Visual Servoing Platform  version 3.3.0
Tutorial: Automatic thresholding

Introduction

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

  • Huang [Huang_imagethresholding]
  • Intermodes [NYAS:NYAS1035]
  • Isodata [article4310039]
  • Mean [Glasbey:1993:AHT:167725.167747]
  • Otsu [article4310076]
  • Triangle [doi:10.1177/25.7.70454]

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/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 = "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
#elif defined(VISP_HAVE_GDI)
#elif defined(VISP_HAVE_OPENCV)
#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);
return EXIT_SUCCESS;
#else
(void)argc;
(void)argv;
return 0;
#endif
}

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.

vpDisplayX
Use the X11 console to display images on unix-like OS. Thus to enable this class X11 should be instal...
Definition: vpDisplayX.h:149
vp::AUTO_THRESHOLD_OTSU
Definition: vpImgproc.h:74
vp::AUTO_THRESHOLD_MEAN
Definition: vpImgproc.h:70
vp::AUTO_THRESHOLD_HUANG
Definition: vpImgproc.h:58
vpImageIo::read
static void read(vpImage< unsigned char > &I, const std::string &filename)
Definition: vpImageIo.cpp:242
vpDisplayGDI
Display for windows using GDI (available on any windows 32 platform).
Definition: vpDisplayGDI.h:127
vpImage::insert
void insert(const vpImage< Type > &src, const vpImagePoint &topLeft)
Definition: vpImage.h:1229
vp::autoThreshold
VISP_EXPORT unsigned char autoThreshold(vpImage< unsigned char > &I, const vp::vpAutoThresholdMethod &method, const unsigned char backgroundValue=0, const unsigned char foregroundValue=255)
Definition: vpThreshold.cpp:373
vpDisplayOpenCV
The vpDisplayOpenCV allows to display image using the OpenCV library. Thus to enable this class OpenC...
Definition: vpDisplayOpenCV.h:140
vp::AUTO_THRESHOLD_TRIANGLE
Definition: vpImgproc.h:78
vpDisplay::display
static void display(const vpImage< unsigned char > &I)
Definition: vpDisplay_uchar.cpp:739
vpDisplay::displayText
static void displayText(const vpImage< unsigned char > &I, const vpImagePoint &ip, const std::string &s, const vpColor &color)
Definition: vpDisplay_uchar.cpp:663
vpImage::getHeight
unsigned int getHeight() const
Definition: vpImage.h:221
vp::AUTO_THRESHOLD_ISODATA
Definition: vpImgproc.h:66
vpImagePoint
Class that defines a 2D point in an image. This class is useful for image processing and stores only ...
Definition: vpImagePoint.h:87
vpDisplayX::init
void init(vpImage< unsigned char > &I, int winx=-1, int winy=-1, const std::string &title="")
Definition: vpDisplayX.cpp:251
vpDisplay::setDownScalingFactor
virtual void setDownScalingFactor(unsigned int scale)
Definition: vpDisplay.cpp:230
vpDisplay::SCALE_2
Definition: vpDisplay.h:179
vp::AUTO_THRESHOLD_INTERMODES
Definition: vpImgproc.h:62
vpDisplay::flush
static void flush(const vpImage< unsigned char > &I)
Definition: vpDisplay_uchar.cpp:715
vpImage< unsigned char >
vpDisplay::getClick
static bool getClick(const vpImage< unsigned char > &I, bool blocking=true)
Definition: vpDisplay_uchar.cpp:764
vpColor::red
static const vpColor red
Definition: vpColor.h:178
vpImage::getWidth
unsigned int getWidth() const
Definition: vpImage.h:279