Find contours in an image and output x,y data structure of all found contours (OpenCV)

build-shape-detect.sh

#!/bin/sh
i="shape-detect.cpp"
            echo "compiling $i"
            g++ -ggdb `pkg-config --cflags opencv` -o `basename $i .cpp` $i `pkg-config --libs opencv`;

shape-detect.cpp

/* 

 Usage: ./shape-detect.cpp image-name-with-shapes-in.jpg
 Feed in an image to process and two windows will display, one with the image to be process with a slider to adjust threshold
 and another window to display the results.

 A JSON like data structure of id,x,y data for each shape detected will be output to the command line. 
 Each adjustment of the threshold will output a new data structure.

 This file is set to draw small circles but the code is there to draw a polygonal contour or bonding rectangles.

 You will need the OpenCV library to use this.

 A build script is also included here.
 
*/

#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <fstream>

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, CV_BGR2GRAY );

  /// adding a blur can sometimes improve results on otherwise noisy images
  /// blur( src_gray, src_gray, Size(3,3) );

  /// Create Window
  char* source_window = "Source";
  namedWindow( source_window, CV_WINDOW_AUTOSIZE );
  imshow( source_window, src );

  createTrackbar( " Threshold:", "Source", &thresh, max_thresh, thresh_callback );
  thresh_callback( 0, 0 );

  waitKey(0);
  return(0);
}


/** @function thresh_callback */
void thresh_callback(int, void* )
{
  Mat threshold_output;
  vector<vector<Point> > contours;
  vector<Vec4i> hierarchy;

  /// Detect edges using Threshold
  threshold( src_gray, threshold_output, thresh, 255, THRESH_BINARY );
  /// Find contours
  findContours( threshold_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );

  /// Approximate contours to polygons + get bounding rects and circles
  vector<vector<Point> > contours_poly( contours.size() );
  vector<Rect> boundRect( contours.size() );
  vector<Point2f>center( contours.size() );
  vector<float>radius( contours.size() );

  for( int i = 0; i < contours.size(); i++ )
     { approxPolyDP( Mat(contours[i]), contours_poly[i], 3, true );
       boundRect[i] = boundingRect( Mat(contours_poly[i]) );
       minEnclosingCircle( contours_poly[i], center[i], radius[i] );
     }


  /// Draw polygonal contour, bonding rects or circles
  Mat drawing = Mat::zeros( threshold_output.size(), CV_8UC3 );
  for( int i = 0; i< contours.size(); i++ )
     {
       // use a random color:     Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
       Scalar color = Scalar( 0, 0, 240 );

       // draw contours:     drawContours( drawing, contours_poly, i, color, 1, 8, vector<Vec4i>(), 0, Point() );
       // draw rectangle:    rectangle( drawing, boundRect[i].tl(), boundRect[i].br(), color, 2, 8, 0 );
       // draw circle with dynamic radious:     circle( drawing, center[i], (int)radius[i], color, 2, 8, 0 );
        circle( drawing, center[i], 2, color, 2, 8, 0 );

       /// print data sctructure to command line to be used in another program etc
       cout << "{id:" << i << ",x:" << center[i].x << ",y:" << center[i].y << "}," << flush;
     }

  /// print some clear lines to command line to seperate output from different runs
  cout << "\n \n" << flush;

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

I'm using OpenCV4Tegra and Linux, Jetson TK1 board.

I met this error 👍
ubuntu@tegra-ubuntu:/FindBiggestBlobWithGithubContours/build$ ./'ZED with OpenCV'
terminate called after throwing an instance of 'std::logic_error'
what(): basic_string::_S_construct null not valid
Aborted
ubuntu@tegra-ubuntu:/FindBiggestBlobWithGithubContours/build$ ./'ZED with OpenCV'
terminate called after throwing an instance of 'std::logic_error'
what(): basic_string::_S_construct null not valid
Aborted