Tutorial: How to display an image and basic drawings in a window

Table of Contents

• Introduction
• Load and display an image
• Display basic drawings in window overlay
  • Display a point in overlay
  • Display a line between 2 points in overlay
  • Display a circle in overlay
  • Display a rectangle in overlay
  • Display a cross in overlay
  • Display text in window overlay
• Export and save the content of a window as an image
• Handle keyboard events in a window
• Next tutorial

Introduction

In this tutorial you will learn how to display basic drawings with ViSP either on Unix-like systems (including OSX, Fedora, Ubuntu, Debian, ...) or on Windows.

Note that all the material (source code and images) described in this tutorial is part of ViSP source code and could be downloaded using the following command:

$ svn export https://github.com/lagadic/visp.git/trunk/tutorial/image

Load and display an image

ViSP gui module provides Graphical User Interfaces capabilities. To this end you may use several optional third-party libraries which are: X11, GDI, OpenCV, GTK, Direct3D. In the next example, we will use the first 3rd party that is available from the previous list.

The following example also available in tutorial-viewer.cpp shows how to read and display an image.

#include <visp3/gui/vpDisplayD3D.h>
#include <visp3/gui/vpDisplayGDI.h>
#include <visp3/gui/vpDisplayGTK.h>
#include <visp3/gui/vpDisplayOpenCV.h>
#include <visp3/gui/vpDisplayX.h>
#include <visp3/io/vpImageIo.h>

int main(int argc, char **argv)
{
  if (argc != 2) {
    printf("Usage: %s <image name.[pgm,ppm,jpeg,png,tiff,bmp,ras,jp2]>\n", argv[0]);
    return -1;
  }

  vpImage<vpRGBa> I;

  try {
    vpImageIo::read(I, argv[1]);
  } catch (...) {
    std::cout << "Cannot read image \"" << argv[1] << "\"" << std::endl;
    return -1;
  }

  try {
#if defined(VISP_HAVE_X11)
    vpDisplayX d(I, vpDisplay::SCALE_AUTO);
#elif defined(VISP_HAVE_GDI)
    vpDisplayGDI d(I, vpDisplay::SCALE_AUTO);
#elif defined(VISP_HAVE_OPENCV)
    vpDisplayOpenCV d(I, vpDisplay::SCALE_AUTO);
#elif defined(VISP_HAVE_GTK)
    vpDisplayGTK d(I, vpDisplay::SCALE_AUTO);
#elif defined(VISP_HAVE_D3D9)
    vpDisplayD3D d(I, vpDisplay::SCALE_AUTO);
#else
    std::cout << "No image viewer is available..." << std::endl;
#endif

    vpDisplay::setTitle(I, "My image");
    vpDisplay::display(I);
    vpDisplay::flush(I);
    std::cout << "A click to quit..." << std::endl;
    vpDisplay::getClick(I);
  } catch (const vpException &e) {
    std::cout << "Catch an exception: " << e << std::endl;
  }
}

Once build, if you run the corresponding binary loading monkey.png image:

$ cd $VISP_WS/visp-build/tutorial/image
$ ./tutorial-viewer monkey.png

It will open a window containing monkey.png image:

Here is the detailed explanation of the source, line by line :

#include <visp3/gui/vpDisplayD3D.h>
#include <visp3/gui/vpDisplayGDI.h>
#include <visp3/gui/vpDisplayGTK.h>
#include <visp3/gui/vpDisplayOpenCV.h>
#include <visp3/gui/vpDisplayX.h>

Include all the headers for image viewers. The two first one are for Windows systems. They require that Direct 3D or the Graphical Device Interface (GDI) coming with the installation of Visual Studio are available. The third one needs GTK that is cross-platform. The fourth is for unix-like systems and requires that libX11 is available. The last one is also cross-platform and requires that OpenCV is available.

#include <visp3/io/vpImageIo.h>

Include the header that allows to read/write PGM, PPM, PNG and JPEG images from the disk using vpImageIo class.

  vpImage<vpRGBa> I;

Create an instance of a color image where each pixel is coded in RGBa.

  try {
    vpImageIo::read(I, argv[1]);
  } catch (...) {
    std::cout << "Cannot read image \"" << argv[1] << "\"" << std::endl;
    return -1;
  }

The image I is initialized by reading an image file from the disk. If the image format is not supported we throw an exception.

#if defined(VISP_HAVE_X11)
    vpDisplayX d(I, vpDisplay::SCALE_AUTO);
#elif defined(VISP_HAVE_GDI)
    vpDisplayGDI d(I, vpDisplay::SCALE_AUTO);
#elif defined(VISP_HAVE_OPENCV)
    vpDisplayOpenCV d(I, vpDisplay::SCALE_AUTO);
#elif defined(VISP_HAVE_GTK)
    vpDisplayGTK d(I, vpDisplay::SCALE_AUTO);
#elif defined(VISP_HAVE_D3D9)
    vpDisplayD3D d(I, vpDisplay::SCALE_AUTO);
#else
    std::cout << "No image viewer is available..." << std::endl;
#endif

Create an instance of an image display window for image I. The first viewer that is available is used. Here we create the link between the image I and the display d. Note that an image can only have one display.

    vpDisplay::setTitle(I, "My image");

The title of the display is then set to "My image".

    vpDisplay::display(I);
    vpDisplay::flush(I);

First we display the content of the image I, then we flush the display to render the image.

    vpDisplay::getClick(I);

Here we handle mouse events. We are waiting for a blocking mouse click to end the program.

Display basic drawings in window overlay

There are a lot of examples in ViSP that show how to display drawings in window overlay. There is testDisplays.cpp that gives an overview.

If you run the corresponding binary:

$ cd $VISP_WS/visp-build/modules/gui
$ ./testDisplays

it will open a window like the following:

Display a point in overlay

As shown in tutorial-draw-point.cpp which source code is given below we use vpDisplay::displayPoint() function to draw a point in the overlay of a windows that displays a 3840 by 2160 grey image that has all the pixels set to 128 gray level.

#include <visp3/gui/vpDisplayGDI.h>
#include <visp3/gui/vpDisplayX.h>

int main()
{
  vpImage<unsigned char> I(2160, 3840, 128);

  try {

#if defined(VISP_HAVE_X11)
    vpDisplayX d(I, vpDisplay::SCALE_AUTO);
#elif defined(VISP_HAVE_GDI)
    vpDisplayGDI d(I, vpDisplay::SCALE_AUTO);
#endif
    
    vpDisplay::setTitle(I, "My image");
    vpDisplay::display(I);
    vpDisplay::displayPoint(I, I.getHeight() / 2, I.getWidth() / 2, vpColor::red, 2);
    vpDisplay::flush(I);
    std::cout << "A click to quit..." << std::endl;
    vpDisplay::getClick(I);
  } catch (const vpException &e) {
    std::cout << "Catch an exception: " << e.getMessage() << std::endl;
  }
}

Here we draw a point at the center of a grey image with red color and thickness 2.

Display a line between 2 points in overlay

As given in tutorial-draw-line.cpp we use vpDisplay::displayLine() function to draw a line segment on the screen.

    vpDisplay::displayLine(I, I.getHeight() / 4, I.getWidth() / 4, (3 * I.getHeight() ) / 4, (3 * I.getWidth() ) / 4, vpColor::red, 10);

Here we draw a red coloured line segment with the specified initial and final coordinates and thickness 10.

Display a circle in overlay

As given in tutorial-image-display-scaled-auto.cpp we use vpDisplay::displayCircle() function to draw a circle on the screen.

    vpDisplay::displayCircle(I, I.getHeight() / 2, I.getWidth() / 2, 200, vpColor::red, true);

Here we draw a red coloured filled circle at the center with radius of 200.

Display a rectangle in overlay

As given in tutorial-draw-rectangle.cpp we use vpDisplay::displayRectangle() function to draw a rectangle on the screen.

    vpDisplay::displayRectangle(I, I.getHeight() / 4, I.getWidth() / 4, I.getWidth() / 2, I.getHeight() / 2, vpColor::red, true);

Here we draw a red coloured filled rectangle with specified top-left coordinates and width and height.

Display a cross in overlay

As given in tutorial-draw-cross.cpp we use vpDisplay::displayCross() function to draw a rectangle on the screen.

    vpDisplay::displayCross(I, I.getHeight() / 2, I.getWidth() / 2, I.getWidth() / 2, vpColor::red, 2);

Here we draw a red coloured cross on the center with speicfied size and thickness 2.

Display text in window overlay

As given in tutorial-draw-text.cpp we use vpDisplay::displayText() function to add text in the window overlay.

    vpDisplay::displayText(I, I.getHeight() / 2, I.getWidth() / 2, "Hello World!", vpColor::yellow);

Here Hello world is displayed in the middle of the image.

