Check out example codes for "PCL RANSAC". It will help you in understanding the concepts better.

Code Example 1

cmake_minimum_required(VERSION 2.8 FATAL_ERROR)

project(random_sample_consensus)

find_package(PCL 1.2 REQUIRED)

include_directories(${PCL_INCLUDE_DIRS})
link_directories(${PCL_LIBRARY_DIRS})
add_definitions(${PCL_DEFINITIONS})

add_executable (random_sample_consensus random_sample_consensus.cpp)
target_link_libraries (random_sample_consensus ${PCL_LIBRARIES})

Code Example 2

#include <iostream>
#include <thread>

#include <pcl/console/parse.h>
#include <pcl/filters/extract_indices.h>
#include <pcl/io/pcd_io.h>
#include <pcl/point_types.h>
#include <pcl/sample_consensus/ransac.h>
#include <pcl/sample_consensus/sac_model_plane.h>
#include <pcl/sample_consensus/sac_model_sphere.h>
#include <pcl/visualization/pcl_visualizer.h>

using namespace std::chrono_literals;

pcl::visualization::PCLVisualizer::Ptr
simpleVis (pcl::PointCloud<pcl::PointXYZ>::ConstPtr cloud)
{
  // --------------------------------------------
  // -----Open 3D viewer and add point cloud-----
  // --------------------------------------------
  pcl::visualization::PCLVisualizer::Ptr viewer (new pcl::visualization::PCLVisualizer ("3D Viewer"));
  viewer->setBackgroundColor (0, 0, 0);
  viewer->addPointCloud<pcl::PointXYZ> (cloud, "sample cloud");
  viewer->setPointCloudRenderingProperties (pcl::visualization::PCL_VISUALIZER_POINT_SIZE, 3, "sample cloud");
  //viewer->addCoordinateSystem (1.0, "global");
  viewer->initCameraParameters ();
  return (viewer);
}

int
main(int argc, char** argv)
{
  // initialize PointClouds
  pcl::PointCloud<pcl::PointXYZ>::Ptr cloud (new pcl::PointCloud<pcl::PointXYZ>);
  pcl::PointCloud<pcl::PointXYZ>::Ptr final (new pcl::PointCloud<pcl::PointXYZ>);

  // populate our PointCloud with points
  cloud->width    = 500;
  cloud->height   = 1;
  cloud->is_dense = false;
  cloud->points.resize (cloud->width * cloud->height);
  for (pcl::index_t i = 0; i < cloud->size (); ++i)
  {
    if (pcl::console::find_argument (argc, argv, "-s") >= 0 || pcl::console::find_argument (argc, argv, "-sf") >= 0)
    {
      (*cloud)[i].x = 1024 * rand () / (RAND_MAX + 1.0);
      (*cloud)[i].y = 1024 * rand () / (RAND_MAX + 1.0);
      if (i % 5 == 0)
        (*cloud)[i].z = 1024 * rand () / (RAND_MAX + 1.0);
      else if(i % 2 == 0)
        (*cloud)[i].z =  sqrt( 1 - ((*cloud)[i].x * (*cloud)[i].x)
                                      - ((*cloud)[i].y * (*cloud)[i].y));
      else
        (*cloud)[i].z =  - sqrt( 1 - ((*cloud)[i].x * (*cloud)[i].x)
                                        - ((*cloud)[i].y * (*cloud)[i].y));
    }
    else
    {
      (*cloud)[i].x = 1024 * rand () / (RAND_MAX + 1.0);
      (*cloud)[i].y = 1024 * rand () / (RAND_MAX + 1.0);
      if( i % 2 == 0)
        (*cloud)[i].z = 1024 * rand () / (RAND_MAX + 1.0);
      else
        (*cloud)[i].z = -1 * ((*cloud)[i].x + (*cloud)[i].y);
    }
  }

  std::vector<int> inliers;

  // created RandomSampleConsensus object and compute the appropriated model
  pcl::SampleConsensusModelSphere<pcl::PointXYZ>::Ptr
    model_s(new pcl::SampleConsensusModelSphere<pcl::PointXYZ> (cloud));
  pcl::SampleConsensusModelPlane<pcl::PointXYZ>::Ptr
    model_p (new pcl::SampleConsensusModelPlane<pcl::PointXYZ> (cloud));
  if(pcl::console::find_argument (argc, argv, "-f") >= 0)
  {
    pcl::RandomSampleConsensus<pcl::PointXYZ> ransac (model_p);
    ransac.setDistanceThreshold (.01);
    ransac.computeModel();
    ransac.getInliers(inliers);
  }
  else if (pcl::console::find_argument (argc, argv, "-sf") >= 0 )
  {
    pcl::RandomSampleConsensus<pcl::PointXYZ> ransac (model_s);
    ransac.setDistanceThreshold (.01);
    ransac.computeModel();
    ransac.getInliers(inliers);
  }

  // copies all inliers of the model computed to another PointCloud
  pcl::copyPointCloud (*cloud, inliers, *final);

  // creates the visualization object and adds either our original cloud or all of the inliers
  // depending on the command line arguments specified.
  pcl::visualization::PCLVisualizer::Ptr viewer;
  if (pcl::console::find_argument (argc, argv, "-f") >= 0 || pcl::console::find_argument (argc, argv, "-sf") >= 0)
    viewer = simpleVis(final);
  else
    viewer = simpleVis(cloud);
  while (!viewer->wasStopped ())
  {
    viewer->spinOnce (100);
    std::this_thread::sleep_for(100ms);
  }
  return 0;
 }

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