How to find convexity defects using opencv?

Question

I have the following code

        #include"opencv2/opencv.hpp"

          #include<iostream>
            #include<math.h>

        using namespace std;
        using namespace cv;

main()
{
Mat img1,img2,sub,gray1,gray2,lab,ycbcr;
int v[3];

int row,col,i,j,t;
VideoCapture cap(0);

namedWindow("current");

cap>>img1;
sub=img1;

row=img1.rows;
col=img1.cols;

cvtColor(img1,gray1,CV_BGR2GRAY);


vector<vector<Point> > cont;

vector<Vec4i> hierarchy;

while (1) {

    cap>>img2;

    cvtColor(img2,gray2,CV_BGR2GRAY);


    for(i=0;i<row;++i)
    {
        for (j=0; j<col; ++j)
        {


            if(abs(gray1.at<uchar>(i,j) - gray2.at<uchar>(i,j))>10)
            {
                sub.at<Vec3b>(i,j)[0] = img2.at<Vec3b>(i,j)[0];
                sub.at<Vec3b>(i,j)[1] = img2.at<Vec3b>(i,j)[1];
                sub.at<Vec3b>(i,j)[2] = img2.at<Vec3b>(i,j)[2];

            }
            else
            {

                sub.at<Vec3b>(i,j)[0]=0;
                sub.at<Vec3b>(i,j)[1]=0;
                sub.at<Vec3b>(i,j)[2]=0;
            }

        }


    }


    cvtColor(sub,ycbcr,CV_BGR2YCrCb);

    inRange(ycbcr,Scalar(7,133,106),Scalar(255,178,129),ycbcr);
    findContours(ycbcr,cont,hierarchy,CV_RETR_LIST,CV_CHAIN_APPROX_SIMPLE);
    Scalar color = CV_RGB(255,255,255);
    vector<vector<Point> >  hullPoints(cont.size());
    for(  i = 0; i < cont.size(); i++ )
    convexHull(cont[i],hullPoints[i],false);

    for(i = 0 ;i >= 0; i = hierarchy[i][0] )
    {
        drawContours( ycbcr, cont, i, color,1, CV_AA, hierarchy );//for drawing contours
        drawContours( ycbcr, hullPoints, i, color,2, CV_AA, hierarchy );//for drawing convex hull
    }

    flip(ycbcr,ycbcr,1);
    imshow("current",ycbcr);

    if(waitKey(33)=='q')
        break;

    img1=img2.clone();
}

}

How to find the convexity defects in this convexHull..cvConvexityDefects() require const cvArr * as arguments.But i have vector point type result from convexHull..So how to type cast ..?

Have a look at [this question](http://stackoverflow.com/questions/6806637/convexity-defects-c-opencv) — mathematical.coffee, Feb 06 '12 at 06:39
What version of OpenCV you are using? AFAIK cvConvexHull function is legacy, and in new versions you should use cvConvexHull2. Also OpenCV have CvPoint, CvMat and CvSequense types - why not to use them, instead of merging std and opencv types? — Mikhail Churbanov, Feb 06 '12 at 06:48

score 1 · Answer 1 · edited May 23 '17 at 12:29

1

You should be using the vector<vector<int>> type for your convexHull calculations:

convexHull(Mat(contours), hullsI, false); // false means it will return the indices at which the hulls are found and not the point of the convex hull

Then call:

convexityDefects(contours, hullsI, defects);

Similar to Convexity defects C++ OpenCv and many other SO questions :) Hope that helps!

edited May 23 '17 at 12:29

Community

1
1

answered Apr 04 '13 at 01:20

Eleanor

11
1

Just the found the SO question that helped me solve the same issue. http://stackoverflow.com/questions/10620981/calculating-convexitydefects-using-opencv-2-4-in-c – Eleanor Apr 04 '13 at 01:34

How to find convexity defects using opencv?

1 Answers1