private void UpdataAABB(double elapsed)
{
aabb.Clear();
float x, y;
for (int i = 0; i < count; i++)
{
x = pointmass_list[i].position.X;
y = pointmass_list[i].position.Y;
aabb.Add(x, y);
x += (float)(pointmass_list[i].velocity.X * elapsed);
y += (float)(pointmass_list[i].velocity.Y * elapsed);
aabb.Add(x, y);
}
}