Visual Servoing Platform  version 3.6.1 under development (2024-10-13)
Tutorial: Automatic thresholding

Introduction

This tutorial will show you how to automatically threshold, binarize an image using different methods:

These functions have been ported from the Auto Threshold ImageJ plugin and you can refer to the corresponding documentation for more information.

Example code

The following example also available in tutorial-autothreshold.cpp will demonstrate on a sample image the result of each of these methods:

#include <cstdlib>
#include <iostream>
#include <visp3/core/vpConfig.h>
#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))
#ifdef ENABLE_VISP_NAMESPACE
using namespace VISP_NAMESPACE_NAME;
#endif
std::string input_filename = "grid36-03.pgm";
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]) == "--help" || std::string(argv[i]) == "-h") {
std::cout << "Usage: " << argv[0] << " [--input <input image>] [--help]" << std::endl;
return EXIT_SUCCESS;
}
}
vpImageIo::read(I, input_filename);
vpImage<unsigned char> I_res(3 * I.getHeight(), 3 * I.getWidth());
I_res.insert(I, vpImagePoint(I.getHeight(), I.getWidth()));
#ifdef VISP_HAVE_X11
vpDisplayX d;
#elif defined(VISP_HAVE_GDI)
#elif defined(HAVE_OPENCV_HIGHGUI)
#endif
d.init(I_res);
I_res.insert(I_huang, vpImagePoint());
vpImage<unsigned char> I_intermodes = I;
I_res.insert(I_intermodes, vpImagePoint(0, I.getWidth()));
vpImage<unsigned char> I_isodata = I;
I_res.insert(I_isodata, vpImagePoint(0, 2 * I.getWidth()));
I_res.insert(I_mean, vpImagePoint(I.getHeight(), 0));
I_res.insert(I_otsu, vpImagePoint(I.getHeight(), 2 * I.getWidth()));
vpImage<unsigned char> I_triangle = I;
I_res.insert(I_triangle, vpImagePoint(2 * I.getHeight(), 0));
vpDisplay::displayText(I_res, 30, 20, "Huang", vpColor::red);
vpDisplay::displayText(I_res, 30, 20 + I.getWidth(), "Intermodes", vpColor::red);
vpDisplay::displayText(I_res, 30, 20 + 2 * I.getWidth(), "IsoData", vpColor::red);
vpDisplay::displayText(I_res, 30 + I.getHeight(), 20, "Mean", vpColor::red);
vpDisplay::displayText(I_res, 30 + I.getHeight(), 20 + I.getWidth(), "Original", vpColor::red);
vpDisplay::displayText(I_res, 30 + I.getHeight(), 20 + 2 * I.getWidth(), "Otsu", vpColor::red);
vpDisplay::displayText(I_res, 30 + 2 * I.getHeight(), 20, "Triangle", vpColor::red);
#else
(void)argc;
(void)argv;
#endif
return EXIT_SUCCESS;
}
static const vpColor red
Definition: vpColor.h:217
Display for windows using GDI (available on any windows 32 platform).
Definition: vpDisplayGDI.h:130
The vpDisplayOpenCV allows to display image using the OpenCV library. Thus to enable this class OpenC...
void init(vpImage< unsigned char > &I, int winx=-1, int winy=-1, const std::string &title="") VP_OVERRIDE
void setDownScalingFactor(unsigned int scale)
static bool getClick(const vpImage< unsigned char > &I, bool blocking=true)
static void display(const vpImage< unsigned char > &I)
static void flush(const vpImage< unsigned char > &I)
static void displayText(const vpImage< unsigned char > &I, const vpImagePoint &ip, const std::string &s, const vpColor &color)
static void read(vpImage< unsigned char > &I, const std::string &filename, int backend=IO_DEFAULT_BACKEND)
Definition: vpImageIo.cpp:147
Class that defines a 2D point in an image. This class is useful for image processing and stores only ...
Definition: vpImagePoint.h:82
unsigned int getWidth() const
Definition: vpImage.h:242
unsigned int getHeight() const
Definition: vpImage.h:181
VISP_EXPORT unsigned char autoThreshold(VISP_NAMESPACE_ADDRESSING vpImage< unsigned char > &I, const vpAutoThresholdMethod &method, const unsigned char backgroundValue=0, const unsigned char foregroundValue=255)

These functions are provided in a vp:: namespace and accessible using this include:

#include <visp3/imgproc/vpImgproc.h>

The code to use is pretty straightword:

The following image presents the results for each method:

Comparison of different binarizations using the threshold value returned by each method

The function vp::autoThreshold(vpImage<unsigned char> &, const vp::vpAutoThresholdMethod &, const unsigned char, const unsigned char) has two parameters to specify the pixel values to use for the background and the foreground. By default, it is (see vpImageTools::binarise(vpImage<Type> &, Type, Type, Type, Type, Type, const bool)):

\[ I_{bin}\left ( i,j \right ) = \left \{ \begin{matrix} 0 \text{ if } I_{src}\left ( i,j \right ) < \text{threshold} \\ 255 \text{ otherwise} \end{matrix} \right. \]

Next tutorial

You can now read the Tutorial: Contours extraction from a binary image, to learn how to extract the contours from a binary image.