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, char** argv ) { src = imread( argv[1], IMREAD_COLOR ); 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: