特征匹配的结果会得到两个特征集合的对应关系列表。第一组特征集被称为训练集，第二组被称为查询集。FLANN在调用匹配函数之前，为了提高匹配速度，训练一个匹配器。训练阶段是为了优化cv::FlannBasedMatcher的性能。train类将会建立特征集的索引树。
而对于暴力匹配，train类不需要做什么，因为没有预处理。
FLANN和SURF搭配匹配特征点
knnMatch（）找每个特征的K近邻匹配
效率不够高，帧率很低。参考《opencv3编程入门》

#include<opencv2/opencv.hpp>
#include<opencv2/features2d/features2d.hpp>
#include<opencv2/xfeatures2d/nonfree.hpp>
#include<opencv2/highgui/highgui.hpp>
#include<iostream>
using namespace std;
using namespace cv;
using namespace cv::xfeatures2d;

int main()
{
    Mat trainImage = imread("3.jpg",1);
    Mat trainImage_gray;
    imshow("原始图",trainImage);
    cvtColor(trainImage,trainImage_gray,CV_BGR2GRAY);

    vector<KeyPoint>train_keypoint;
    Mat trainDescriptor;
    Ptr<SurfFeatureDetector> Detector = SurfFeatureDetector::create(80);
    Detector->detectAndCompute(trainImage_gray,Mat(),train_keypoint,trainDescriptor);

    vector<Mat> train_desc_collection(1,trainDescriptor);
    FlannBasedMatcher matcher;
    matcher.add(train_desc_collection);
    matcher.train();

    VideoCapture cap(0);
    unsigned int frameCount = 0;

    while (char(waitKey(1) != 'q'))
    {
        int64 time0 = getTickCount();
        Mat testImage, testImage_gray;
        cap >> testImage;
        if (testImage.empty())
            continue;
        cvtColor(testImage,testImage_gray,CV_BGR2GRAY);
        vector<KeyPoint>test_keypoint;
        Mat testDescriptor;
        Detector->detectAndCompute(testImage_gray,Mat(),test_keypoint,testDescriptor);

        vector<vector<DMatch>>matches;
        matcher.knnMatch(testDescriptor,matches,2);

        vector<DMatch>goodMatches;
        for (unsigned int i = 0; i < matches.size(); i++)
        {
            if (matches[i][0].distance < 0.6*matches[i][1].distance)
                goodMatches.push_back(matches[i][0]);
        }

        Mat dstImage;
        drawMatches(testImage,test_keypoint,trainImage,train_keypoint,goodMatches,dstImage);
        imshow("匹配窗口",dstImage);
        cout << "当前帧率为" << getTickFrequency() / (getTickCount() - time0) << endl;
    }
    return 0;
}