codeigniter:fail to create captcha

Question

I am using captcha and I want to create a captcha using the cpatcha helper!I read the user guide and write a piece of code,but it fails to create an captcha,also here is no error displayed on the page,so I just can't find where I am wrong,my code is as follows:

public function captcha(){
        $this->load->helper('captcha');

        $vals=array(
            'word'=>'0123456789',
            'img_path'=>base_url('asserts/cpatcha').'/',
            'img_url'=>base_url('asserts/cpatcha').'/',
            'img_width'=>150,
            'img_height'=>30,
            'expiration'=>7200
            );

        var_dump($vals);
        $cap=create_captcha($vals);
        var_dump($cap['image']);
    }

as I print the two variables,the result displayed on the page is as follows:

array (size=6)
  'word' => string '0123456789' (length=10)
  'img_path' => string 'http://wrecruit.tudouya.com/asserts/cpatcha/' (length=44)
  'img_url' => string 'http://wrecruit.tudouya.com/asserts/cpatcha/' (length=44)
  'img_width' => int 150
  'img_height' => int 30
  'expiration' => int 7200

null

Answer 1

You may change $cap=create_captcha(); to $cap = create_captcha($vals); and also change var_dump($cap['image']); to echo $cap['image'];

Answer 2

I solve this problem by myself and I post my solution at here,hope this can help others!
at first,the reason why the captcha can't display correctly due to the uncorrect config.it's this:'img_path'=>base_url('asserts/cpatcha').'/',.
I read the source code of captcha in ci and I find in the line 66-69,it will check if the img_path is a dir,just as follows:

if ( ! @is_dir($img_path))
{
    return FALSE;
}

in my config,I set the img_path to "base_url('asserts/cpatcha').'/'",this is not a dir,so it will fail to create the captcha.
the way to solve this is just setting the correct path which will store the captcha image. hope this can help you!