Cluster Recognition and 6DOF Pose Estimation using VFH descriptors

As previously described in Estimating VFH signatures for a set of points, Viewpoint Feature Histograms (VFH) are powerful meta-local descriptors, created for the purpose of recognition and pose estimation for clusters of points. We here refer to a cluster as a collection of 3D points, most of the time representing a particular object or part of a scene, obtained through some segmentation or detection mechanisms (please see Euclidean Cluster Extraction for an example).

Our goal here is not to provide an ultimate recognition tool, but rather a mechanism for obtaining candidates that could potentially be the cluster/object that is searched for, together with its 6DOF pose in space. With this in mind, we will be formulating the recognition problem as a nearest neighbor estimation problem. So given a set of training data, we will use efficient nearest neighbor search structures such as kd-trees and return a set of potential candidates with sorted distances to the query object, rather than an absolute “this is the object that we were searching for” kind of response. The reader can imagine that such a system becomes much more useful as we can explicitly reason about failures (false positives, or true negatives).

For the purpose of this tutorial, the application example could be formulated as follows:

  • Training stage:

    • given a scene with 1 object that is easily separable as a cluster;

    • use a ground-truth system to obtain its pose (see the discussion below);

    • rotate around the object or rotate the object with respect to the camera, and compute a VFH descriptor for each view;

    • store the views, and build a kd-tree representation.

  • Testing stage:

    • given a scene with objects that can be separated as individual clusters, first extract the clusters;

    • for each cluster, compute a VFH descriptor from the current camera position;

    • use the VFH descriptor to search for candidates in the trained kd-tree.

We hope the above makes sense. Basically we’re first going to create the set of objects that we try to later on recognize, and then we will use that to obtain valid candidates for objects in the scene.

A good example of a ground-truth system could be a simple rotating pan-tilt unit such as the one in the figure below. Placing an object on the unit, and moving it with some increments in both horizontal and vertical, can result in a perfect ground-truth system for small objects. A cheaper solution could be to use a marker-based system (e.g., checkerboard) and rotate the camera/table manually.

_images/pan_tilt.jpg

Our Kd-Tree implementation of choice for the purpose of this tutorial is of course, FLANN.

Training

We begin the training by assuming that the objects are already separated as individual clusters (see Euclidean Cluster Extraction), as shown in the figure below:

_images/scene_raw.jpg _images/scene_segmented.jpg

Since we’re only trying to cover the explicit training/testing of VFH signatures in this tutorial, we provide a set of datasets already collected at: vfh_recognition_tutorial_data.tbz. The data is a subset of the objects presented in the figure below (left), and look like the point clouds on the right. We used the pan-tilt table shown above to acquire the data.

_images/objects.jpg _images/training.jpg

Next, copy and paste the following code into your editor and save it as build_tree.cpp.

