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Image Moments

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Goal

In this tutorial you will learn how to:

Theory

Code

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

#include <iostream>
#include <iomanip>
using namespace cv;
using namespace std;
Mat src_gray;
int thresh = 100;
RNG rng(12345);
void thresh_callback(int, void* );
int main( int argc, char** argv )
{
CommandLineParser parser( argc, argv, "{@input | stuff.jpg | input image}" );
Mat src = imread( samples::findFile( parser.get<String>( "@input" ) ) );
if( src.empty() )
{
cout << "Could not open or find the image!\n" << endl;
cout << "usage: " << argv[0] << " <Input image>" << endl;
return -1;
}
cvtColor( src, src_gray, COLOR_BGR2GRAY );
blur( src_gray, src_gray, Size(3,3) );
const char* source_window = "Source";
namedWindow( source_window );
imshow( source_window, src );
const int max_thresh = 255;
createTrackbar( "Canny thresh:", source_window, &thresh, max_thresh, thresh_callback );
thresh_callback( 0, 0 );
return 0;
}
void thresh_callback(int, void* )
{
Mat canny_output;
Canny( src_gray, canny_output, thresh, thresh*2, 3 );
vector<vector<Point> > contours;
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] );
}
vector<Point2f> mc( contours.size() );
for( size_t i = 0; i < contours.size(); i++ )
{
//add 1e-5 to avoid division by zero
mc[i] = Point2f( static_cast<float>(mu[i].m10 / (mu[i].m00 + 1e-5)),
static_cast<float>(mu[i].m01 / (mu[i].m00 + 1e-5)) );
cout << "mc[" << i << "]=" << mc[i] << endl;
}
Mat drawing = Mat::zeros( canny_output.size(), CV_8UC3 );
for( size_t i = 0; i< contours.size(); i++ )
{
Scalar color = Scalar( rng.uniform(0, 256), rng.uniform(0,256), rng.uniform(0,256) );
drawContours( drawing, contours, (int)i, color, 2 );
circle( drawing, mc[i], 4, color, -1 );
}
imshow( "Contours", drawing );
cout << "\t Info: Area and Contour Length \n";
for( size_t i = 0; i < contours.size(); i++ )
{
cout << " * Contour[" << i << "] - Area (M_00) = " << std::fixed << std::setprecision(2) << mu[i].m00
<< " - Area OpenCV: " << contourArea(contours[i]) << " - Length: " << arcLength( contours[i], true ) << endl;
}
}

Explanation

Result

Here it is:

Moments_Source_Image.jpg
Moments_Result1.jpg
Moments_Result2.jpg