Visual Servoing Platform  version 3.6.1 under development (2024-05-21)
Tutorial: Automatic thresholding


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/vpImage.h>
#include <visp3/gui/vpDisplayGDI.h>
#include <visp3/gui/vpDisplayOpenCV.h>
#include <visp3/gui/vpDisplayX.h>
#include <visp3/io/vpImageIo.h>
#include <visp3/imgproc/vpImgproc.h>
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;
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(HAVE_OPENCV_HIGHGUI)
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);
static const vpColor red
Definition: vpColor.h:211
Display for windows using GDI (available on any windows 32 platform).
Definition: vpDisplayGDI.h:128
The vpDisplayOpenCV allows to display image using the OpenCV library. Thus to enable this class OpenC...
Use the X11 console to display images on unix-like OS. Thus to enable this class X11 should be instal...
Definition: vpDisplayX.h:128
void init(vpImage< unsigned char > &I, int win_x=-1, int win_y=-1, const std::string &win_title="") vp_override
static bool getClick(const vpImage< unsigned char > &I, bool blocking=true)
virtual void setDownScalingFactor(unsigned int scale)
Definition: vpDisplay.cpp:227
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:143
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:245
unsigned int getHeight() const
Definition: vpImage.h:184
VISP_EXPORT unsigned char autoThreshold(vpImage< unsigned char > &I, const vp::vpAutoThresholdMethod &method, const unsigned char backgroundValue=0, const unsigned char foregroundValue=255)
Definition: vpImgproc.h:76
Definition: vpImgproc.h:68
Definition: vpImgproc.h:72
Definition: vpImgproc.h:88
Definition: vpImgproc.h:80
Definition: vpImgproc.h:84

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.