  1#include <pcl/point_types.h>
  2#include <pcl/point_cloud.h>
  3#include <pcl/common/pcl_filesystem.h>
  4#include <pcl/console/print.h>
  5#include <pcl/io/pcd_io.h>
  6
  7#include <flann/flann.h>
  8#include <flann/io/hdf5.h>
  9#include <fstream>
 10
 11
 12typedef std::pair<std::string, std::vector<float> > vfh_model;
 13
 14/** \brief Loads an n-D histogram file as a VFH signature
 15  * \param path the input file name
 16  * \param vfh the resultant VFH model
 17  */
 18bool
 19loadHist (const pcl_fs::path &path, vfh_model &vfh)
 20{
 21  int vfh_idx;
 22  // Load the file as a PCD
 23  try
 24  {
 25    pcl::PCLPointCloud2 cloud;
 26    int version;
 27    Eigen::Vector4f origin;
 28    Eigen::Quaternionf orientation;
 29    pcl::PCDReader r;
 30    int type; unsigned int idx;
 31    r.readHeader (path.string (), cloud, origin, orientation, version, type, idx);
 32
 33    vfh_idx = pcl::getFieldIndex (cloud, "vfh");
 34    if (vfh_idx == -1)
 35      return (false);
 36    if ((int)cloud.width * cloud.height != 1)
 37      return (false);
 38  }
 39  catch (const pcl::InvalidConversionException&)
 40  {
 41    return (false);
 42  }
 43
 44  // Treat the VFH signature as a single Point Cloud
 45  pcl::PointCloud <pcl::VFHSignature308> point;
 46  pcl::io::loadPCDFile (path.string (), point);
 47  vfh.second.resize (308);
 48
 49  std::vector <pcl::PCLPointField> fields;
 50  pcl::getFieldIndex<pcl::VFHSignature308> ("vfh", fields);
 51
 52  for (std::size_t i = 0; i < fields[vfh_idx].count; ++i)
 53  {
 54    vfh.second[i] = point[0].histogram[i];
 55  }
 56  vfh.first = path.string ();
 57  return (true);
 58}
 59
 60/** \brief Load a set of VFH features that will act as the model (training data)
 61  * \param argc the number of arguments (pass from main ())
 62  * \param argv the actual command line arguments (pass from main ())
 63  * \param extension the file extension containing the VFH features
 64  * \param models the resultant vector of histogram models
 65  */
 66void
 67loadFeatureModels (const pcl_fs::path &base_dir, const std::string &extension, 
 68                   std::vector<vfh_model> &models)
 69{
 70  if (!pcl_fs::exists (base_dir) && !pcl_fs::is_directory (base_dir))
 71    return;
 72
 73  for (pcl_fs::directory_iterator it (base_dir); it != pcl_fs::directory_iterator (); ++it)
 74  {
 75    if (pcl_fs::is_directory (it->status ()))
 76    {
 77      std::stringstream ss;
 78      ss << it->path ();
 79      pcl::console::print_highlight ("Loading %s (%lu models loaded so far).\n", ss.str ().c_str (), (unsigned long)models.size ());
 80      loadFeatureModels (it->path (), extension, models);
 81    }
 82    if (pcl_fs::is_regular_file (it->status ()) && it->path ().extension ().string () == extension)
 83    {
 84      vfh_model m;
 85      if (loadHist (base_dir / it->path ().filename (), m))
 86        models.push_back (m);
 87    }
 88  }
 89}
 90
 91int
 92main (int argc, char** argv)
 93{
 94  if (argc < 2)
 95  {
 96    PCL_ERROR ("Need at least two parameters! Syntax is: %s [model_directory] [options]\n", argv[0]);
 97    return (-1);
 98  }
 99
100  std::string extension (".pcd");
101  transform (extension.begin (), extension.end (), extension.begin (), (int(*)(int))tolower);
102
103  std::string kdtree_idx_file_name = "kdtree.idx";
104  std::string training_data_h5_file_name = "training_data.h5";
105  std::string training_data_list_file_name = "training_data.list";
106
107  std::vector<vfh_model> models;
108
109  // Load the model histograms
110  loadFeatureModels (argv[1], extension, models);
111  pcl::console::print_highlight ("Loaded %d VFH models. Creating training data %s/%s.\n", 
112      (int)models.size (), training_data_h5_file_name.c_str (), training_data_list_file_name.c_str ());
113
114  // Convert data into FLANN format
115  flann::Matrix<float> data (new float[models.size () * models[0].second.size ()], models.size (), models[0].second.size ());
116
117  for (std::size_t i = 0; i < data.rows; ++i)
118    for (std::size_t j = 0; j < data.cols; ++j)
119      data[i][j] = models[i].second[j];
120
121  // Save data to disk (list of models)
122  flann::save_to_file (data, training_data_h5_file_name, "training_data");
123  std::ofstream fs;
124  fs.open (training_data_list_file_name.c_str ());
125  for (std::size_t i = 0; i < models.size (); ++i)
126    fs << models[i].first << "\n";
127  fs.close ();
128 
129  // Build the tree index and save it to disk
130  pcl::console::print_error ("Building the kdtree index (%s) for %d elements...\n", kdtree_idx_file_name.c_str (), (int)data.rows);
131  flann::Index<flann::ChiSquareDistance<float> > index (data, flann::LinearIndexParams ());
132  //flann::Index<flann::ChiSquareDistance<float> > index (data, flann::KDTreeIndexParams (4));
133  index.buildIndex ();
134  index.save (kdtree_idx_file_name);
135  delete[] data.ptr ();
136
137  return (0);
138}

In the following paragraphs we will explain what the above code does (or should do). We’ll begin with the main function.

We begin by loading a set of feature models from a directory given as the first command line argument (see details for running the example below). The loadFeatureModels method does nothing but recursively traverse a set of directories and subdirectories, and loads in all .PCD files it finds. In loadFeatureModels, we call loadHist, which will attempt to open each PCD file found, read its header, and check whether it contains a VFH signature or not. Together with the VFH signature we also store the PCD file name into a vfh_model pair.

Once all VFH features have been loaded, we convert them to FLANN format, using:

  // Convert data into FLANN format
  flann::Matrix<float> data (new float[models.size () * models[0].second.size ()], models.size (), models[0].second.size ());

  for (std::size_t i = 0; i < data.rows; ++i)
    for (std::size_t j = 0; j < data.cols; ++j)
      data[i][j] = models[i].second[j];

Since we’re lazy, and we want to use this data (and not reload it again by crawling the directory structure in the testing phase), we dump the data to disk:

  // Save data to disk (list of models)
  flann::save_to_file (data, training_data_h5_file_name, "training_data");
  std::ofstream fs;
  fs.open (training_data_list_file_name.c_str ());
  for (std::size_t i = 0; i < models.size (); ++i)
    fs << models[i].first << "\n";
  fs.close ();

Finally, we create the KdTree, and save its structure to disk:

  pcl::console::print_error ("Building the kdtree index (%s) for %d elements...\n", kdtree_idx_file_name.c_str (), (int)data.rows);
  flann::Index<flann::ChiSquareDistance<float> > index (data, flann::LinearIndexParams ());
  //flann::Index<flann::ChiSquareDistance<float> > index (data, flann::KDTreeIndexParams (4));
  index.buildIndex ();
  index.save (kdtree_idx_file_name);

Here we will use a LinearIndex, which does a brute-force search using a Chi-Square distance metric (see [VFH] for more information). For building a proper kd-tree, comment line 1 and uncomment line 2 in the code snippet above. The most important difference between a LinearIndex and a KDTreeIndex in FLANN is that the KDTree will be much faster, while producing approximate nearest neighbor results, rather than absolute.

So, we’re done with training. To summarize:

  1. we crawled a directory structure, looked at all the .PCD files we found, tested them whether they are VFH signatures and loaded them in memory;

  2. we converted the data into FLANN format and dumped it to disk;

  3. we built a kd-tree structure and dumped it to disk.

Testing

In the testing phase, we will illustrate how the system works by randomly loading one of the files used in the training phase (feel free to supply your own file here!), and checking the results of the tree.

Begin by copying and pasting the following code into your editor and save it as nearest_neighbors.cpp.

