Image Moments

Goal

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

Theory

Code

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

#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>
#include <stdlib.h>

using namespace cv;
using namespace std;

Mat src; Mat src_gray;
int thresh = 100;
int max_thresh = 255;
RNG rng(12345);

void thresh_callback(int, void* );

int main( int, char** argv )
{
 src = imread( argv[1], 1 );

 cvtColor( src, src_gray, COLOR_BGR2GRAY );
 blur( src_gray, src_gray, Size(3,3) );

 const char* source_window = "Source";
 namedWindow( source_window, WINDOW_AUTOSIZE );
 imshow( source_window, src );

 createTrackbar( " Canny thresh:", "Source", &thresh, max_thresh, thresh_callback );
 thresh_callback( 0, 0 );

 waitKey(0);
 return(0);
}

void thresh_callback(int, void* )
{
 Mat canny_output;
 vector<vector<Point> > contours;
 vector<Vec4i> hierarchy;

 Canny( src_gray, canny_output, thresh, thresh*2, 3 );
 findContours( canny_output, contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE, Point(0, 0) );

 vector<Moments> mu(contours.size() );
 for( size_t i = 0; i < contours.size(); i++ )
 { mu[i] = moments( contours[i], false ); }

 vector<Point2f> mc( contours.size() );
 for( size_t i = 0; i < contours.size(); i++ )
 { mc[i] = Point2f( static_cast<float>(mu[i].m10/mu[i].m00) , static_cast<float>(mu[i].m01/mu[i].m00) ); }

 Mat drawing = Mat::zeros( canny_output.size(), CV_8UC3 );
 for( size_t i = 0; i< contours.size(); i++ )
 {
 Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
 drawContours( drawing, contours, (int)i, color, 2, 8, hierarchy, 0, Point() );
 circle( drawing, mc[i], 4, color, -1, 8, 0 );
 }

 namedWindow( "Contours", WINDOW_AUTOSIZE );
 imshow( "Contours", drawing );

 printf("\t Info: Area and Contour Length \n");
 for( size_t i = 0; i< contours.size(); i++ )
 {
 printf(" * Contour[%d] - Area (M_00) = %.2f - Area OpenCV: %.2f - Length: %.2f \n", (int)i, mu[i].m00, contourArea(contours[i]), arcLength( contours[i], true ) );
 Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
 drawContours( drawing, contours, (int)i, color, 2, 8, hierarchy, 0, Point() );
 circle( drawing, mc[i], 4, color, -1, 8, 0 );
 }
}

Explanation

Result

Here it is: