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I have a std::vector of a couple million points (cv::Point2d) and I'd like to find, for every point, all other points within a 2 pixel radius.

Since my project already requires OpenCV, I thought it would be useful to use the cv::flann module. However, I haven't made much progress with my attempts so far. In particular, I'm not sure how to present my data to the index constructor. Here is my best attempt:

#include <iostream>
#include <vector>
#include <opencv2/opencv.hpp>
#include <opencv2/flann.hpp>


int main(int argc, char* argv[]) {

  // Sample data
  std::vector<cv::Point2d> points;
  points.emplace_back(438.6,268.8);
  points.emplace_back(439.1,268.6);
  points.emplace_back(438.2,268.1);
  points.emplace_back(498.3,285.9);
  points.emplace_back(312.9,245.9);
  points.emplace_back(313.4,245.7);
  points.emplace_back(313.1,245.5);
  points.emplace_back(312.5,245.4);
  points.emplace_back(297.6,388.1);
  points.emplace_back(291.7,309.8);
  points.emplace_back(194.1,369.8);
  points.emplace_back(439.9,314.9);
  points.emplace_back(312.8,246.0);

  // Create an empty Mat for the features that includes dimensional
  // space for an x and y coordinate
  cv::Mat_<double> features(0,2);

  for(auto && point : points) {

    //Fill matrix
    cv::Mat row = (cv::Mat_<double>(1, 2) << point.x, point.y);
    features.push_back(row);
  }
  std::cout << features << std::endl;

  cv::flann::Index flann_index(features, cv::flann::KDTreeIndexParams(1));


  unsigned int max_neighbours = 10;
  cv::Mat query = (cv::Mat_<double>(1, 2) << 313.0, 245.6);
  //Ensure indices and dists have enough space for at least max_neighbours
  cv::Mat_<double> indices(max_neighbours, 2) ;
  cv::Mat_<double> dists(max_neighbours, 2);
  double radius= 2.0;

  flann_index.radiusSearch(query, indices, dists, radius, max_neighbours,
      cv::flann::SearchParams(32));
}

The code compiles, but I receive a runtime error:

OpenCV Error: Unsupported format or combination of formats (type=6
) in buildIndex_, file /opt/opencv/modules/flann/src/miniflann.cpp, line 315
terminate called after throwing an instance of 'cv::Exception'
  what():  /opt/opencv/modules/flann/src/miniflann.cpp:315: error: (-210) type=6
 in function buildIndex_

Aborted (core dumped)

Can anyone please help me figure out how to use radiusSearch? And how can I configure the radiusSearch to find all neighbours, instead of just max_neighbours? Thanks in advance.