[OpenCV Tutorial] $14 - Finding contours in your image

Goal

In this tutorial you will learn how to:

• Use the OpenCV function cv::findContours
• Use the OpenCV function cv::drawContours

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);

if (src.empty())

{

cerr << "No image supplied ..." << 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, 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) );

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() );

}

namedWindow( "Contours", WINDOW_AUTOSIZE );

imshow( "Contours", drawing );

}

Explanation

Result

Here it is: