How to find the corners of a Rect object in openCV?

Question

I am using openCV library on the android platform. I have successfully detected the largest rectangle from the image but since my application will be used for the scanning p

Answer 1

1. find the coordinate of left ,right, top,bottom point of the rect
2. min_x = min(left.x right.x,top.x,bottom.x)
   min_y = min(left.y right.y,top.y,bottom.y)
   max_x = max(left.x right.x,top.x,bottom.x)
   max_y = max(left.y right.y,top.y,bottom.y)
3. the corners's coordinate is the point with the min_x, min_y ,max_x, max_y

Answer 2

I am very exciting to answer my question ! It was easy but it happens when u just begin with something with not so proper relevant documentation .

I was trying hard to get the corners of a general rectangle which was not defined in the implementation of openCV and hence was almost impossible.

I followed the standard code on stackoverflow for the largest Square detection. and corners can be easily find out using the approxCurve itself.

//convert the image to black and white

        Imgproc.cvtColor(imgSource, imgSource, Imgproc.COLOR_BGR2GRAY);

        //convert the image to black and white does (8 bit)
        Imgproc.Canny(imgSource, imgSource, 50, 50);

        //apply gaussian blur to smoothen lines of dots
        Imgproc.GaussianBlur(imgSource, imgSource, new  org.opencv.core.Size(5, 5), 5);

        //find the contours
        List<MatOfPoint> contours = new ArrayList<MatOfPoint>();
        Imgproc.findContours(imgSource, contours, new Mat(), Imgproc.RETR_LIST, Imgproc.CHAIN_APPROX_SIMPLE);

        double maxArea = -1;
        int maxAreaIdx = -1;
        Log.d("size",Integer.toString(contours.size()));
        MatOfPoint temp_contour = contours.get(0); //the largest is at the index 0 for starting point
        MatOfPoint2f approxCurve = new MatOfPoint2f();
        MatOfPoint largest_contour = contours.get(0);
        //largest_contour.ge
        List<MatOfPoint> largest_contours = new ArrayList<MatOfPoint>();
        //Imgproc.drawContours(imgSource,contours, -1, new Scalar(0, 255, 0), 1);

        for (int idx = 0; idx < contours.size(); idx++) {
            temp_contour = contours.get(idx);
            double contourarea = Imgproc.contourArea(temp_contour);
            //compare this contour to the previous largest contour found
            if (contourarea > maxArea) {
                //check if this contour is a square
                MatOfPoint2f new_mat = new MatOfPoint2f( temp_contour.toArray() );
                int contourSize = (int)temp_contour.total();
                MatOfPoint2f approxCurve_temp = new MatOfPoint2f();
                Imgproc.approxPolyDP(new_mat, approxCurve_temp, contourSize*0.05, true);
                if (approxCurve_temp.total() == 4) {
                    maxArea = contourarea;
                    maxAreaIdx = idx;
                    approxCurve=approxCurve_temp;
                    largest_contour = temp_contour;
                }
            }
        }

       Imgproc.cvtColor(imgSource, imgSource, Imgproc.COLOR_BayerBG2RGB);
       sourceImage =Highgui.imread(Environment.getExternalStorageDirectory().
                 getAbsolutePath() +"/scan/p/1.jpg");
       double[] temp_double;
       temp_double = approxCurve.get(0,0);       
       Point p1 = new Point(temp_double[0], temp_double[1]);
       //Core.circle(imgSource,p1,55,new Scalar(0,0,255));
       //Imgproc.warpAffine(sourceImage, dummy, rotImage,sourceImage.size());
       temp_double = approxCurve.get(1,0);       
       Point p2 = new Point(temp_double[0], temp_double[1]);
      // Core.circle(imgSource,p2,150,new Scalar(255,255,255));
       temp_double = approxCurve.get(2,0);       
       Point p3 = new Point(temp_double[0], temp_double[1]);
       //Core.circle(imgSource,p3,200,new Scalar(255,0,0));
       temp_double = approxCurve.get(3,0);       
       Point p4 = new Point(temp_double[0], temp_double[1]);
      // Core.circle(imgSource,p4,100,new Scalar(0,0,255));
       List<Point> source = new ArrayList<Point>();
       source.add(p1);
       source.add(p2);
       source.add(p3);
       source.add(p4);
       Mat startM = Converters.vector_Point2f_to_Mat(source);
       Mat result=warp(sourceImage,startM);
       return result;

and the function used for the perspective transform is given below :

 public Mat warp(Mat inputMat,Mat startM) {
            int resultWidth = 1000;
            int resultHeight = 1000;

            Mat outputMat = new Mat(resultWidth, resultHeight, CvType.CV_8UC4);



            Point ocvPOut1 = new Point(0, 0);
            Point ocvPOut2 = new Point(0, resultHeight);
            Point ocvPOut3 = new Point(resultWidth, resultHeight);
            Point ocvPOut4 = new Point(resultWidth, 0);
            List<Point> dest = new ArrayList<Point>();
            dest.add(ocvPOut1);
            dest.add(ocvPOut2);
            dest.add(ocvPOut3);
            dest.add(ocvPOut4);
            Mat endM = Converters.vector_Point2f_to_Mat(dest);      

            Mat perspectiveTransform = Imgproc.getPerspectiveTransform(startM, endM);

            Imgproc.warpPerspective(inputMat, 
                                    outputMat,
                                    perspectiveTransform,
                                    new Size(resultWidth, resultHeight), 
                                    Imgproc.INTER_CUBIC);

            return outputMat;
        }