| OpenCV 3.4.1 Open Source Computer Vision

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.hpp"
#include "opencv2/imgproc.hpp"
#include <iostream>
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 argc, char** argv )
{
CommandLineParser parser( argc, argv, "{@input | ../data/stuff.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;
exit(0);
}
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;
Canny( src_gray, canny_output, thresh, thresh*2, 3 );
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], 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, LINE_8 );
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, LINE_8 );
circle( drawing, mc[i], 4, color, -1, 8, 0 );
}
}

## Explanation

## Result

Here it is: