;
                if (displacement != Vector2.Zero)
                {
                    // Calculate penetration as only in X or Y direction.
                    // Whichever is lower.
                    var dispx = new Vector2(displacement.X, 0);
                    var dispy = new Vector2(0, displacement.Y);
                    dispx.Normalize();
                    dispy.Normalize();

                    dispx *= (circ.Radius + rect.Width / 2);
                    dispy *= (circ.Radius + rect.Height / 2);

                    if (dispx.LengthSquared() < dispy.LengthSquared())
                    {
                        desiredDisplacement = dispx;
                        displacement.Y      = 0;
                    }
                    else
                    {
                        desiredDisplacement = dispy;
                        displacement.X      = 0;
                    }
                }
                else
                {
                    desiredDisplacement = -Vector2.UnitY * (circ.Radius + rect.Height / 2);
                }

                var penetration = displacement - desiredDisplacement;
                return(penetration);
            }
            else
            {
                var penetration = circ.Radius * cToCollPoint.NormalizedCopy() - cToCollPoint;
                return(penetration);
            }
        }
        private static Vector2 PenetrationVector(CircleF circ1, CircleF circ2)
        {
            Debug.Assert(circ1.Intersects(circ2));

            var displacement = PointF.Displacement(circ1.Center, circ2.Center);

            Vector2 desiredDisplacement;

            if (displacement != Vector2.Zero)
            {
                desiredDisplacement = displacement.NormalizedCopy() * (circ1.Radius + circ2.Radius);
            }
            else
            {
                desiredDisplacement = -Vector2.UnitY * (circ1.Radius + circ2.Radius);
            }


            var penetration = displacement - desiredDisplacement;

            return(penetration);
        }