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: