This code is used for simple triangulation. It uses findEssentialMat, recoverPose, triangulatePoints in OpenCV. For viewer, PCL is used. You can watch 3D points and 2 camera poses. I checked alcatraz2.jpg and alcatraz1.jpg in pcv_data.zip (https://www.oreilly.co.jp/pub/9784873116075/). Perhaps there is something wrong ( actually it looks working…

simple_triangulation.cc

#include <opencv2/opencv.hpp>

#include <pcl/common/common_headers.h>
#include <pcl/io/pcd_io.h>
#include <pcl/point_types.h>
#include <pcl/point_cloud.h>
#include <pcl/visualization/pcl_visualizer.h>

#include <Eigen/Core>
#include <Eigen/LU>
#include <Eigen/Geometry>

using namespace std;

int main(int argc, char* argv[]) {
  if(argc != 3) {
    cout << 
      "usage: this.out [/path/to/image1] [path/to/image2] "
      << endl;
    return -1;
  }

  cv::Mat image1 = cv::imread(argv[1]);
  cv::Mat image2 = cv::imread(argv[2]);

  // Camera intristic parameter matrix
  // I did not calibration
  cv::Mat K = (cv::Mat_<float>(3,3) <<  500.f,   0.f, image1.cols / 2.f,
                                          0.f, 500.f, image1.rows / 2.f,
                                          0.f,   0.f,               1.f);

  vector<cv::KeyPoint> kpts_vec1, kpts_vec2;
  cv::Mat desc1, desc2;
  cv::Ptr<cv::AKAZE> akaze = cv::AKAZE::create();

  // extract feature points and calculate descriptors
  akaze -> detectAndCompute(image1, cv::noArray(), kpts_vec1, desc1);
  akaze -> detectAndCompute(image2, cv::noArray(), kpts_vec2, desc2);


  cv::BFMatcher* matcher = new cv::BFMatcher(cv::NORM_L2, false); 
  // cross check flag set to false
  // because i do cross-ratio-test match
  vector< vector<cv::DMatch> > matches_2nn_12, matches_2nn_21;
  matcher->knnMatch( desc1, desc2, matches_2nn_12, 2 );
  matcher->knnMatch( desc2, desc1, matches_2nn_21, 2 );
  const double ratio = 0.8;

  vector<cv::Point2f> selected_points1, selected_points2;

  for(int i = 0; i < matches_2nn_12.size(); i++) { // i is queryIdx
    if( matches_2nn_12[i][0].distance/matches_2nn_12[i][1].distance < ratio 
        and 
        matches_2nn_21[matches_2nn_12[i][0].trainIdx][0].distance 
          / matches_2nn_21[matches_2nn_12[i][0].trainIdx][1].distance < ratio ) 
    {
      if(matches_2nn_21[matches_2nn_12[i][0].trainIdx][0].trainIdx 
            == matches_2nn_12[i][0].queryIdx) 
      {
        selected_points1.push_back(kpts_vec1[matches_2nn_12[i][0].queryIdx].pt);
        selected_points2.push_back(
            kpts_vec2[matches_2nn_21[matches_2nn_12[i][0].trainIdx][0].queryIdx].pt
            );
      }
    }
  }

  if(true) {
    cv::Mat src;
    cv::hconcat(image1, image2, src);
    for(int i = 0; i < selected_points1.size(); i++) {
      cv::line( src, selected_points1[i],
                cv::Point2f(selected_points2[i].x + image1.cols, selected_points2[i].y),
                1, 1, 0 );
    }
    cv::imwrite("match-result.png", src);
  }

  cv::Mat Kd;
  K.convertTo(Kd, CV_64F);

  cv::Mat mask; // unsigned char array
  cv::Mat E = cv::findEssentialMat(selected_points1, selected_points2, Kd.at<double>(0,0),
                           // cv::Point2f(0.f, 0.f),
                           cv::Point2d(image1.cols/2., image1.rows/2.),
                           cv::RANSAC, 0.999, 1.0, mask);
  // E is CV_64F not 32F

  vector<cv::Point2f> inlier_match_points1, inlier_match_points2;
  for(int i = 0; i < mask.rows; i++) {
    if(mask.at<unsigned char>(i)){
      inlier_match_points1.push_back(selected_points1[i]);
      inlier_match_points2.push_back(selected_points2[i]);
    }
  }

  if(true) {
    cv::Mat src;
    cv::hconcat(image1, image2, src);
    for(int i = 0; i < inlier_match_points1.size(); i++) {
      cv::line( src, inlier_match_points1[i],
                cv::Point2f(inlier_match_points2[i].x + image1.cols, inlier_match_points2[i].y),
                1, 1, 0 );
    }
    cv::imwrite("inlier_match_points.png", src);
  }

