OpenCV code doesn't work on specific image - c++

Question

I am trying to run the followin code (based on this page) on an image, but it doesn't work:
Mat src=imread("img.jpg",1);
Mat tmp,thr;
cvtColor(src,tmp,CV_BGR2GRAY);
threshold(tmp,thr,200,255,THRESH_BINARY_INV);
vector< vector <Point> > contours;
vector< Vec4i > hierarchy;
Mat dst(src.rows,src.cols,CV_8UC1,Scalar::all(0));//Ceate Mat to draw contour
int box_w=10; // Define box width here
int box_h=10; // Define box height here
int threshold_perc=25; //perceantage value for eliminating the box according to pixel count inside the box
int threshold=(box_w*box_h*threshold_perc)/100;
findContours( thr, contours, hierarchy,CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE ); //Find contour
for( int i = 0; i< contours.size(); i++ ){
drawContours( dst,contours, i, Scalar(255,255,255),CV_FILLED, 8, hierarchy ); // Draw contour with thickness = filled
Rect r= boundingRect(contours[i]); // Find bounding rect
// Scan the image with in bounding box
for(int j=r.x;j<r.x+r.width;j=j+box_w){
for(int k=r.y;k<r.y+r.height;k=k+box_h){
Rect roi_rect(j,k,box_w,box_h);
Mat roi = dst(roi_rect);
int count = countNonZero(roi);
if(count > threshold)
rectangle(src, roi_rect, Scalar(255,0,0),1,8,0 );
}
}
}
imshow("src",src);
waitKey();
It works fine for any normal image, but for the images below, it either breaks or doesn't find the contour and draws boxes all over the image.
It says:
Unhandled exception at 0x00007FF9A72DA388 in test2.exe: Microsoft C++ exception: cv::Exception at memory location 0x000000FECC9DEAC0.
It breaks and points to here:
inline
Mat Mat::operator()( const Rect& roi ) const
{
return Mat(*this, roi);
}
in mat.inl.hpp.
What is wrong with my image? I have changed it from Gray-scale to RGB, but didn't help.
On the following image, it works fine:

Answer 1

As I commented, you're trying to access a region of the image that doesn't exist by using a rectangle of fixed size.
By intersecting the roi with the rectangle, you can avoid this problem:
Mat roi = dst(roi_rect & r);

Answer 2

The problem was that in the first images, the contour gets close to the boundaries of the image and in the bottom for loop of the program, it exceeds the coordinates. It was fixed with this:
// Scan the image with in bounding box
for (int j = r.x;j<r.x + r.width;j = j + box_w) {
for (int k = r.y;k<r.y + r.height;k = k + box_h) {
Rect roi_rect(j, k, box_w, box_h);
if (j + box_w < dst.cols && k + box_h < dst.rows)
{
Mat roi = dst(roi_rect);
int count = countNonZero(roi);
if (count > threshold)
rectangle(src, roi_rect, Scalar(0,0,255), 1, 8, 0);
}
}
}