MomentsDemo.java

import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.Image;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;

import javax.swing.BoxLayout;
import javax.swing.ImageIcon;
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.CvType;
import org.opencv.core.Mat;
import org.opencv.core.MatOfPoint;
import org.opencv.core.MatOfPoint2f;
import org.opencv.core.Point;
import org.opencv.core.Scalar;
import org.opencv.core.Size;
import org.opencv.highgui.HighGui;
import org.opencv.imgcodecs.Imgcodecs;
import org.opencv.imgproc.Imgproc;
import org.opencv.imgproc.Moments;

class MomentsClass {
    private Mat srcGray = new Mat();
    private JFrame frame;
    private JLabel imgSrcLabel;
    private JLabel imgContoursLabel;
    private static final int MAX_THRESHOLD = 255;
    private int threshold = 100;
    private Random rng = new Random(12345);

    public MomentsClass(String[] args) {
        //! [setup]
        /// Load source image
        String filename = args.length > 0 ? args[0] : "../data/stuff.jpg";
        Mat src = Imgcodecs.imread(filename);
        if (src.empty()) {
            System.err.println("Cannot read image: " + filename);
            System.exit(0);
        }

        /// Convert image to gray and blur it
        Imgproc.cvtColor(src, srcGray, Imgproc.COLOR_BGR2GRAY);
        Imgproc.blur(srcGray, srcGray, new Size(3, 3));
        //! [setup]

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

    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));

        //! [trackbar]
        sliderPanel.add(new JLabel("Canny threshold: "));
        JSlider slider = new JSlider(0, MAX_THRESHOLD, threshold);
        slider.setMajorTickSpacing(20);
        slider.setMinorTickSpacing(10);
        slider.setPaintTicks(true);
        slider.setPaintLabels(true);
        slider.addChangeListener(new ChangeListener() {
            @Override
            public void stateChanged(ChangeEvent e) {
                JSlider source = (JSlider) e.getSource();
                threshold = source.getValue();
                update();
            }
        });
        //! [trackbar]
        sliderPanel.add(slider);
        pane.add(sliderPanel, BorderLayout.PAGE_START);

        JPanel imgPanel = new JPanel();
        imgSrcLabel = new JLabel(new ImageIcon(img));
        imgPanel.add(imgSrcLabel);

        Mat blackImg = Mat.zeros(srcGray.size(), CvType.CV_8U);
        imgContoursLabel = new JLabel(new ImageIcon(HighGui.toBufferedImage(blackImg)));
        imgPanel.add(imgContoursLabel);

        pane.add(imgPanel, BorderLayout.CENTER);
    }

    private void update() {
        //! [Canny]
        /// Detect edges using Canny
        Mat cannyOutput = new Mat();
        Imgproc.Canny(srcGray, cannyOutput, threshold, threshold * 2);
        //! [Canny]

        //! [findContours]
        /// Find contours
        List<MatOfPoint> contours = new ArrayList<>();
        Mat hierarchy = new Mat();
        Imgproc.findContours(cannyOutput, contours, hierarchy, Imgproc.RETR_TREE, Imgproc.CHAIN_APPROX_SIMPLE);
        //! [findContours]

        /// Get the moments
        List<Moments> mu = new ArrayList<>(contours.size());
        for (int i = 0; i < contours.size(); i++) {
            mu.add(Imgproc.moments(contours.get(i)));
        }

        /// Get the mass centers
        List<Point> mc = new ArrayList<>(contours.size());
        for (int i = 0; i < contours.size(); i++) {
            //add 1e-5 to avoid division by zero
            mc.add(new Point(mu.get(i).m10 / (mu.get(i).m00 + 1e-5), mu.get(i).m01 / (mu.get(i).m00 + 1e-5)));
        }

        //! [zeroMat]
        /// Draw contours
        Mat drawing = Mat.zeros(cannyOutput.size(), CvType.CV_8UC3);
        //! [zeroMat]
        //! [forContour]
        for (int i = 0; i < contours.size(); i++) {
            Scalar color = new Scalar(rng.nextInt(256), rng.nextInt(256), rng.nextInt(256));
            Imgproc.drawContours(drawing, contours, i, color, 2);
            Imgproc.circle(drawing, mc.get(i), 4, color, -1);
        }
        //! [forContour]

        //! [showDrawings]
        /// Show in a window
        imgContoursLabel.setIcon(new ImageIcon(HighGui.toBufferedImage(drawing)));
        frame.repaint();
        //! [showDrawings]

        /// Calculate the area with the moments 00 and compare with the result of the OpenCV function
        System.out.println("\t Info: Area and Contour Length \n");
        for (int i = 0; i < contours.size(); i++) {
            System.out.format(" * Contour[%d] - Area (M_00) = %.2f - Area OpenCV: %.2f - Length: %.2f\n", i,
                    mu.get(i).m00, Imgproc.contourArea(contours.get(i)),
                    Imgproc.arcLength(new MatOfPoint2f(contours.get(i).toArray()), true));
        }
    }
}

public class MomentsDemo {
    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 MomentsClass(args);
            }
        });
    }
}