private function prime($x, $y) {
        $x = round($x);
        $y = round($y);
        $px = floor($x / 2);
        $py = floor($y / 4);

        if (!isset($this->chars[$py][$px])) {
            $this->chars[$py][$px] = 0;
        }

        return [$x, $y, $px, $py, $this->chars[$py][$px]];
    }