如何使用codelgniter框架验证码类库文件？

<?php
class Authcode
{
 var $CI;
 var $fontPath;//字体路径
 var $image;
 var $charLen  = 4; //生成几位验证码
 var $arrChr  = array();//验证码字符
 var $width  = 83; //图片宽
 var $height  = 24; //图片高
 var $bgcolor  = "#ffffff"; //背景色
 var $showNoisePix = true; //生成杂点
 var $noiseNumPix = 80; //生成杂点数量
 var $showNoiseLine = true; //生成杂线
 var $noiseNumLine = 2; //生成杂线数量
 var $showBorder = true; //边框，当杂点、线一起作用的时候，边框容易受干扰
 var $borderColor = "#000000";
 function Authcode()
 {
 $this->CI = & get_instance();
 $this->fontPath = realpath(dirname(__FILE__) . '/fonts/'); //字体文件
 //$this->arrChr  = array_merge(range(1, 9) , range('A', 'Z'));//数字字母验证码
 //$this->arrChr  = range('A', 'Z');//纯字母验证码
 $this->arrChr = range(0, 9);//纯数字验证码
 }
 /**
 * 显示验证码
 *
 */
 function show()
 {
 $this->image = imageCreate($this->width, $this->height);
 $this->back = $this->getColor($this->bgcolor);
 imageFilledRectangle($this->image, 0, 0, $this->width, $this->height, $this->back);
 $size = $this->width / $this->charLen - 4;
 if ($size > $this->height) {
  $size = $this->height;
 }
 $left = ($this->width - $this->charLen * ($size + $size / 10)) / $size + 5;
 $code = '';
 for($i = 0; $i < $this->charLen; $i ++) {
  $randKey = rand(0, count($this->arrChr) - 1);
  $randText = $this->arrChr[$randKey];
  $code .= $randText;
  $textColor = imageColorAllocate($this->image, rand(0, 100), rand(0, 100), rand(0, 100));
  $font = $this->fontPath . '/' . rand(1, 5) . ".ttf";
  $randsize = rand($size - $size / 10, $size + $size / 10);
  $location = $left + ($i * $size + $size / 10);
  @imagettftext($this->image, $randsize, rand(- 18, 18), $location, rand($size - $size / 10, $size + $size / 10) + 2, $textColor, $font, $randText);
 }
 if ($this->showNoisePix == true) {
  $this->setNoisePix();
 }
 if ($this->showNoiseLine == true) {
  $this->setNoiseLine();
 }
 if ($this->showBorder == true) {
  $this->borderColor = $this->getColor($this->borderColor);
  imageRectangle($this->image, 0, 0, $this->width - 1, $this->height - 1, $this->borderColor);
 }
 $this->CI->session->set_userdata('auth_code', $code);
 ob_clean();
 header("Content-type: image/jpeg");
 imagejpeg($this->image);
 imagedestroy($this->image);
 }
 /**
 * 显示验证码的JS调用
 *
 */
 function showscript()
 {
 //显示验证码
 echo "var img_src = '/imgauthcode/show/?';\n";
 echo "document.writeln('<img id=\"img_authcode\" src=\"' + img_src + Math.random() + '\" style=\"cursor:hand;\" onclick=\"this.src=img_src + Math.random();\" alt=\"点击更换图片\">');";
 }
 /**
 * 检查验证码是否正确
 *
 * @param string $auth_code
 * @return bool
 */
 function check($auth_code = null)
 {
 return ($this->CI->session->userdata('auth_code') && $auth_code) ? ($this->CI->session->userdata('auth_code') === $auth_code) : false;
 }
 function getColor($color)
 {
 $color = eregi_replace("^#", "", $color);
 $r = $color[0] . $color[1];
 $r = hexdec($r);
 $b = $color[2] . $color[3];
 $b = hexdec($b);
 $g = $color[4] . $color[5];
 $g = hexdec($g);
 $color = imagecolorallocate($this->image, $r, $b, $g);
 return $color;
 }
 function setNoisePix()
 {
 for($i = 0; $i < $this->noiseNumPix; $i ++) {
  $randColor = imageColorAllocate($this->image, rand(0, 255), rand(0, 255), rand(0, 255));
  imageSetPixel($this->image, rand(0, $this->width), rand(0, $this->height), $randColor);
 }
 }
 function setNoiseLine()
 {
 for($i = 0; $i < $this->noiseNumLine; $i ++) {
  $randColor = imageColorAllocate($this->image, rand(0, 255), rand(0, 255), rand(0, 255));
  imageline($this->image, rand(1, $this->width), rand(1, $this->height), rand(1, $this->width), rand(1, $this->height), $randColor);
 }
 }
}

Class Admin extends CI_Controller{
 function __construct()
 {
 parent::__construct();
 $this->load->library('Authcode');
 }
function captcha(){
 if($_POST){
  if ($this->authcode->check($this->input->post('gd_pic'))) {
  echo "right";
  } else {
  echo '验证码不正确，请重新输入';
  }
 }else{
  $this->load->view('demo');
 }
 }
 function show_captcha(){ //此方法用于显示验证码图片，归一个view中的img的src调用
 $this->authcode->show();
 }
}

<?php echo form_open('c=admin&m=captcha');?>
<input type="text" name="gd_pic" />
<img src="<?php echo base_url('?c=admin&m=show_captcha');?>" ><br>
<input type="submit" name="submit" value="验证" />
<?php echo form_close();?>