Eroding and Dilating

Goal

In this tutorial you will learn how to:

• Apply two very common morphological operators: Erosion and Dilation. For this purpose, you will use the following OpenCV functions:
  • cv::erode
  • cv::dilate

Note

The explanation below belongs to the book Learning OpenCV by Bradski and Kaehler.

Morphological Operations

• In short: A set of operations that process images based on shapes. Morphological operations apply a structuring element to an input image and generate an output image.
• The most basic morphological operations are: Erosion and Dilation. They have a wide array of uses, i.e. :
  • Removing noise
  • Isolation of individual elements and joining disparate elements in an image.
  • Finding of intensity bumps or holes in an image

• We will explain dilation and erosion briefly, using the following image as an example:

  

Dilation

• This operations consists of convolving an image \(A\) with some kernel ( \(B\)), which can have any shape or size, usually a square or circle.
• The kernel \(B\) has a defined anchor point, usually being the center of the kernel.
• As the kernel \(B\) is scanned over the image, we compute the maximal pixel value overlapped by \(B\) and replace the image pixel in the anchor point position with that maximal value. As you can deduce, this maximizing operation causes bright regions within an image to "grow" (therefore the name dilation).
• The dilatation operation is: \(\texttt{dst} (x,y) = \max _{(x',y'): \, \texttt{element} (x',y') \ne0 } \texttt{src} (x+x',y+y')\)

• Take the above image as an example. Applying dilation we can get:

  
• The bright area of the letter dilates around the black regions of the background.

Erosion

• This operation is the sister of dilation. It computes a local minimum over the area of given kernel.
• As the kernel \(B\) is scanned over the image, we compute the minimal pixel value overlapped by \(B\) and replace the image pixel under the anchor point with that minimal value.
• The erosion operation is: \(\texttt{dst} (x,y) = \min _{(x',y'): \, \texttt{element} (x',y') \ne0 } \texttt{src} (x+x',y+y')\)

• Analagously to the example for dilation, we can apply the erosion operator to the original image (shown above). You can see in the result below that the bright areas of the image get thinner, whereas the dark zones gets bigger.

  

Code

C++

This tutorial's code is shown below. You can also download it here

#include "opencv2/imgproc.hpp"
#include "opencv2/highgui.hpp"
#include <iostream>

using namespace cv;
using namespace std;

Mat src, erosion_dst, dilation_dst;

int erosion_elem = 0;
int erosion_size = 0;
int dilation_elem = 0;
int dilation_size = 0;
int const max_elem = 2;
int const max_kernel_size = 21;

void Erosion( int, void* );
void Dilation( int, void* );

int main( int argc, char** argv )
{
  CommandLineParser parser( argc, argv, "{@input | ../data/LinuxLogo.jpg | input image}" );
  src = imread( parser.get<String>( "@input" ), IMREAD_COLOR );
  if( src.empty() )
  {
    cout << "Could not open or find the image!\n" << endl;
    cout << "Usage: " << argv[0] << " <Input image>" << endl;
    return -1;
  }

  namedWindow( "Erosion Demo", WINDOW_AUTOSIZE );
  namedWindow( "Dilation Demo", WINDOW_AUTOSIZE );
  moveWindow( "Dilation Demo", src.cols, 0 );

  createTrackbar( "Element:\n 0: Rect \n 1: Cross \n 2: Ellipse", "Erosion Demo",
          &erosion_elem, max_elem,
          Erosion );

  createTrackbar( "Kernel size:\n 2n +1", "Erosion Demo",
          &erosion_size, max_kernel_size,
          Erosion );

  createTrackbar( "Element:\n 0: Rect \n 1: Cross \n 2: Ellipse", "Dilation Demo",
          &dilation_elem, max_elem,
          Dilation );

  createTrackbar( "Kernel size:\n 2n +1", "Dilation Demo",
          &dilation_size, max_kernel_size,
          Dilation );

  Erosion( 0, 0 );
  Dilation( 0, 0 );

  waitKey(0);
  return 0;
}


void Erosion( int, void* )
{
  int erosion_type = 0;
  if( erosion_elem == 0 ){ erosion_type = MORPH_RECT; }
  else if( erosion_elem == 1 ){ erosion_type = MORPH_CROSS; }
  else if( erosion_elem == 2) { erosion_type = MORPH_ELLIPSE; }

  Mat element = getStructuringElement( erosion_type,
                       Size( 2*erosion_size + 1, 2*erosion_size+1 ),
                       Point( erosion_size, erosion_size ) );

  erode( src, erosion_dst, element );
  imshow( "Erosion Demo", erosion_dst );
}


void Dilation( int, void* )
{
  int dilation_type = 0;
  if( dilation_elem == 0 ){ dilation_type = MORPH_RECT; }
  else if( dilation_elem == 1 ){ dilation_type = MORPH_CROSS; }
  else if( dilation_elem == 2) { dilation_type = MORPH_ELLIPSE; }

  Mat element = getStructuringElement( dilation_type,
                       Size( 2*dilation_size + 1, 2*dilation_size+1 ),
                       Point( dilation_size, dilation_size ) );

  dilate( src, dilation_dst, element );
  imshow( "Dilation Demo", dilation_dst );
}

Java

This tutorial's code is shown below. You can also download it here

import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.Image;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.BoxLayout;
import javax.swing.ImageIcon;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JSlider;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;

import org.opencv.core.Core;
import org.opencv.core.Mat;
import org.opencv.core.Point;
import org.opencv.core.Size;
import org.opencv.highgui.HighGui;
import org.opencv.imgcodecs.Imgcodecs;
import org.opencv.imgproc.Imgproc;

public class MorphologyDemo1 {
    private static final String[] ELEMENT_TYPE = { "Rectangle", "Cross", "Ellipse" };
    private static final String[] MORPH_OP = { "Erosion", "Dilatation" };
    private static final int MAX_KERNEL_SIZE = 21;
    private Mat matImgSrc;
    private Mat matImgDst = new Mat();
    private int elementType = Imgproc.CV_SHAPE_RECT;
    private int kernelSize = 0;
    private boolean doErosion = true;
    private JFrame frame;
    private JLabel imgLabel;

    public MorphologyDemo1(String[] args) {
        String imagePath = args.length > 0 ? args[0] : "../data/LinuxLogo.jpg";
        matImgSrc = Imgcodecs.imread(imagePath);
        if (matImgSrc.empty()) {
            System.out.println("Empty image: " + imagePath);
            System.exit(0);
        }

        // Create and set up the window.
        frame = new JFrame("Erosion and dilatation demo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        // Set up the content pane.
        Image img = HighGui.toBufferedImage(matImgSrc);
        addComponentsToPane(frame.getContentPane(), img);
        // Use the content pane's default BorderLayout. No need for
        // setLayout(new BorderLayout());
        // Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    private void addComponentsToPane(Container pane, Image img) {
        if (!(pane.getLayout() instanceof BorderLayout)) {
            pane.add(new JLabel("Container doesn't use BorderLayout!"));
            return;
        }

        JPanel sliderPanel = new JPanel();
        sliderPanel.setLayout(new BoxLayout(sliderPanel, BoxLayout.PAGE_AXIS));

        JComboBox<String> elementTypeBox = new JComboBox<>(ELEMENT_TYPE);
        elementTypeBox.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                @SuppressWarnings("unchecked")
                JComboBox<String> cb = (JComboBox<String>)e.getSource();
                if (cb.getSelectedIndex() == 0) {
                    elementType = Imgproc.CV_SHAPE_RECT;
                } else if (cb.getSelectedIndex() == 1) {
                    elementType = Imgproc.CV_SHAPE_CROSS;
                } else if (cb.getSelectedIndex() == 2) {
                    elementType = Imgproc.CV_SHAPE_ELLIPSE;
                }
                update();
            }
        });
        sliderPanel.add(elementTypeBox);

        sliderPanel.add(new JLabel("Kernel size: 2n + 1"));
        JSlider slider = new JSlider(0, MAX_KERNEL_SIZE, 0);
        slider.setMajorTickSpacing(5);
        slider.setMinorTickSpacing(5);
        slider.setPaintTicks(true);
        slider.setPaintLabels(true);
        slider.addChangeListener(new ChangeListener() {
            @Override
            public void stateChanged(ChangeEvent e) {
                JSlider source = (JSlider) e.getSource();
                kernelSize = source.getValue();
                update();
            }
        });
        sliderPanel.add(slider);

