CodeIgniter图像剪裁

/**
  * 剪裁头像
  */
 public function facecrop(){
  $sourcefilename = $this -> input -> post('crop_filename');
  $sourcepath = UPLOADPATH.'/images/day_'.date('ymd').'/'.$sourcefilename;
  $x = $this -> input -> post('x');
  $y = $this -> input -> post('y');
  $w = $this -> input -> post('w');
  $h = $this -> input -> post('h');
  
  $config['image_library'] = 'gd2';
     $config['x_axis'] = $x;
        $config['y_axis'] = $y;
  $config['width'] = $w;
  $config['height'] = $h;
  $config['source_image'] = $sourcepath;
  $config['maintain_ratio'] = false;
  
  $this->load->library('Image_lib', $config);
  $r = $this->image_lib->crop();
  echo $r;
 }