Tutorial: Image filtering

Table of Contents

• Gaussian blur
• Gradients computation
• Canny edge detector
• Convolution
• Gaussian image pyramid

This tutorial supposes that you have followed the Tutorial: Getting started.

Introduction

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

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

#include <visp/vpDisplayD3D.h>
#include <visp/vpDisplayGDI.h>
#include <visp/vpDisplayGTK.h>
#include <visp/vpDisplayX.h>
#include <visp/vpDisplayOpenCV.h>
#include <visp/vpImageIo.h>
#include <visp/vpImageFilter.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(VISP_HAVE_OPENCV)
  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.c_str());
  vpDisplay::display(I);
  vpDisplay::displayCharString(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 -1;
    }

    vpImage<unsigned char> I;

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

    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");


#if (VISP_HAVE_OPENCV_VERSION >= 0x020100)
    vpImage<unsigned char> C;
    vpImageFilter::canny(I, C, 5, 15, 3);
    display(C, "Canny");
#endif

    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 0;
  }
  catch(vpException &e) {
    std::cout << "Catch an exception: " << e << std::endl;
    return 1;
  }
}

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

To see the resulting filtered images, just run:

./tutorial-image-filter lena.pgm

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

Gaussian blur

Lena 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 is only available if ViSP was build with OpenCV 2.1 or higher.

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

#if (VISP_HAVE_OPENCV_VERSION >= 0x020100)
    vpImage<unsigned char> C;
    vpImageFilter::canny(I, C, 5, 15, 3);
#endif

Where:

• 5: is the low threshold
• 15: is the high threshold set in the program as three times the lower 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:

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