轮廓检测函数：
Imgproc. findContours(image, contours, hierarchy, mode, method)
通过轮廓检测找到面积最大的轮廓并绘制轮廓：

new ArrayList<>();
        Imgproc.findContours(blurredImage,contours,new Mat(),Imgproc.RETR_LIST,Imgproc.CHAIN_APPROX_SIMPLE);
        double maxVal = 0;
        int maxValIdx = 0;
        for (int contourIdx = 0; contourIdx < contours.size(); contourIdx++)
        {
            double contourArea = Imgproc.contourArea(contours.get(contourIdx));
            if (maxVal < contourArea)
            {
                maxVal = contourArea;
                maxValIdx = contourIdx;
            }
        }
 Mat mRgba=new Mat();
        mRgba.create(blurredImage.rows(), blurredImage.cols(), CvType.CV_8UC3);
        //绘制检测到的轮廓
         Imgproc.drawContours(mRgba, contours, maxAreaIdx, new Scalar(0,255,0), 5);

//通过轮廓检测获取4个交点

public static List<Point> getCornersByContour(Mat imgsource){
        List<MatOfPoint> contours=new ArrayList<>();
        //轮廓检测
        Imgproc.findContours(imgsource,contours,new Mat(),Imgproc.RETR_LIST,Imgproc.CHAIN_APPROX_SIMPLE);
        double maxArea=-1;
        int maxAreaIdx=-1;
        MatOfPoint temp_contour=contours.get(0);//假设最大的轮廓在index=0处
        MatOfPoint2f approxCurve=new MatOfPoint2f();
        for (int idx=0;idx<contours.size();idx++){
            temp_contour=contours.get(idx);
            double contourarea=Imgproc.contourArea(temp_contour);
            //当前轮廓面积比最大的区域面积大就检测是否为四边形
            if (contourarea>maxArea){
                //检测contour是否是四边形
                MatOfPoint2f new_mat=new MatOfPoint2f(temp_contour.toArray());
                int contourSize= (int) temp_contour.total();
                MatOfPoint2f approxCurve_temp=new MatOfPoint2f();
                //对图像轮廓点进行多边形拟合
                Imgproc.approxPolyDP(new_mat,approxCurve_temp,contourSize*0.05,true);
                if (approxCurve_temp.total()==4){
                    maxArea=contourarea;
                    maxAreaIdx=idx;
                    approxCurve=approxCurve_temp;
                }
            }
        }
        double[] temp_double=approxCurve.get(0,0);
        Point point1=new Point(temp_double[0],temp_double[1]);

        temp_double=approxCurve.get(1,0);
        Point point2=new Point(temp_double[0],temp_double[1]);

        temp_double=approxCurve.get(2,0);
        Point point3=new Point(temp_double[0],temp_double[1]);
        temp_double=approxCurve.get(3,0);

        Point point4=new Point(temp_double[0],temp_double[1]);

        List<Point> source=new ArrayList<>();
        source.add(point1);
        source.add(point2);
        source.add(point3);
        source.add(point4);
        //对4个点进行排序
        Point centerPoint=new Point(0,0);//质心
        for (Point corner:source){
            centerPoint.x+=corner.x;
            centerPoint.y+=corner.y;
        }
        centerPoint.x=centerPoint.x/source.size();
        centerPoint.y=centerPoint.y/source.size();
        Point lefttop=new Point();
        Point righttop=new Point();
        Point leftbottom=new Point();
        Point rightbottom=new Point();
        for (int i=0;i<source.size();i++){
            if (source.get(i).x<centerPoint.x&&source.get(i).y<centerPoint.y){
                lefttop=source.get(i);
            }else if (source.get(i).x>centerPoint.x&&source.get(i).y<centerPoint.y){
                righttop=source.get(i);
            }else if (source.get(i).x<centerPoint.x&& source.get(i).y>centerPoint.y){
                leftbottom=source.get(i);
            }else if (source.get(i).x>centerPoint.x&&source.get(i).y>centerPoint.y){
                rightbottom=source.get(i);
            }
        }
        source.clear();
        source.add(lefttop);
        source.add(righttop);
        source.add(leftbottom);
        source.add(rightbottom);
        return

ps：轮廓检测的输入图像必须是二值化的图片否则只能检测一个轮廓。