  1#include <pcl/point_types.h>
  2#include <pcl/point_cloud.h>
  3#include <pcl/common/common.h>
  4#include <pcl/common/centroid.h> // for compute3DCentroid
  5#include <pcl/visualization/pcl_visualizer.h>
  6#include <pcl/common/pcl_filesystem.h>
  7#include <pcl/console/parse.h>
  8#include <pcl/console/print.h>
  9#include <pcl/io/pcd_io.h>
 10#include <iostream>
 11#include <limits>
 12#include <flann/flann.h>
 13#include <flann/io/hdf5.h>
 14
 15#include <boost/algorithm/string/replace.hpp> // for replace_last
 16typedef std::pair<std::string, std::vector<float> > vfh_model;
 17
 18/** \brief Loads an n-D histogram file as a VFH signature
 19  * \param path the input file name
 20  * \param vfh the resultant VFH model
 21  */
 22bool
 23loadHist (const pcl_fs::path &path, vfh_model &vfh)
 24{
 25  int vfh_idx;
 26  // Load the file as a PCD
 27  try
 28  {
 29    pcl::PCLPointCloud2 cloud;
 30    int version;
 31    Eigen::Vector4f origin;
 32    Eigen::Quaternionf orientation;
 33    pcl::PCDReader r;
 34    int type; unsigned int idx;
 35    r.readHeader (path.string (), cloud, origin, orientation, version, type, idx);
 36
 37    vfh_idx = pcl::getFieldIndex (cloud, "vfh");
 38    if (vfh_idx == -1)
 39      return (false);
 40    if ((int)cloud.width * cloud.height != 1)
 41      return (false);
 42  }
 43  catch (const pcl::InvalidConversionException&)
 44  {
 45    return (false);
 46  }
 47
 48  // Treat the VFH signature as a single Point Cloud
 49  pcl::PointCloud <pcl::VFHSignature308> point;
 50  pcl::io::loadPCDFile (path.string (), point);
 51  vfh.second.resize (308);
 52
 53  std::vector <pcl::PCLPointField> fields;
 54  pcl::getFieldIndex<pcl::VFHSignature308> ("vfh", fields);
 55
 56  for (std::size_t i = 0; i < fields[vfh_idx].count; ++i)
 57  {
 58    vfh.second[i] = point[0].histogram[i];
 59  }
 60  vfh.first = path.string ();
 61  return (true);
 62}
 63
 64
 65/** \brief Search for the closest k neighbors
 66  * \param index the tree
 67  * \param model the query model
 68  * \param k the number of neighbors to search for
 69  * \param indices the resultant neighbor indices
 70  * \param distances the resultant neighbor distances
 71  */
 72inline void
 73nearestKSearch (flann::Index<flann::ChiSquareDistance<float> > &index, const vfh_model &model, 
 74                int k, flann::Matrix<int> &indices, flann::Matrix<float> &distances)
 75{
 76  // Query point
 77  flann::Matrix<float> p = flann::Matrix<float>(new float[model.second.size ()], 1, model.second.size ());
 78  memcpy (&p.ptr ()[0], &model.second[0], p.cols * p.rows * sizeof (float));
 79
 80  indices = flann::Matrix<int>(new int[k], 1, k);
 81  distances = flann::Matrix<float>(new float[k], 1, k);
 82  index.knnSearch (p, indices, distances, k, flann::SearchParams (512));
 83  delete[] p.ptr ();
 84}
 85
 86/** \brief Load the list of file model names from an ASCII file
 87  * \param models the resultant list of model name
 88  * \param filename the input file name
 89  */
 90bool
 91loadFileList (std::vector<vfh_model> &models, const std::string &filename)
 92{
 93  std::ifstream fs;
 94  fs.open (filename.c_str ());
 95  if (!fs.is_open () || fs.fail ())
 96    return (false);
 97
 98  std::string line;
 99  while (!fs.eof ())
100  {
101    std::getline (fs, line);
102    if (line.empty ())
103      continue;
104    vfh_model m;
105    m.first = line;
106    models.push_back (m);
107  }
108  fs.close ();
109  return (true);
110}
111
112int
113main (int argc, char** argv)
114{
115  int k = 6;
116
117  double thresh = std::numeric_limits<double>::max();     // No threshold, disabled by default
118
119  if (argc < 2)
120  {
121    pcl::console::print_error 
122      ("Need at least three parameters! Syntax is: %s <query_vfh_model.pcd> [options] {kdtree.idx} {training_data.h5} {training_data.list}\n", argv[0]);
123    pcl::console::print_info ("    where [options] are:  -k      = number of nearest neighbors to search for in the tree (default: "); 
124    pcl::console::print_value ("%d", k); pcl::console::print_info (")\n");
125    pcl::console::print_info ("                          -thresh = maximum distance threshold for a model to be considered VALID (default: "); 
126    pcl::console::print_value ("%f", thresh); pcl::console::print_info (")\n\n");
127    return (-1);
128  }
129
130  std::string extension (".pcd");
131  transform (extension.begin (), extension.end (), extension.begin (), (int(*)(int))tolower);
132
133  // Load the test histogram
134  std::vector<int> pcd_indices = pcl::console::parse_file_extension_argument (argc, argv, ".pcd");
135  vfh_model histogram;
136  if (!loadHist (argv[pcd_indices.at (0)], histogram))
137  {
138    pcl::console::print_error ("Cannot load test file %s\n", argv[pcd_indices.at (0)]);
139    return (-1);
140  }
141
142  pcl::console::parse_argument (argc, argv, "-thresh", thresh);
143  // Search for the k closest matches
144  pcl::console::parse_argument (argc, argv, "-k", k);
145  pcl::console::print_highlight ("Using "); pcl::console::print_value ("%d", k); pcl::console::print_info (" nearest neighbors.\n");
146
147  std::string kdtree_idx_file_name = "kdtree.idx";
148  std::string training_data_h5_file_name = "training_data.h5";
149  std::string training_data_list_file_name = "training_data.list";
150
151  std::vector<vfh_model> models;
152  flann::Matrix<int> k_indices;
153  flann::Matrix<float> k_distances;
154  flann::Matrix<float> data;
155  // Check if the data has already been saved to disk
156  if (!pcl_fs::exists ("training_data.h5") || !pcl_fs::exists ("training_data.list"))
157  {
158    pcl::console::print_error ("Could not find training data models files %s and %s!\n", 
159        training_data_h5_file_name.c_str (), training_data_list_file_name.c_str ());
160    return (-1);
161  }
162  else
163  {
164    loadFileList (models, training_data_list_file_name);
165    flann::load_from_file (data, training_data_h5_file_name, "training_data");
166    pcl::console::print_highlight ("Training data found. Loaded %d VFH models from %s/%s.\n", 
167        (int)data.rows, training_data_h5_file_name.c_str (), training_data_list_file_name.c_str ());
168  }
169
170  // Check if the tree index has already been saved to disk
171  if (!pcl_fs::exists (kdtree_idx_file_name))
172  {
173    pcl::console::print_error ("Could not find kd-tree index in file %s!", kdtree_idx_file_name.c_str ());
174    return (-1);
175  }
176  else
177  {
178    flann::Index<flann::ChiSquareDistance<float> > index (data, flann::SavedIndexParams ("kdtree.idx"));
179    index.buildIndex ();
180    nearestKSearch (index, histogram, k, k_indices, k_distances);
181  }
182
183  // Output the results on screen
184  pcl::console::print_highlight ("The closest %d neighbors for %s are:\n", k, argv[pcd_indices[0]]);
185  for (int i = 0; i < k; ++i)
186    pcl::console::print_info ("    %d - %s (%d) with a distance of: %f\n", 
187        i, models.at (k_indices[0][i]).first.c_str (), k_indices[0][i], k_distances[0][i]);
188
189  // Load the results
190  pcl::visualization::PCLVisualizer p (argc, argv, "VFH Cluster Classifier");
191  int y_s = (int)std::floor (sqrt ((double)k));
192  int x_s = y_s + (int)std::ceil ((k / (double)y_s) - y_s);
193  double x_step = (double)(1 / (double)x_s);
194  double y_step = (double)(1 / (double)y_s);
195  pcl::console::print_highlight ("Preparing to load "); 
196  pcl::console::print_value ("%d", k); 
197  pcl::console::print_info (" files ("); 
198  pcl::console::print_value ("%d", x_s);    
199  pcl::console::print_info ("x"); 
200  pcl::console::print_value ("%d", y_s); 
201  pcl::console::print_info (" / ");
202  pcl::console::print_value ("%f", x_step); 
203  pcl::console::print_info ("x"); 
204  pcl::console::print_value ("%f", y_step); 
205  pcl::console::print_info (")\n");
206
207  int viewport = 0, l = 0, m = 0;
208  for (int i = 0; i < k; ++i)
209  {
210    std::string cloud_name = models.at (k_indices[0][i]).first;
211    boost::replace_last (cloud_name, "_vfh", "");
212
213    p.createViewPort (l * x_step, m * y_step, (l + 1) * x_step, (m + 1) * y_step, viewport);
214    l++;
215    if (l >= x_s)
216    {
217      l = 0;
218      m++;
219    }
220
221    pcl::PCLPointCloud2 cloud;
222    pcl::console::print_highlight (stderr, "Loading "); pcl::console::print_value (stderr, "%s ", cloud_name.c_str ());
223    if (pcl::io::loadPCDFile (cloud_name, cloud) == -1)
224      break;
225
226    // Convert from blob to PointCloud
227    pcl::PointCloud<pcl::PointXYZ> cloud_xyz;
228    pcl::fromPCLPointCloud2 (cloud, cloud_xyz);
229
230    if (cloud_xyz.size () == 0)
231      break;
232
233    pcl::console::print_info ("[done, "); 
234    pcl::console::print_value ("%zu", static_cast<std::size_t>(cloud_xyz.size ()));
235    pcl::console::print_info (" points]\n");
236    pcl::console::print_info ("Available dimensions: "); 
237    pcl::console::print_value ("%s\n", pcl::getFieldsList (cloud).c_str ());
238
239    // Demean the cloud
240    Eigen::Vector4f centroid;
241    pcl::compute3DCentroid (cloud_xyz, centroid);
242    pcl::PointCloud<pcl::PointXYZ>::Ptr cloud_xyz_demean (new pcl::PointCloud<pcl::PointXYZ>);
243    pcl::demeanPointCloud<pcl::PointXYZ> (cloud_xyz, centroid, *cloud_xyz_demean);
244    // Add to renderer*
245    p.addPointCloud (cloud_xyz_demean, cloud_name, viewport);
246    
247    // Check if the model found is within our inlier tolerance
248    std::stringstream ss;
249    ss << k_distances[0][i];
250    if (k_distances[0][i] > thresh)
251    {
252      p.addText (ss.str (), 20, 30, 1, 0, 0, ss.str (), viewport);  // display the text with red
253
254      // Create a red line
255      pcl::PointXYZ min_p, max_p;
256      pcl::getMinMax3D (*cloud_xyz_demean, min_p, max_p);
257      std::stringstream line_name;
258      line_name << "line_" << i;
259      p.addLine (min_p, max_p, 1, 0, 0, line_name.str (), viewport);
260      p.setShapeRenderingProperties (pcl::visualization::PCL_VISUALIZER_LINE_WIDTH, 5, line_name.str (), viewport);
261    }
262    else
263      p.addText (ss.str (), 20, 30, 0, 1, 0, ss.str (), viewport);
264
265    // Increase the font size for the score*
266    p.setShapeRenderingProperties (pcl::visualization::PCL_VISUALIZER_FONT_SIZE, 18, ss.str (), viewport);
267
268    // Add the cluster name
269    p.addText (cloud_name, 20, 10, cloud_name, viewport);
270  }
271  // Add coordinate systems to all viewports
272  p.addCoordinateSystem (0.1, "global", 0);
273
274  p.spin ();
275  return (0);
276}

