public double intersect(Ray r)
    {
        Vector v = r.origin.sub(pos);
        double b = 2 * r.direction.dot(v);
        double c = v.dot(v) - radius * radius;
        double d = b * b - 4 * c;
        if(d > 0)
        {
            double x1 = (-b - Math.sqrt(d)) / 2;
            double x2 = (-b + Math.sqrt(d)) / 2;
            if (x1 >= 0 && x2 >= 0)
                return x1;
            if (x1 < 0 && x2 >= 0)
                return x2;
        }
        return -1;
    }