How to find borders and crop the image

edit

try this code.

i get these images ( to get second image uncomment commented lines )

Note: with C++ the easiest way is to use findNonZero() and boundingRect() functions to achieve this like explained here

                Mat src = Imgcodecs.imread("d:/bt.png");
                Mat bt = new Mat();
                Imgproc.cvtColor(src, bt, Imgproc.COLOR_BGR2GRAY);
                Imgproc.threshold(bt, bt, 0, 255, Imgproc.THRESH_BINARY);
                Imgcodecs.imwrite("d:/bw.png",bt);

                int xmin = bt.cols();
                int ymin = bt.rows();
                int xmax=0;                
                int ymax=0;

                double[] pixel = new double[0];
                for (int x = 0; x < bt.cols(); x++) {
                    for (int y = 0; y < bt.rows(); y++) {

                        pixel = bt.get(y, x);

                        if( pixel[0] == 0)
                        {
                            if( x < xmin )
                            {
                                xmin = x;
                            }

                            if( y < ymin )
                            {
                                ymin = y;
                            }

                            if( x > xmax )
                            {
                                xmax = x;
                            }

                            if( y > ymax )
                            {
                                ymax = y;
                            }
                        }

                    }
                }
                //xmin = xmin - 3;  // you can try to uncomment these lines
                //ymin = ymin - 3;
                //xmax = xmax + 3;
                //ymax = ymax + 3;

                Rect roi = new Rect(xmin, ymin, xmax-xmin, ymax-ymin);
                Mat cr = new Mat(src, roi);