The above code snippet is slightly larger, because we also included some visualization routines and some other “eye candy” stuff.

In lines:

  std::vector<int> pcd_indices = pcl::console::parse_file_extension_argument (argc, argv, ".pcd");
  vfh_model histogram;
  if (!loadHist (argv[pcd_indices.at (0)], histogram))
  {
    pcl::console::print_error ("Cannot load test file %s\n", argv[pcd_indices.at (0)]);
    return (-1);
  }

  pcl::console::parse_argument (argc, argv, "-thresh", thresh);
  // Search for the k closest matches
  pcl::console::parse_argument (argc, argv, "-k", k);
  pcl::console::print_highlight ("Using "); pcl::console::print_value ("%d", k); pcl::console::print_info (" nearest neighbors.\n");

we load the first given user histogram (and ignore the rest). Then we proceed at checking two command line parameters, namely -k which will define how many nearest neighbors to check and display on screen, and -thresh which defines a maximum distance metric after which we will start displaying red lines (i.e., crossing) over the k models found on screen (eye candy!).

In lines:

    loadFileList (models, training_data_list_file_name);
    flann::load_from_file (data, training_data_h5_file_name, "training_data");

we load the training data from disk, together with the list of file names that we previously stored in build_tree.cpp. Then, we read the kd-tree and rebuild the index:

    flann::Index<flann::ChiSquareDistance<float> > index (data, flann::SavedIndexParams ("kdtree.idx"));
    index.buildIndex ();

Here we need to make sure that we use the exact distance metric (ChiSquareDistance in this case), as the one that we used while creating the tree. The most important part of the code comes here:

    nearestKSearch (index, histogram, k, k_indices, k_distances);

Inside nearestKSearch, we first convert the query point to FLANN format:

  flann::Matrix<float> p = flann::Matrix<float>(new float[model.second.size ()], 1, model.second.size ());
  memcpy (&p.ptr ()[0], &model.second[0], p.cols * p.rows * sizeof (float));

Followed by obtaining the resultant nearest neighbor indices and distances for the query in:

  indices = flann::Matrix<int>(new int[k], 1, k);
  distances = flann::Matrix<float>(new float[k], 1, k);
  index.knnSearch (p, indices, distances, k, flann::SearchParams (512));

Lines:

    index.buildIndex ();
    nearestKSearch (index, histogram, k, k_indices, k_distances);
  }

  // Output the results on screen
  pcl::console::print_highlight ("The closest %d neighbors for %s are:\n", k, argv[pcd_indices[0]]);
  for (int i = 0; i < k; ++i)
    pcl::console::print_info ("    %d - %s (%d) with a distance of: %f\n", 
        i, models.at (k_indices[0][i]).first.c_str (), k_indices[0][i], k_distances[0][i]);

  // Load the results
  pcl::visualization::PCLVisualizer p (argc, argv, "VFH Cluster Classifier");
  int y_s = (int)std::floor (sqrt ((double)k));
  int x_s = y_s + (int)std::ceil ((k / (double)y_s) - y_s);
  double x_step = (double)(1 / (double)x_s);

create a PCLVisualizer object, and sets up a set of different viewports (e.g., splits the screen into different chunks), which will be enabled in:

    p.createViewPort (l * x_step, m * y_step, (l + 1) * x_step, (m + 1) * y_step, viewport);

Using the file names representing the models that we previously obtained in loadFileList, we proceed at loading the model file names using:

    pcl::PCLPointCloud2 cloud;
    pcl::console::print_highlight (stderr, "Loading "); pcl::console::print_value (stderr, "%s ", cloud_name.c_str ());
    if (pcl::io::loadPCDFile (cloud_name, cloud) == -1)
      break;

    // Convert from blob to PointCloud
    pcl::PointCloud<pcl::PointXYZ> cloud_xyz;
    pcl::fromPCLPointCloud2 (cloud, cloud_xyz);

For visualization purposes, we demean the point cloud by computing its centroid and then subtracting it:

    Eigen::Vector4f centroid;
    pcl::compute3DCentroid (cloud_xyz, centroid);
    pcl::PointCloud<pcl::PointXYZ>::Ptr cloud_xyz_demean (new pcl::PointCloud<pcl::PointXYZ>);
    pcl::demeanPointCloud<pcl::PointXYZ> (cloud_xyz, centroid, *cloud_xyz_demean);
    // Add to renderer*
    p.addPointCloud (cloud_xyz_demean, cloud_name, viewport);

Finally we check if the distance obtained by nearestKSearch is larger than the user given threshold, and if it is, we display a red line over the cloud that is being rendered in the viewport:

      // Create a red line
      pcl::PointXYZ min_p, max_p;
      pcl::getMinMax3D (*cloud_xyz_demean, min_p, max_p);
      std::stringstream line_name;
      line_name << "line_" << i;
      p.addLine (min_p, max_p, 1, 0, 0, line_name.str (), viewport);
      p.setShapeRenderingProperties (pcl::visualization::PCL_VISUALIZER_LINE_WIDTH, 5, line_name.str (), viewport);

