//缩略图（固定长高容器，等比例缩略图，扩容填充，裁剪）
    public function thumb($new_width=0,$new_height=0) {
        if (empty($new_width) && empty($new_height)){
            $new_width = $this->width;
            $new_height = $this->height;
        }
        if (!is_numeric($new_width) || !is_numeric($new_width)){
            $new_width = $this->width;
            $new_height = $this->height;
        }
        //创建一个容器
        $_n_w = $new_width;
        $_n_h = $new_height;
        //创建裁剪点
        $_cut_width = 0;
        $_cut_height = 0;
        //等比例缩略
        if ($this->width < $this->height) {
            $new_width = ($new_height / $this->height) * $this->width;
        } else {
            $new_height = ($new_width / $this->width) * $this->height;
        }
        if ($new_width < $_n_w) { //如果新高度小于新容器高度
            $r = $_n_w / $new_width; //按长度求出等比例因子
            $new_width *= $r; //扩展填充后的长度
            $new_height *= $r; //扩展填充后的高度
            $_cut_height = ($new_height - $_n_h) / 2; //求出裁剪点的高度
        }
        if ($new_height < $_n_h) { //如果新高度小于容器高度
            $r = $_n_h / $new_height; //按高度求出等比例因子
            $new_width *= $r; //扩展填充后的长度
            $new_height *= $r; //扩展填充后的高度
            $_cut_width = ($new_width - $_n_w    ) / 2; //求出裁剪点的长度
        }
        $this->new = imagecreatetruecolor($_n_w,$_n_h);
        imagecopyresampled($this->new,$this->img,0,0,$_cut_width,$_cut_height,$new_width,$new_height,$this->width,$this->height);
    }