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I think this is bug. I have wrote a simple script to test this. Please if someone want to confirm It would help.

This script extract first 100 keypoints on an image then extract 1 keypoint. Both set of descriptors are stored inside the matcher with .add.

Then if you run it you will see that using .match the code works. Using knnMatch the matcher doesnt' return anything:

Ptr<FeatureDetector> d = new OrbFeatureDetector();  
Ptr<DescriptorExtractor> e = new OrbDescriptorExtractor();
Ptr<DescriptorMatcher> m = new BFMatcher(NORM_HAMMING);


//> The first iteration will extract 100 keypoints and store them
//> The second iteration will extract 1 keypoint
for(int i=100;i>0;i=i-99) {
    d = new OrbFeatureDetector(i);
    Mat image = imread("train.png",0);
    vector<KeyPoint> kp;
    d->detect(image,kp);
    cout << "detected: "<<kp.size()<<endl;

    Mat desc;
    e->compute(image,kp,desc);
    cout << "descriptor size: "<<desc.size()<<endl;

    vector<Mat> addDesc;
    addDesc.push_back(desc);
    m->add( addDesc );

}

//> Test match and knnMatch
d = new OrbFeatureDetector(100);
Mat image = imread("query.png",0);
vector<KeyPoint> kp;

d->detect(image,kp);
Mat queryDesc;
e->compute(image,kp,queryDesc);



vector<DMatch> matches;
m->match(queryDesc,matches);


cout << "Parsing matches: "<<matches.size()<<endl;
for(int i=0;i<matches.size();i++)
    cout << "Matches[i].distance: "<<matches[i].distance<<endl;


vector<vector<DMatch>> matches2;
m->knnMatch(queryDesc,matches2,2);


cout << "Parsing matches: "<<matches2.size()<<endl;
for(int i=0;i<matches2.size();i++)
    cout << "matches size: "<<matches2[i].size()<<endl; //> THIS SHOULDN'T OUTPUT size: 0!

With this code, the call to .match will return correct result. But knnMatch fails!