Compiling and running the code

Create a new CMakeLists.txt file, and put the following content into it

 1cmake_minimum_required(VERSION 3.5 FATAL_ERROR)
 2
 3project(vfh_cluster_classifier)
 4
 5find_package(PCL 1.2 REQUIRED)
 6include_directories(${PCL_INCLUDE_DIRS})
 7link_directories(${PCL_LIBRARY_DIRS})
 8add_definitions(${PCL_DEFINITIONS})
 9
10find_package(HDF5 REQUIRED)
11find_package(FLANN REQUIRED)
12
13include_directories(SYSTEM
14  ${HDF5_INCLUDE_DIR}
15)
16
17add_executable(build_tree build_tree.cpp)
18target_link_libraries(build_tree ${PCL_LIBRARIES} ${Boost_LIBRARIES}
19                                 FLANN::FLANN ${HDF5_LIBRARIES})
20
21add_executable(nearest_neighbors nearest_neighbors.cpp)
22target_link_libraries(nearest_neighbors ${PCL_LIBRARIES} ${Boost_LIBRARIES} FLANN::FLANN ${HDF5_LIBRARIES})

Note

If you are running this tutorial on Windows, you have to install (HDF5 1.8.7 Shared Library). If CMake is not able to find HDF5, you can manually supply the include directory in HDF5_INCLUDE_DIR variable and the full path of hdf5dll.lib in HDF5_hdf5_LIBRARY variable. Make sure that the needed dlls are in the same folder as the executables.

The above assumes that your two source files (build_tree.cpp and nearest_neighbors.cpp) are stored into the src/ subdirectory.

Then, make sure that the datasets you downloaded (vfh_recognition_tutorial_data.tbz) are unpacked in this directory, thus creating a data/ subdirectory.

After you have made the executable, you can run them like so:

$ ./build/build_tree data/

You should see the following output on screen:

> Loading data/001.324.25 (0 models loaded so far).
> Loading data/800.919.49 (13 models loaded so far).
> Loading data/100.922.16 (27 models loaded so far).
> Loading data/901.125.07 (47 models loaded so far).
> Loading data/000.580.67 (65 models loaded so far).
> Loading data/463.156.00 (81 models loaded so far).
> Loading data/401.431.44 (97 models loaded so far).
> Loading data/100.919.00 (113 models loaded so far).
> Loading data/401.324.52 (134 models loaded so far).
> Loading data/201.327.78 (150 models loaded so far).
> Loading data/300.151.23 (166 models loaded so far).
> Loading data/200.921.07 (180 models loaded so far).
> Loaded 195 VFH models. Creating training data training_data.h5/training_data.list.
Building the kdtree index (kdtree.idx) for 195 elements...

The above crawled the data/ subdirectory, and created a kd-tree with 195 entries. To run the nearest neighbor testing example, you have two options:

  1. Either run the following command manually, and select one of the datasets that we provided as a testing sample, like this:

    ./build/nearest_neighbors -k 16 -thresh 50 data/000.580.67/1258730231333_cluster_0_nxyz_vfh.pcd

  2. Or, if you are on a linux system, you can place the following on a bash script file (e.g., test.sh):

    #!/bin/bash

# Example directory containing _vfh.pcd files
DATA=data

# Inlier distance threshold
thresh=50

# Get the closest K nearest neighbors
k=16

for i in `find $DATA -type d -name "*"`
do
  echo $i
  for j in `find $i -type f \( -iname "*cluster*_vfh.pcd" \) | sort -R`
  do
    echo $j
    ./build/nearest_neighbors -k $k -thresh $thresh $j -cam "0.403137,0.868471/0,0,0/-0.0932051,-0.201608,-0.518939/-0.00471487,-0.931831,0.362863/1464,764/6,72"
  done
done

and run the script like this:

bash test.sh

You should see recognition examples like the ones shown below:

_images/vfh_example1.jpg _images/vfh_example2.jpg _images/vfh_example3.jpg