Tutorial: Connected-components labeling

Table of Contents

• Introduction
• Example code
• Next tutorial

Introduction

This tutorial will show you how to perform a connected-components labeling.

Example code

The corresponding code is available in tutorial-connected-components.cpp:

#include <cstdlib>
#include <iostream>
#include <visp3/core/vpImage.h>
#include <visp3/gui/vpDisplayGDI.h>
#include <visp3/gui/vpDisplayOpenCV.h>
#include <visp3/gui/vpDisplayX.h>
#include <visp3/io/vpImageIo.h>

#if defined(VISP_HAVE_MODULE_IMGPROC)
#include <visp3/imgproc/vpImgproc.h>
#endif

int main(int argc, const char **argv)
{
#if defined(VISP_HAVE_MODULE_IMGPROC) && (defined(VISP_HAVE_X11) || defined(VISP_HAVE_GDI) || defined(VISP_HAVE_OPENCV))

  std::string input_filename = "img.pgm";
  vpImageMorphology::vpConnexityType connexity = vpImageMorphology::CONNEXITY_4;

  for (int i = 1; i < argc; i++) {
    if (std::string(argv[i]) == "--input" && i + 1 < argc) {
      input_filename = std::string(argv[i + 1]);
    } else if (std::string(argv[i]) == "--connexity" && i + 1 < argc) {
      connexity = (vpImageMorphology::vpConnexityType)atoi(argv[i + 1]);
    } else if (std::string(argv[i]) == "--help" || std::string(argv[i]) == "-h") {
      std::cout << "Usage: " << argv[0]
                << " [--input <input image>] [--connexity <0: 4-connexity, "
                   "1: 8-connexity>] [--help]"
                << std::endl;
      return EXIT_SUCCESS;
    }
  }

  vpImage<unsigned char> I;
  vpImageIo::read(I, input_filename);

#ifdef VISP_HAVE_X11
  vpDisplayX d, d2;
#elif defined(VISP_HAVE_GDI)
  vpDisplayGDI d, d2;
#elif defined(VISP_HAVE_OPENCV)
  vpDisplayOpenCV d, d2;
#endif
  d.init(I, 0, 0, "Input image");

  vpImage<int> labels;
  int nbComponents = 0;
  vp::connectedComponents(I, labels, nbComponents, connexity);
  std::cout << "nbComponents=" << nbComponents << std::endl;

  vpImage<vpRGBa> I_conn(I.getHeight(), I.getWidth());
  for (unsigned int i = 0; i < I_conn.getHeight(); i++) {
    for (unsigned int j = 0; j < I_conn.getWidth(); j++) {
      if (labels[i][j] != 0) {
        I_conn[i][j] =
            vpRGBa(vpColor::getColor((unsigned int)labels[i][j]).R, vpColor::getColor((unsigned int)labels[i][j]).G,
                   vpColor::getColor((unsigned int)labels[i][j]).B);
      }
    }
  }
  d2.init(I_conn, I.getWidth(), 10, "Connected components");

  vpDisplay::display(I);
  vpDisplay::display(I_conn);
  vpDisplay::displayText(I_conn, 20, 20, "Click to quit.", vpColor::red);
  vpDisplay::flush(I);
  vpDisplay::flush(I_conn);
  vpDisplay::getClick(I_conn);

  return EXIT_SUCCESS;
#else
  (void)argc;
  (void)argv;
  return 0;
#endif
}

The function is provided in a vp:: namespace and accessible using this include:

#include <visp3/imgproc/vpImgproc.h>

The first step is to read an image:

  vpImage<unsigned char> I;
  vpImageIo::read(I, input_filename);

Input image

The connected-components labeling can be done with:

  vpImage<int> labels;
  int nbComponents = 0;
  vp::connectedComponents(I, labels, nbComponents, connexity);
  std::cout << "nbComponents=" << nbComponents << std::endl;

Each pixel other than the background (0 pixel value in the original image) is assigned a label stored in vpImage<int> variable. The number of connected-components is returned in nbComponents variable. The connexity can be 4-connexity or 8-connexity.

To visualize the labeling, we can use these lines of code:

  vpImage<vpRGBa> I_conn(I.getHeight(), I.getWidth());
  for (unsigned int i = 0; i < I_conn.getHeight(); i++) {
    for (unsigned int j = 0; j < I_conn.getWidth(); j++) {
      if (labels[i][j] != 0) {
        I_conn[i][j] =
            vpRGBa(vpColor::getColor((unsigned int)labels[i][j]).R, vpColor::getColor((unsigned int)labels[i][j]).G,
                   vpColor::getColor((unsigned int)labels[i][j]).B);
      }
    }
  }

Each label is assigned a specific color. The result image is:

Connected-components labeling

Note

As you can see, the input image does not need to be a binary image but must be a grayscale image.

Next tutorial

You can now read the Tutorial: Flood fill algorithm, to learn how to do a flood fill.