        JComboBox<String> morphOpBox = new JComboBox<>(MORPH_OP);
        morphOpBox.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                @SuppressWarnings("unchecked")
                JComboBox<String> cb = (JComboBox<String>)e.getSource();
                doErosion = cb.getSelectedIndex() == 0;
                update();
            }
        });
        sliderPanel.add(morphOpBox);

        pane.add(sliderPanel, BorderLayout.PAGE_START);
        imgLabel = new JLabel(new ImageIcon(img));
        pane.add(imgLabel, BorderLayout.CENTER);
    }

    private void update() {
        Mat element = Imgproc.getStructuringElement(elementType, new Size(2 * kernelSize + 1, 2 * kernelSize + 1),
                new Point(kernelSize, kernelSize));

        if (doErosion) {
            Imgproc.erode(matImgSrc, matImgDst, element);
        } else {
            Imgproc.dilate(matImgSrc, matImgDst, element);
        }
        Image img = HighGui.toBufferedImage(matImgDst);
        imgLabel.setIcon(new ImageIcon(img));
        frame.repaint();
    }

    public static void main(String[] args) {
        // Load the native OpenCV library
        System.loadLibrary(Core.NATIVE_LIBRARY_NAME);

        // Schedule a job for the event dispatch thread:
        // creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new MorphologyDemo1(args);
            }
        });
    }
}

Python

This tutorial's code is shown below. You can also download it here

from __future__ import print_function
import cv2 as cv
import numpy as np
import argparse

erosion_size = 0
max_elem = 2
max_kernel_size = 21
title_trackbar_element_type = 'Element:\n 0: Rect \n 1: Cross \n 2: Ellipse'
title_trackbar_kernel_size = 'Kernel size:\n 2n +1'
title_erosion_window = 'Erosion Demo'
title_dilatation_window = 'Dilation Demo'

def erosion(val):
    erosion_size = cv.getTrackbarPos(title_trackbar_kernel_size, title_erosion_window)
    erosion_type = 0
    val_type = cv.getTrackbarPos(title_trackbar_element_type, title_erosion_window)
    if val_type == 0:
        erosion_type = cv.MORPH_RECT
    elif val_type == 1:
        erosion_type = cv.MORPH_CROSS
    elif val_type == 2:
        erosion_type = cv.MORPH_ELLIPSE

    element = cv.getStructuringElement(erosion_type, (2*erosion_size + 1, 2*erosion_size+1), (erosion_size, erosion_size))
    erosion_dst = cv.erode(src, element)
    cv.imshow(title_erosion_window, erosion_dst)

def dilatation(val):
    dilatation_size = cv.getTrackbarPos(title_trackbar_kernel_size, title_dilatation_window)
    dilatation_type = 0
    val_type = cv.getTrackbarPos(title_trackbar_element_type, title_dilatation_window)
    if val_type == 0:
        dilatation_type = cv.MORPH_RECT
    elif val_type == 1:
        dilatation_type = cv.MORPH_CROSS
    elif val_type == 2:
        dilatation_type = cv.MORPH_ELLIPSE

    element = cv.getStructuringElement(dilatation_type, (2*dilatation_size + 1, 2*dilatation_size+1), (dilatation_size, dilatation_size))
    dilatation_dst = cv.dilate(src, element)
    cv.imshow(title_dilatation_window, dilatation_dst)

parser = argparse.ArgumentParser(description='Code for Eroding and Dilating tutorial.')
parser.add_argument('--input', help='Path to input image.', default='../data/LinuxLogo.jpg')
args = parser.parse_args()

