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Hi there..Im new to this opencv and really need the explanation how to solve the problem. I try to get a multiple of ROI (array) from a known coordinate but keep on getting this error.

Anyone can figure it out what the problem is and how to solve it?? This is my code:

#include "opencv2/core/core.hpp"
#include "opencv2/features2d/features2d.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/nonfree/features2d.hpp"

using namespace cv;

int main( )
{

  Mat img_1 = imread( "Colour2.bmp", CV_LOAD_IMAGE_GRAYSCALE );
  Mat img_2 = imread( "Greyscale.bmp", CV_LOAD_IMAGE_GRAYSCALE );
  int i;

  if( !img_1.data || !img_2.data )
  { std::cout<< " --(!) Error reading images " << std::endl; return -1; }

  //-- Step 1: Detect the keypoints using SURF Detector
  int minHessian = 400;

  SurfFeatureDetector detector( minHessian );

  std::vector<KeyPoint> keypoints_1;
  std::vector<KeyPoint> keypoints_2;

  detector.detect( img_1, keypoints_1 );
  detector.detect( img_2, keypoints_2 );

  //-- Draw keypoints
  Mat img_keypoints_1,img_keypoints_2;

  drawKeypoints( img_1, keypoints_1, img_keypoints_1, Scalar::all(-1), DrawMatchesFlags::DEFAULT );
  drawKeypoints( img_2, keypoints_2, img_keypoints_2, Scalar::all(-1), DrawMatchesFlags::DEFAULT );

  //-- Show detected (drawn) keypoints
  imshow("Input", img_keypoints_1 );
  imshow("Input2", img_keypoints_2 );

  //-- Step 2: Calculate descriptors (feature vectors)
  SurfDescriptorExtractor extractor;

  Mat descriptors_object, descriptors_scene;

  extractor.compute( img_1, keypoints_1, descriptors_object );
  extractor.compute( img_2, keypoints_2, descriptors_scene );

  //-- Step 3: Matching descriptor vectors using FLANN matcher
  FlannBasedMatcher matcher;  
  //BFMatcher matcher(NORM_L1);
  std::vector< DMatch > matches;
  matcher.match( descriptors_object, descriptors_scene, matches );

  double max_dist = 0; double min_dist = 150; double dist;

  //Quick calculation of min and max distances between keypoints
  for(int i=0; i<descriptors_object.rows; i++)
  {
    dist = matches[i].distance;
    if( dist < min_dist ) min_dist = dist;
    if( dist > max_dist ) max_dist = dist;
  }

  vector< DMatch > good_matches;

  for(int i = 0; i < descriptors_object.rows; i++)
  {
        if( matches[i].distance < 3*min_dist) 
            good_matches.push_back( matches[i] );
  }

  Point c1, c2;
  std::vector<cv::Mat> ROI;
  Mat img_3 = cv::Mat::zeros(img_2.rows, img_2.cols, CV_8UC1);

  for( unsigned int i = 0; i < good_matches.size(); i++ )
  {
    //-- Get the keypoints from the good matches
        c1.x =  keypoints_1[ good_matches[i].queryIdx ].pt.x;
        c1.y =  keypoints_1[ good_matches[i].queryIdx ].pt.y;

        c2.x=  keypoints_2[ good_matches[i].trainIdx ].pt.x;
        c2.y=  keypoints_2[ good_matches[i].trainIdx ].pt.y;

        //Check the coordinate of good matches
        printf( "-- Good Match [%d] Keypoint 1.x: %d  -- Keypoint 1.y: %d  \n", i, c1.x, c1.y ); 

        /* This is where the problems begin
        //Get@Crop ROI for each good matches on keypoints_1 with area 20x20
        ROI[i] = img_1( cv::Rect(c1.x, c1.y, 20, 20)); 

        //Paste the previous ROI on another image with the same coordinate in img_2
        ROI[i].copyTo(img_3( cv::Rect(c2.x, c2.y, 20, 20)));
        */
  }
  //show final image
  imshow("Final ",img_3);

  waitKey(0);

  return 0;
}