我试图使用特征检测器的通用接口构造FREAK,但是当然不起作用.我应该如何修改代码以获得结果？

#include <stdio.h>
#include <iostream>
#include <opencv\cxcore.h>
#include <opencv2\nonfree\features2d.hpp>
#include <opencv\highgui.h>
#include <opencv2\features2d\features2d.hpp>
#include <vector>
 
using namespace std;
using namespace cv;
 
int main(){
    Mat mat1;
    mat1 = imread("Testimg06.jpg",0);
    vector<KeyPoint> P1;
    Ptr<FeatureDetector> freakdes;
    Ptr<DescriptorExtractor> descriptorExtractor; 
    freakdes = FeatureDetector::create("FREAK"); 
 
    freakdes->detect(mat1,P1);
 
    Mat keypoint_img;
 
    drawKeypoints( mat1,P1,keypoint_img,Scalar::all(-1),DrawMatchesFlags::DEFAULT );
     imshow("Keypoints 1",keypoint_img );
    cvWaitKey(0);
 
}