  mask.release();
  cv::Mat R, t;
  cv::recoverPose(E, 
                  inlier_match_points1,
                  inlier_match_points2, 
                  R, t, Kd.at<double>(0,0), 
                  // cv::Point2f(0, 0),
                  cv::Point2d(image1.cols/2., image1.rows/2.),
                  mask);
  // R,t is CV_64F not 32F

  vector<cv::Point2d> triangulation_points1, triangulation_points2;
  for(int i = 0; i < mask.rows; i++) {
    if(mask.at<unsigned char>(i)){
      triangulation_points1.push_back 
                   (cv::Point2d((double)inlier_match_points1[i].x,(double)inlier_match_points1[i].y));
      triangulation_points2.push_back 
                   (cv::Point2d((double)inlier_match_points2[i].x,(double)inlier_match_points2[i].y));
    }
  }

  if(true) {
    cv::Mat src;
    cv::hconcat(image1, image2, src);
    for(int i = 0; i < triangulation_points1.size(); i++) {
      cv::line( src, triangulation_points1[i],
                cv::Point2f((float)triangulation_points2[i].x + (float)image1.cols,
                            (float)triangulation_points2[i].y),
                1, 1, 0 );
    }
    cv::imwrite("triangulatedPoints.png", src);
  }

  cv::Mat Rt0 = cv::Mat::eye(3, 4, CV_64FC1);
  cv::Mat Rt1 = cv::Mat::eye(3, 4, CV_64FC1);
  R.copyTo(Rt1.rowRange(0,3).colRange(0,3));
  t.copyTo(Rt1.rowRange(0,3).col(3));


  cv::Mat point3d_homo;
  cv::triangulatePoints(Kd * Rt0, Kd * Rt1, 
                        triangulation_points1, triangulation_points2,
                        point3d_homo);
  //point3d_homo is 64F
  //available input type is here
  //https://stackoverflow.com/questions/16295551/how-to-correctly-use-cvtriangulatepoints

  assert(point3d_homo.cols == triangulation_points1.size());

  // prepare a viewer
	pcl::visualization::PCLVisualizer viewer("Viewer");
  viewer.setBackgroundColor (255, 255, 255);

  // create point cloud
	pcl::PointCloud<pcl::PointXYZRGB>::Ptr cloud (new pcl::PointCloud<pcl::PointXYZRGB>);
	cloud->points.resize (point3d_homo.cols);

  for(int i = 0; i < point3d_homo.cols; i++) {
    pcl::PointXYZRGB &point = cloud->points[i];
    cv::Mat p3d;
    cv::Mat _p3h = point3d_homo.col(i);
    convertPointsFromHomogeneous(_p3h.t(), p3d);
    point.x = p3d.at<double>(0);
    point.y = p3d.at<double>(1);
    point.z = p3d.at<double>(2);
    point.r = 0;
    point.g = 0;
    point.b = 255;
  }

  viewer.addPointCloud(cloud, "Triangulated Point Cloud");
  viewer.setPointCloudRenderingProperties (pcl::visualization::PCL_VISUALIZER_POINT_SIZE,
                                            3,
                                            "Triangulated Point Cloud");
  viewer.addCoordinateSystem (1.0);

  // add the second camera pose 
  Eigen::Matrix4f eig_mat;
  Eigen::Affine3f cam_pose;

  R.convertTo(R, CV_32F);
  t.convertTo(t, CV_32F);

  //this shows how a camera moves
  cv::Mat Rinv = R.t(); 
  cv::Mat T = -Rinv * t;

  eig_mat(0,0) = Rinv.at<float>(0,0);eig_mat(0,1) = Rinv.at<float>(0,1);eig_mat(0,2) = Rinv.at<float>(0,2);
  eig_mat(1,0) = Rinv.at<float>(1,0);eig_mat(1,1) = Rinv.at<float>(1,1);eig_mat(1,2) = Rinv.at<float>(1,2);
  eig_mat(2,0) = Rinv.at<float>(2,0);eig_mat(2,1) = Rinv.at<float>(2,1);eig_mat(2,2) = Rinv.at<float>(2,2);
  eig_mat(3,0) = 0.f; eig_mat(3,1) = 0.f; eig_mat(3,2) = 0.f;
  eig_mat(0, 3) = T.at<float>(0);
  eig_mat(1, 3) = T.at<float>(1);
  eig_mat(2, 3) = T.at<float>(2);
  eig_mat(3, 3) = 1.f;

  cam_pose = eig_mat;

  //cam_pose should be Affine3f, Affine3d cannot be used
  viewer.addCoordinateSystem(1.0, cam_pose, "2nd cam");

  viewer.initCameraParameters ();
  while (!viewer.wasStopped ()) {
    viewer.spin();
  }

  return 0;
}