How to interpret the distances in matches of descriptors

edit

It is the classical L2 distance for floating point descriptor and HAMMING like distance for binary descriptor.

You can check it with the following code:

  cv::Mat img1 = cv::imread("opencv-3.1.0/samples/data/aero1.jpg");
  cv::Mat img2 = cv::imread("opencv-3.1.0/samples/data/aero3.jpg");

  cv::Ptr<cv::Feature2D> sift = cv::xfeatures2d::SIFT::create();
  std::vector<cv::KeyPoint> trainKeypoints;
  cv::Mat trainDescriptors;
  sift->detectAndCompute(img1, cv::Mat(), trainKeypoints, trainDescriptors);

  std::vector<cv::KeyPoint> queryKeypoints;
  cv::Mat queryDescriptors;
  sift->detectAndCompute(img2, cv::Mat(), queryKeypoints, queryDescriptors);

  cv::Ptr<cv::DescriptorMatcher> matcher = cv::DescriptorMatcher::create("BruteForce");
  std::vector<cv::DMatch> matches;
  matcher->match(queryDescriptors, trainDescriptors, matches);
  std::cout << "matches[0]: trainIdx=" << matches[0].trainIdx << " ; queryIdx="
      << matches[0].queryIdx << " ; dist=" << matches[0].distance << std::endl;

  double my_dist = 0.0;
  for(int j = 0; j < trainDescriptors.cols; j++) {
    my_dist += (trainDescriptors.at<float>(matches[0].trainIdx, j) - queryDescriptors.at<float>(matches[0].queryIdx, j))
        * (trainDescriptors.at<float>(matches[0].trainIdx, j) - queryDescriptors.at<float>(matches[0].queryIdx, j));
  }
  std::cout << "my_dist=" << cv::sqrt(my_dist) << std::endl;

Edit: the type of distance that could be used is present in the documentation.