src = cv.imread(args.input)
if src is None:
    print('Could not open or find the image: ', args.input)
    exit(0)

cv.namedWindow(title_erosion_window)
cv.createTrackbar(title_trackbar_element_type, title_erosion_window , 0, max_elem, erosion)
cv.createTrackbar(title_trackbar_kernel_size, title_erosion_window , 0, max_kernel_size, erosion)

cv.namedWindow(title_dilatation_window)
cv.createTrackbar(title_trackbar_element_type, title_dilatation_window , 0, max_elem, dilatation)
cv.createTrackbar(title_trackbar_kernel_size, title_dilatation_window , 0, max_kernel_size, dilatation)

erosion(0)
dilatation(0)
cv.waitKey()

Explanation

1. Most of the material shown here is trivial (if you have any doubt, please refer to the tutorials in previous sections). Let's check the general structure of the C++ program:

   • Load an image (can be BGR or grayscale)
   • Create two windows (one for dilation output, the other for erosion)
   • Create a set of two Trackbars for each operation:
     • The first trackbar "Element" returns either erosion_elem or dilation_elem
     • The second trackbar "Kernel size" return erosion_size or dilation_size for the corresponding operation.
   • Every time we move any slider, the user's function Erosion or Dilation will be called and it will update the output image based on the current trackbar values.

   Let's analyze these two functions:

2. erosion:

   
   void Erosion( int, void* )
   {
     int erosion_type = 0;
     if( erosion_elem == 0 ){ erosion_type = MORPH_RECT; }
     else if( erosion_elem == 1 ){ erosion_type = MORPH_CROSS; }
     else if( erosion_elem == 2) { erosion_type = MORPH_ELLIPSE; }
   
     Mat element = getStructuringElement( erosion_type,
                          Size( 2*erosion_size + 1, 2*erosion_size+1 ),
                          Point( erosion_size, erosion_size ) );
   
     erode( src, erosion_dst, element );
     imshow( "Erosion Demo", erosion_dst );
   }

   • The function that performs the erosion operation is cv::erode . As we can see, it receives three arguments:
     • src: The source image
     • erosion_dst: The output image

     • element: This is the kernel we will use to perform the operation. If we do not specify, the default is a simple 3x3 matrix. Otherwise, we can specify its shape. For this, we need to use the function cv::getStructuringElement :

         Mat element = getStructuringElement( erosion_type,
                              Size( 2*erosion_size + 1, 2*erosion_size+1 ),
                              Point( erosion_size, erosion_size ) );

       We can choose any of three shapes for our kernel:

       • Rectangular box: MORPH_RECT
       • Cross: MORPH_CROSS
       • Ellipse: MORPH_ELLIPSE

       Then, we just have to specify the size of our kernel and the anchor point. If not specified, it is assumed to be in the center.

   • That is all. We are ready to perform the erosion of our image.

     Note

     Additionally, there is another parameter that allows you to perform multiple erosions (iterations) at once. However, We haven't used it in this simple tutorial. You can check out the reference for more details.

3. dilation:

   The code is below. As you can see, it is completely similar to the snippet of code for erosion. Here we also have the option of defining our kernel, its anchor point and the size of the operator to be used.

   
   void Dilation( int, void* )
   {
     int dilation_type = 0;
     if( dilation_elem == 0 ){ dilation_type = MORPH_RECT; }
     else if( dilation_elem == 1 ){ dilation_type = MORPH_CROSS; }
     else if( dilation_elem == 2) { dilation_type = MORPH_ELLIPSE; }
   
     Mat element = getStructuringElement( dilation_type,
                          Size( 2*dilation_size + 1, 2*dilation_size+1 ),
                          Point( dilation_size, dilation_size ) );
   
     dilate( src, dilation_dst, element );
     imshow( "Dilation Demo", dilation_dst );
   }

   Results

Compile the code above and execute it with an image as argument. For instance, using this image:

We get the results below. Varying the indices in the Trackbars give different output images, naturally. Try them out! You can even try to add a third Trackbar to control the number of iterations.