Find and draw largest rect OpenCV - c++

Question

I need to find the largest contour/rect on this image which should be the card.
I try to use the following code but I get no drawing:
int largest_area=0;
int largest_contour_index=0;
cv::Rect bounding_rect;
Mat thr(src.rows,src.cols,CV_8UC1);
Mat dst(src.rows,src.cols,CV_8UC1,Scalar::all(0));
cvtColor(src,thr,CV_BGR2GRAY); //Convert to gray
threshold(thr, thr,25, 255,THRESH_BINARY); //Threshold the gray
vector<vector<cv::Point>> contours; // Vector for storing contour
vector<Vec4i> hierarchy;
findContours( thr, contours, hierarchy,CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE ); // Find the contours in the image
for( int i = 0; i< contours.size(); i++ ) // iterate through each contour.
{
double a=contourArea( contours[i],false); // Find the area of contour
if(a>largest_area){
largest_area=a;
largest_contour_index=i; //Store the index of largest contour
bounding_rect=boundingRect(contours[i]); // Find the bounding rectangle for biggest contour
}
}
Scalar color( 255,255,255);
drawContours( dst, contours,largest_contour_index, color, CV_FILLED, 8, hierarchy ); // Draw the largest contour using previously stored index.
rectangle(src, bounding_rect, Scalar(0,255,0),1, 8,0);
Could someone provide me with an example using this image to find the largest rect?

Answer 1

You can try yourself by increasing the threshold.
Here You are finding biggest contour on thresholded image, so display thr just after threshold() using imshow() and see what going on , and how it's look like.
See the result by increasing the threshold to little higher value.
threshold(thr, thr,100, 255,THRESH_BINARY); //Threshold the gray
Threshold image
Bounding rect for biggest contour