Question

I’m developing project using javacv and I was able to identify polygon and I store that details in CvSeq. I try to access the points in that structure but it doesn’t work for me. So please can some one explain how to access points in CvSeq structure in javacv?

For example I need to access the 8 edge points of following image. So i could be able to access the lengths of each side of the polygon.

Answer 1

You can go through following class to identify the points of CvSeq. This method is use to identify the points in CvSeq.

        for(int i = 0; i < rslt.total(); i++){
                CvPoint v=new CvPoint(cvGetSeqElem(rslt, i));
                cvDrawCircle(image, v, 5, CvScalar.BLUE, -1, 8, 0);
                System.out.println(" X value = "+v.x()+" ; Y value ="+v.y());
        }

In above method rslt is CvSeq that contain Contour details and in cvDrawCircle() method it will draw circle on each point with Blue color. and next line will print the x,y coordinates of each point.

    import com.googlecode.javacpp.Loader;
    import com.googlecode.javacv.CanvasFrame;
    import static com.googlecode.javacpp.Loader.*;
    import static com.googlecode.javacv.cpp.opencv_core.*;
    import static com.googlecode.javacv.cpp.opencv_highgui.*;
    import static com.googlecode.javacv.cpp.opencv_imgproc.*;

    public class Test {
        public static void main(String[] args) {
            CvMemStorage storage=CvMemStorage.create();
            CvSeq squares = new CvContour();
            squares = cvCreateSeq(0, sizeof(CvContour.class), sizeof(CvSeq.class), storage);
            String path="C:/Users/Smash/Desktop/A.PNG";
            IplImage src = cvLoadImage(path);//hear path is actual path to image
            IplImage gry=cvCreateImage(cvGetSize(src),IPL_DEPTH_8U,1);
            cvCvtColor(src, gry, CV_BGR2GRAY);
            cvFindContours(gry, storage, squares, Loader.sizeof(CvContour.class), CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);
            CvSeq ss=null;
            for (int i=0; i<1; i++)
            {
                cvDrawContours(gry, squares, CvScalar.WHITE, CV_RGB(248, 18, 18), 1, -1, 8);
                ss=cvApproxPoly(squares, sizeof(CvContour.class), storage, CV_POLY_APPROX_DP, 8, 0);
            }
            if(!ss.isNull()){
                drawPoly(src, ss);
            }
        }
        public static void drawPoly( IplImage image, final CvSeq Poly )
        {
            CvSeq rslt=cvApproxPoly(Poly, Loader.sizeof(CvContour.class), cvCreateMemStorage(0), CV_POLY_APPROX_DP, cvContourPerimeter(Poly)*0.02, 0);
            System.out.println(rslt.total());
            for(int i = 0; i < rslt.total(); i++){
                    CvPoint v=new CvPoint(cvGetSeqElem(rslt, i));
                    cvDrawCircle(image, v, 5, CvScalar.BLUE, -1, 8, 0);
                    System.out.println(" X value = "+v.x()+" ; Y value ="+v.y());
            }
            final CanvasFrame canvas = new CanvasFrame("Point Identify");
            canvas.setDefaultCloseOperation(javax.swing.JFrame.EXIT_ON_CLOSE);
            canvas.showImage(image);
        }
    }

This is the output image

This are the related x,y coordinates

 X value = 72 ; Y value =61
 X value = 72 ; Y value =177
 X value = 211 ; Y value =178
 X value = 211 ; Y value =380
 X value = 315 ; Y value =380
 X value = 316 ; Y value =177
 X value = 465 ; Y value =177
 X value = 465 ; Y value =61