public override void ComputeAABB(out AABB aabb, Transform xf)
{
Vector2 v1 = xf.TransformPoint(_v1);
Vector2 v2 = xf.TransformPoint(_v2);
Vector2 r = new Vector2(_radius, _radius);
aabb.LowerBound = Vector2.Min(v1, v2) - r;
aabb.UpperBound = Vector2.Max(v1, v2) + r;
}