public function png(&$img)
    {
        if (is_null($this->center) || is_null($this->radius)) {
            throw new \RuntimeException('Before exporting to PNG, you must give center and radius!');
        }

        imagefilledellipse(
            $img,
            $this->center->x,
            $this->center->y,
            $this->radius->w,
            $this->radius->h,
            $this->color->gd($img)
        );
    }