How to delete a QR CODE white margins？

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1. Create a mask image for the area to be cropped

• BGR2GRAY conversion.
• Threshold with CV_THRESH_BINARY_INV.
• Morphology Transformations with MORPH_CLOSE option, here you should use large kernel.

2.Find the bounding box for mask, you should do the following

• Find contour.
• Bounding Rect.

3.Crop the source using bound rect.

Code:

   Mat src=imread("barcode.png",1);
   Mat thr,gray;
   cvtColor(src,gray,CV_BGR2GRAY);
   threshold( gray, thr, 50, 255, CV_THRESH_BINARY_INV );


   Size kernalSize (21,21);
   Mat element = getStructuringElement (MORPH_RECT, kernalSize, Point(1,1)  );
   morphologyEx( thr, thr, MORPH_CLOSE, element );

   vector< vector <Point> > contours;
   vector< Vec4i > hierarchy;
   findContours( thr, contours, hierarchy,CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE );
   Rect ROI=boundingRect(contours[0]); //Contour size  should bealways 1

   Mat crop=src(ROI);
   imwrite("crop.jpg",crop);