How to get ORB descriptors out of openCV as efficent as possible?

Question

I want to store ORB descriptors calculated by openCV into a std::bitset<256> .

As there are several descriptors per frame/image I want to use std::vector<std::bitset<256>> per frame.

And as there are multiple frames in a video, the structure std::vector<std::vector<std::bitset<256>>> is used at end.

As openCV stores the desciptors into cv::Mat , I'm questioning myself how to get the descriptors as fast/efficent as possible out of it? So I digged around and found an answer which does not use logic operators like SHIFT or AND . Is there any faster way?

Maybe there's a better struct based on std as std::vector<std::vector<std::bitset<256>>> to speed it up too?

Here's a short piece of code for easier understanding of the problem.

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

int main(int argc, char ** argv)
{
  // opencv
  cv::Mat color, gray, descs;

  std::vector<cv::KeyPoint> kpts;

  cv::Ptr<cv::ORB> detector = cv::ORB::create();

  cv::VideoCapture video(argv[1]);

  // my desired datastruct
  using my_desc_t = std::bitset<256>; // one descriptor

  using my_frame_t = std::vector<my_desc_t>; // one frame

  using my_video_t = std::vector<my_frame_t>; // one video

  my_video_t my_video;

  my_video.resize(video.get(CV_CAP_PROP_FRAME_COUNT));

  // processing
  for (size_t i=0,j=video.get(CV_CAP_PROP_FRAME_COUNT); i!=j; ++i)
  {
    if (video.read(color))
    {
      // detect and compute
      cv::cvtColor(color,gray,CV_BGR2GRAY);
      detector->detectAndCompute(gray,cv::Mat(),kpts,descs);

      // fill
      // TODO

      // 8 (uchar) * 32 = 256 bits each orb descriptor.
      // how to use logical operators to copy it
      // as fast as possible into my_video?
    }
  }
}

I compile it with clang++ -std=c++11 experiment.cpp -o experiment -lopencv_core -lopencv_imgproc -lopencv_videoio -lopencv_features2d

Answer 1

Here's the answer

// bitset to Mat:

std::bitset<256> bs(11); // some demo value
Mat m(1,32,CV_8U, reinterpret_cast<uchar*>(&bs)); // 32 bytes

cerr << bs << endl;
cerr << m  << endl;

// Mat to bitset

std::bitset<256> bs2;
memcpy(&bs2, m.data, 32);

cerr << bs2 << endl;