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Original author | Ana Huamán |
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Compatibility | OpenCV >= 3.0 |
Goals
In this tutorial you will learn how to
- Use the OpenCV function cv::moments
- Use the OpenCV function cv::contourArea
- Use the OpenCV function cv::arcLength
Code
C++
The tutorial code is shown below. You can also download it from here
#include "opencv2/imgcodecs.hpp" #include "opencv2/highgui.hpp" #include "opencv2/imgproc.hpp" #include <iostream> #include <iomanip> using namespace cv; using namespace std; Mat src_gray; int thresh = 100; RNG rng(12345); void thresh_callback(int, void* ); int main(int argc, char** argv) {<!-- --> CommandLineParser parser( argc, argv, "{@input | stuff.jpg | input image}" ); Mat src = imread( samples::findFile( parser.get<String>( "@input" ) ) ); if( src.empty() ) {<!-- --> cout << "Could not open or find the image!\ " << endl; cout << "usage: " << argv[0] << " <Input image>" << endl; return -1; } cvtColor( src, src_gray, COLOR_BGR2GRAY ); blur( src_gray, src_gray, Size(3,3) ); const char* source_window = "Source"; namedWindow( source_window ); imshow(source_window, src); const int max_thresh = 255; createTrackbar( "Canny thresh:", source_window, & amp;thresh, max_thresh, thresh_callback ); thresh_callback(0, 0); waitKey(); return 0; } void thresh_callback(int, void* ) {<!-- --> Mat canny_output; Canny(src_gray, canny_output, thresh, thresh*2, 3); vector<vector<Point> > contours; findContours( canny_output, contours, RETR_TREE, CHAIN_APPROX_SIMPLE ); vector<Moments> mu(contours.size() ); for( size_t i = 0; i < contours.size(); i + + ) {<!-- --> mu[i] = moments( contours[i] ); } vector<Point2f> mc( contours.size() ); for( size_t i = 0; i < contours.size(); i + + ) {<!-- --> // Add 1e-5 to avoid dividing by zero mc[i] = Point2f( static_cast<float>(mu[i].m10 / (mu[i].m00 + 1e-5)), static_cast<float>(mu[i].m01 / (mu[i].m00 + 1e-5)) ); cout << "mc[" << i << "]=" << mc[i] << endl; } Mat drawing = Mat::zeros( canny_output.size(), CV_8UC3 ); for( size_t i = 0; i< contours.size(); i + + ) {<!-- --> Scalar color = Scalar( rng.uniform(0, 256), rng.uniform(0,256), rng.uniform(0,256) ); drawContours( drawing, contours, (int)i, color, 2 ); circle(drawing, mc[i], 4, color, -1); } imshow( "Contours", drawing ); cout << "\t Info: Area and Contour Length \ "; for( size_t i = 0; i < contours.size(); i + + ) {<!-- --> cout << " * Contour[" << i << "] - Area (M_00) = " << std::fixed << std::setprecision(2) << mu[i].m00 << " - Area OpenCV: " << contourArea(contours[i]) << " - Length: " << arcLength( contours[i], true ) << endl; } }
Java
The tutorial code is shown below. You can also download it from here
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) {<!-- --> 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); } Imgproc.cvtColor(src, srcGray, Imgproc.COLOR_BGR2GRAY); Imgproc.blur(srcGray, srcGray, new Size(3, 3)); // 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); // Use the content pane's default border layout. No need // setLayout(new BorderLayout()); //Show 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)); 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(); } }); 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() {<!-- --> Mat cannyOutput = new Mat(); Imgproc.Canny(srcGray, cannyOutput, threshold, threshold * 2); List<MatOfPoint> contours = new ArrayList<>(); Mat hierarchy = new Mat(); Imgproc.findContours(cannyOutput, contours, hierarchy, Imgproc.RETR_TREE, Imgproc.CHAIN_APPROX_SIMPLE); List<Moments> mu = new ArrayList<>(contours.size()); for (int i = 0; i < contours.size(); i + + ) {<!-- --> mu.add(Imgproc.moments(contours.get(i))); } List<Point> mc = new ArrayList<>(contours.size()); for (int i = 0; i < contours.size(); i + + ) {<!-- --> //Add 1e-5 to avoid dividing 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))); } Mat drawing = Mat.zeros(cannyOutput.size(), CvType.CV_8UC3); 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); } imgContoursLabel.setIcon(new ImageIcon(HighGui.toBufferedImage(drawing))); frame.repaint(); System.out.println("\t Info: Area and Contour Length \ "); for (int i = 0; i < contours.size(); i + + ) {<!-- --> System.out.format(" * Contour[%d] - Area (M_00) = %.2f - Area OpenCV: %.2f - Length: %.2f\ ", 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 local OpenCV library System.loadLibrary(Core.NATIVE_LIBRARY_NAME); // Arrange tasks for the event dispatch thread: // Create and display the graphical user interface for this application. javax.swing.SwingUtilities.invokeLater(new Runnable() {<!-- --> @Override public void run() {<!-- --> new MomentsClass(args); } }); } }
Python
The tutorial code is shown below. You can also download it from here
from __future__ import print_function from __future__ import division import cv2 as cv import numpy as np import argparse import random as rng rng.seed(12345) def thresh_callback(val): threshold = val canny_output = cv.Canny(src_gray, threshold, threshold * 2) contours, _ = cv.findContours(canny_output, cv.RETR_TREE, cv.CHAIN_APPROX_SIMPLE) # Get moments mu = [None]*len(contours) for i in range(len(contours)): mu[i] = cv.moments(contours[i]) # Get the center of mass mc = [None]*len(contours) for i in range(len(contours)): # Add 1e-5 to avoid dividing by zero mc[i] = (mu[i]['m10'] / (mu[i]['m00'] + 1e-5), mu[i]['m01'] / (mu[i]['m00 '] + 1e-5)) # Draw contour lines drawing = np.zeros((canny_output.shape[0], canny_output.shape[1], 3), dtype=np.uint8) for i in range(len(contours)): color = (rng.randint(0,256), rng.randint(0,256), rng.randint(0,256)) cv.drawContours(drawing, contours, i, color, 2) cv.circle(drawing, (int(mc[i][0]), int(mc[i][1])), 4, color, -1) cv.imshow('Contours', drawing) # Calculate the area using matrix 00 and compare with the result of the OpenCV function for i in range(len(contours)): print(' * Contour[%d] - Area (M_00) = %.2f - Area OpenCV: %.2f - Length: %.2f' % (i, mu[i]['m00'], cv.contourArea( contours[i]), cv.arcLength(contours[i], True))) parser = argparse.ArgumentParser(description='Code for Image Moments tutorial.') parser.add_argument('--input', help='Path to input image.', default='stuff.jpg') args = parser.parse_args() src = cv.imread(cv.samples.findFile(args.input)) if src is None: print('Could not open or find the image:', args.input) exit(0) # Convert image to gray and blur src_gray = cv.cvtColor(src, cv.COLOR_BGR2GRAY) src_gray = cv.blur(src_gray, (3,3)) source_window = 'Source' cv.namedWindow(source_window) cv.imshow(source_window, src) max_thresh = 255 thresh = 100 # Initial threshold cv.createTrackbar('Canny Thresh:', source_window, thresh, max_thresh, thresh_callback) thresh_callback(thresh) cv.waitKey()
Results
right here