public function arc($x1, $y1, $x2, $y2, $x3, $y3)
    {
        $h = $this->h;
        $this->out(
            sprintf(
                '%.2F %.2F %.2F %.2F %.2F %.2F c ',
                $x1 * $this->k,
                ($h - $y1) * $this->k,
                $x2 * $this->k,
                ($h - $y2) * $this->k,
                $x3 * $this->k,
                ($h - $y3) * $this->k
            )
        );
    }