opencv/doc/tutorials/imgproc/shapedescriptors/moments/moments.markdown

Image Moments

Goal

In this tutorial you will learn how to:

• Use the OpenCV function @ref cv::moments
• Use the OpenCV function @ref cv::contourArea
• Use the OpenCV function @ref cv::arcLength

Theory

Code

This tutorial code's is shown lines below. You can also download it from here @code{.cpp} #include "opencv2/highgui.hpp" #include "opencv2/imgproc.hpp" #include #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);

/// Function header void thresh_callback(int, void* );

/* @function main / int main( int argc, char* argv ) { /// Load source image and convert it to gray src = imread( argv[1], 1 );

/// Convert image to gray and blur it cvtColor( src, src_gray, COLOR_BGR2GRAY ); blur( src_gray, src_gray, Size(3,3) );

/// Create Window 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); }

/* @function thresh_callback / void thresh_callback(int, void ) { Mat canny_output; vector<vector > contours; vector hierarchy;

/// Detect edges using canny Canny( src_gray, canny_output, thresh, thresh*2, 3 ); /// Find contours findContours( canny_output, contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE, Point(0, 0) );

/// Get the moments vector mu(contours.size() ); for( int i = 0; i < contours.size(); i++ ) { mu[i] = moments( contours[i], false ); }

/// Get the mass centers: vector mc( contours.size() ); for( int i = 0; i < contours.size(); i++ ) { mc[i] = Point2f( mu[i].m10/mu[i].m00 , mu[i].m01/mu[i].m00 ); }

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

/// Show in a window namedWindow( "Contours", WINDOW_AUTOSIZE ); imshow( "Contours", drawing );

/// Calculate the area with the moments 00 and compare with the result of the OpenCV function printf("\t Info: Area and Contour Length \n"); for( int i = 0; i< contours.size(); i++ ) { printf(" * Contour[%d] - Area (M_00) = %.2f - Area OpenCV: %.2f - Length: %.2f \n", 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, i, color, 2, 8, hierarchy, 0, Point() ); circle( drawing, mc[i], 4, color, -1, 8, 0 ); } } @endcode Explanation

Result

1. Here it is:

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   |MU_0| |MU_1| |MU_2|

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