Tutorial: Image filtering

Table of Contents

• Introduction
• Gaussian blur
• Gradients computation
• Canny edge detector
• Convolution
• Gaussian image pyramid
• Next tutorial

This tutorial supposes that you have followed the Tutorial: How to create and build a project that uses ViSP and CMake on Unix or Windows.

Introduction

In this tutorial you will learn how to use ViSP filtering functions implemented in vpImageFilter class.

Note that all the material (source code and images) described in this tutorial is part of ViSP source code (in tutorial/image folder) and could be found in https://github.com/lagadic/visp/tree/master/tutorial/image.

Let us consider the following source code that comes from tutorial-image-filter.cpp.

 
#include <visp3/core/vpImageFilter.h>
#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>
 
void display(vpImage<unsigned char> &I, const std::string &title);
void display(vpImage<double> &D, const std::string &title);
 
void display(vpImage<unsigned char> &I, const std::string &title)
{
#if defined(VISP_HAVE_X11)
  vpDisplayX d(I);
#elif defined(HAVE_OPENCV_HIGHGUI)
  vpDisplayOpenCV d(I);
#elif defined(VISP_HAVE_GTK)
  vpDisplayGTK d(I);
#elif defined(VISP_HAVE_GDI)
  vpDisplayGDI d(I);
#elif defined(VISP_HAVE_D3D9)
  vpDisplayD3D d(I);
#else
  std::cout << "No image viewer is available..." << std::endl;
#endif
 
  vpDisplay::setTitle(I, title);
  vpDisplay::display(I);
  vpDisplay::displayText(I, 15, 15, "Click to continue...", vpColor::red);
  vpDisplay::flush(I);
  vpDisplay::getClick(I);
}
 
void display(vpImage<double> &D, const std::string &title)
{
  vpImage<unsigned char> I; // Image to display
  vpImageConvert::convert(D, I);
  display(I, title);
}
 
int main(int argc, char **argv)
{
  try {
    if (argc != 2) {
      printf("Usage: %s <image name.[pgm,ppm,jpeg,png,bmp]>\n", argv[0]);
      return EXIT_FAILURE;
    }
    vpImage<unsigned char> I;
 
    try {
      vpImageIo::read(I, argv[1]);
    }
    catch (...) {
      std::cout << "Cannot read image \"" << argv[1] << "\"" << std::endl;
      return EXIT_FAILURE;
    }
 
    display(I, "Original image");
 
    vpImage<double> F;
    vpImageFilter::gaussianBlur(I, F);
    display(F, "Blur (default)");
 
    vpImageFilter::gaussianBlur(I, F, 7, 2.);
    display(F, "Blur (var=2)");
 
    vpImage<double> dIx;
    vpImageFilter::getGradX(I, dIx);
    display(dIx, "Gradient dIx");
 
    vpImage<double> dIy;
    vpImageFilter::getGradY(I, dIy);
    display(dIy, "Gradient dIy");
 
    vpImage<unsigned char> C;
    vpImageFilter::canny(I, C, 5, -1., 3);
    display(C, "Canny");
 
    vpMatrix K(3, 3); // Sobel kernel along x
    K[0][0] = 1;
    K[0][1] = 0;
    K[0][2] = -1;
    K[1][0] = 2;
    K[1][1] = 0;
    K[1][2] = -2;
    K[2][0] = 1;
    K[2][1] = 0;
    K[2][2] = -1;
    vpImage<double> Gx;
    vpImageFilter::filter(I, Gx, K);
    display(Gx, "Sobel x");
 
    size_t nlevel = 3;
    std::vector<vpImage<unsigned char> > pyr(nlevel);
    pyr[0] = I;
    for (size_t i = 1; i < nlevel; i++) {
      vpImageFilter::getGaussPyramidal(pyr[i - 1], pyr[i]);
      display(pyr[i], "Pyramid");
    }
    return EXIT_SUCCESS;
  }
  catch (const vpException &e) {
    std::cout << "Catch an exception: " << e << std::endl;
    return EXIT_FAILURE;
  }
}

Once build, you should have tutorial-image-filter binary. It shows how to apply different filters on an input image. Here we will consider monkey.pgm as input image.

To see the resulting filtered images, just run:

./tutorial-image-filter monkey.pgm

The following sections give a line by line explanation of the source code dedicated to image filtering capabilities.

Gaussian blur

Monkey input image is read from disk and is stored in I which is a gray level image declared as

    vpImage<unsigned char> I;

To apply a Gaussian blur to this image we first have to declare a resulting floating-point image F. Then the blurred image could be obtained using the default Gaussian filter:

    vpImage<double> F;
    vpImageFilter::gaussianBlur(I, F);

The resulting image is the following:

It is also possible to specify the Gaussian filter kernel size and the Gaussian standard deviation (sigma) using:

vpImageFilter::gaussianBlur(I, F, 7, 2); // Kernel size: 7, sigma: 2

We thus obtain the following image:

Gradients computation

To compute the gradients or the spatial derivative along X use:

    vpImage<double> dIx;
    vpImageFilter::getGradX(I, dIx);

Gradients along Y could be obtained using:

    vpImage<double> dIy;
    vpImageFilter::getGradY(I, dIy);

The resulting floating-point images dIx, dIy are the following:

Canny edge detector

Canny edge detector function relies on OpenCV if ViSP was build with OpenCV 2.1 or higher. Otherwise, it relies on the ViSP implementation in vpCannyEdgeDetector class.

After the declaration of a new image container C, Canny edge detector is applied using:

    vpImage<unsigned char> C;
    vpImageFilter::canny(I, C, 5, -1., 3);
    display(C, "Canny");

Where:

• 5: is the size of the Gaussian kernel used to blur the image before applying the Canny edge detector.
• -1.: is the upper threshold set in the program. Setting it to a negative value asks ViSP to compute automatically the lower and upper thresholds. Otherwise, the lower threshold is set to be equal to one third of the upper threshold, following Canny’s recommendation.
• 3: is the size of the Sobel kernel used internally.

The resulting image C is the following:

Convolution

To apply a convolution to an image, we first have to define a kernel. For example, let us consider the 3x3 Sobel kernel defined in K.

    vpMatrix K(3, 3); // Sobel kernel along x
    K[0][0] = 1;
    K[0][1] = 0;
    K[0][2] = -1;
    K[1][0] = 2;
    K[1][1] = 0;
    K[1][2] = -2;
    K[2][0] = 1;
    K[2][1] = 0;
    K[2][2] = -1;

After the declaration of a new floating-point image Gx, the convolution is obtained using:

    vpImage<double> Gx;
    vpImageFilter::filter(I, Gx, K);

The content of the filtered image Gx is the following.

Gaussian image pyramid

To construct a pyramid of Gaussian filtered images as a vector of images implemented in pyr[] you may use:

    size_t nlevel = 3;
    std::vector<vpImage<unsigned char> > pyr(nlevel);
    pyr[0] = I;
    for (size_t i = 1; i < nlevel; i++) {
      vpImageFilter::getGaussPyramidal(pyr[i - 1], pyr[i]);
      display(pyr[i], "Pyramid");
    }

The content of pyr[0], pyr[1], pyr[2] is the following:

Next tutorial

You are now ready to see the next Tutorial: Blob tracking.