特征描述子与匹配

       图像特征描述子：即图像中每个像素位置的描述，通过此描述去匹配另一张图像是否含有相同特征。一般用来

大图找小图，具有旋转不变性和尺度不变性。

代码示例：

#include <opencv2/opencv.hpp>
#include <opencv2/xfeatures2d.hpp>
#include <iostream>

using namespace cv;
using namespace std;
using namespace cv::xfeatures2d;

int main(int argc, char** argv) 
{
	Mat img1 = imread("D:/cv400/data/box.png", IMREAD_GRAYSCALE);
	Mat img2 = imread("D:/cv400/data/box_in_scene.png", IMREAD_GRAYSCALE);
	if (img1.empty() || img2.empty())
	{
		cout << "Load image error..." << endl;
		return -1;
	}
	imshow("image1", img1);
	imshow("image2", img2);

	int minHessian = 400;
	Ptr<SURF> detector = SURF::create(minHessian);
	vector<KeyPoint> keypoints_1;
	vector<KeyPoint> keypoints_2;
	//检测计算描述子
	Mat descriptor_1, descriptor_2;
	detector->detectAndCompute(img1, Mat(), keypoints_1, descri