protected function output()
    {
        if ($this->canSendHeaders() || $this->send_headers == false) {
            if ($this->send_headers) {
                // only send the content-type headers if no headers have been output
                // this will ease debugging on misconfigured servers where warnings
                // may have been output which break the image and prevent easily viewing
                // source to see the error.
                header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");
                header("Last-Modified: " . gmdate("D, d M Y H:i:s") . "GMT");
                header("Cache-Control: no-store, no-cache, must-revalidate");
                header("Cache-Control: post-check=0, pre-check=0", false);
                header("Pragma: no-cache");
            }

            switch ($this->image_type) {
                case self::SI_IMAGE_JPEG:
                    if ($this->send_headers) header("Content-Type: image/jpeg");
                    imagejpeg($this->im, null, 90);
                    break;
                case self::SI_IMAGE_GIF:
                    if ($this->send_headers) header("Content-Type: image/gif");
                    imagegif($this->im);
                    break;
                default:
                    if ($this->send_headers) header("Content-Type: image/png");
                    imagepng($this->im);
                    break;
            }
        } else {
            echo '<hr /><strong>'
                .'Failed to generate captcha image, content has already been '
                .'output.<br />This is most likely due to misconfiguration or '
                .'a PHP error was sent to the browser.</strong>';
        }

        imagedestroy($this->im);
        restore_error_handler();

        if (!$this->no_exit) exit;
    }