Export and save the content of a window as an image

As given in tutorial-export-image.cpp which source code is given below, we use vpDisplay::getImage() function to export the image with the whole drawings in overlay. Then we use vpImageIo::write() to save the image in png format.

#include <visp3/io/vpImageIo.h>
#include <visp3/gui/vpDisplayX.h>
#include <visp3/gui/vpDisplayGTK.h>
#include <visp3/gui/vpDisplayGDI.h>
#include <visp3/gui/vpDisplayD3D.h>
#include <visp3/gui/vpDisplayOpenCV.h>

int main()
{
  vpImage<unsigned char> I(240, 320, 255); // Create a black grey level image
  vpImage<vpRGBa> Ioverlay;

  // Depending on the detected third party libraries, we instantiate here the
  // first video device which is available
#if defined(VISP_HAVE_X11)
  vpDisplay *d = new vpDisplayX;
#elif defined(VISP_HAVE_GTK)
  vpDisplay *d = new vpDisplayGTK;
#elif defined(VISP_HAVE_GDI)
  vpDisplay *d = new vpDisplayGDI;
#elif defined(VISP_HAVE_D3D9)
  vpDisplay *d = new vpDisplayD3D;
#elif defined(VISP_HAVE_OPENCV)
  vpDisplay *d = new vpDisplayOpenCV;
#endif
  // Initialize the display with the image I. Display and image are
  // now link together.
#ifdef VISP_HAVE_DISPLAY
  d->init(I);
#endif
  // Set the display background with image I content
  vpDisplay::display(I);
  // Draw a red rectangle in the display overlay (foreground)
  vpDisplay::displayRectangle(I, 10, 10, 100, 20, vpColor::red, true);
  // Flush the foreground and background display
  vpDisplay::flush(I);
  // Updates the color image with the original loaded image and the overlay
  vpDisplay::getImage(I, Ioverlay) ;
  // Write the color image on the disk
  std::cout << "Save image in overlay.ppm" << std::endl;
  std::string ofilename("overlay.png");
  vpImageIo::write(Ioverlay, ofilename) ;
  // Wait for a click in the display window
  std::cout << "A click to quit..." << std::endl;
  vpDisplay::getClick(I);
#ifdef VISP_HAVE_DISPLAY
  delete d;
#endif
}

Handle keyboard events in a window

As given in tutorial-event-keyboard.cpp which code is given below, we use vpDisplay::getKeyboardEvent() function to get the value of the key pressed.

#include <visp3/gui/vpDisplayOpenCV.h>
#include <visp3/gui/vpDisplayX.h>
#include <visp3/gui/vpDisplayGTK.h>
#include <visp3/gui/vpDisplayGDI.h>
#include <visp3/gui/vpDisplayD3D.h>

int main()
{
  vpImage<unsigned char> I(240, 320); // Create a black image
#if defined(VISP_HAVE_X11)
  vpDisplay *d = new vpDisplayX;
#elif defined(VISP_HAVE_GTK)
  vpDisplay *d = new vpDisplayGTK;
#elif defined(VISP_HAVE_GDI)
  vpDisplay *d = new vpDisplayGDI;
#elif defined(VISP_HAVE_D3D9)
  vpDisplay *d = new vpDisplayD3D;
#elif defined(VISP_HAVE_OPENCV)
  vpDisplay *d = new vpDisplayOpenCV;
#else
  std::cout << "Sorry, no video device is available" << std::endl;
  return -1;
#endif
  // Initialize the display with the image I. Display and image are
  // now link together.
#ifdef VISP_HAVE_DISPLAY
  d->init(I);
#endif
  // Set the display background with image I content
  vpDisplay::display(I);
  // Flush the foreground and background display
  vpDisplay::flush(I);
  // Wait for keyboard event
  std::cout << "Waiting a keyboard event..." << std::endl;
  vpDisplay::getKeyboardEvent(I, true);
  std::cout << "A keyboard event was detected" << std::endl;
  // Non blocking keyboard event loop
  int cpt_event = 0;
  char key[10];
  std::cout << "Enter a non blocking keyboard event detection loop..." << std::endl;
  do {
    bool event = vpDisplay::getKeyboardEvent(I, &key[0], false);
    if (event) {
      std::cout << "Key detected: " << key << std::endl;
      cpt_event ++;
    }
    vpTime::wait(5); // wait 5 ms
  } while(cpt_event < 5);
#ifdef VISP_HAVE_DISPLAY
  delete d;
#endif
}

Next tutorial

You are now ready to see how to continue with Tutorial: How to modify an image to insert basic drawings.