protected function drawRing($image, $arcwidth)
    {
        $color = $this->randomColor($image);
        $transparency = $this->transparentColor($image);

        $start = $this->rand(20, 360);
        $stop = $this->rand(20, 360);
        if($stop < $start) list($start, $stop) = array($stop, $start);

        imagefilledarc($image, $this->center, $this->center, $arcwidth, $arcwidth, $stop, $start, $color, IMG_ARC_PIE);
        imagefilledellipse($image, $this->center, $this->center, $arcwidth - $this->ringwidth,
            $arcwidth - $this->ringwidth, $transparency);

        imagecolordeallocate($image, $color);
        imagecolordeallocate($image, $transparency);
    }