Convexity defects C++ OpenCv

Question

I would be grateful to you if you could help me with this issue :)

Relating to this question cvConvexityDefects in OpenCV 2.X / C++?, I have the same problem. The OpenCV C++ wrapper has not the function cvConvexityDefects that appears in the C version, so I tried to write my own version.

Part of the code is (please note that both countour and hull are vector< Point >, calculated separately :

CvSeq* contourPoints;
CvSeq* hullPoints;
CvSeq* defects;
CvMemStorage* storage;
CvMemStorage* strDefects;
CvMemStorage* contourStr;
CvMemStorage* hullStr;
CvConvexityDefect *defectArray = 0;

strDefects = cvCreateMemStorage();
defects = cvCreateSeq( CV_SEQ_KIND_GENERIC|CV_32SC2, sizeof(CvSeq),sizeof(CvPoint), strDefects );

//We start converting vector<Point> resulting from findContours
contourStr = cvCreateMemStorage();
contourPoints = cvCreateSeq(CV_SEQ_KIND_GENERIC|CV_32SC2, sizeof(CvSeq), sizeof(CvPoint), contourStr);
printf("Metiendo valores\n");
for(int i=0; i<(int)contour.size(); i++) {
    CvPoint cp = {contour[i].x,  contour[i].y};
    cvSeqPush(contourPoints, &cp);
}
//Now, the hull points obtained from convexHull c++
hullStr = cvCreateMemStorage(0);
hullPoints = cvCreateSeq(CV_SEQ_KIND_GENERIC|CV_32SC2, sizeof(CvSeq), sizeof(CvPoint), hullStr);
for(int i=0; i<(int)hull.size(); i++) {
    CvPoint cp = {hull[i].x,  hull[i].y};
    cvSeqPush(hullPoints, &cp);
}

//And we compute convexity defects
storage = cvCreateMemStorage(0);
defects = cvConvexityDefects(contourPoints, hullPoints, storage);

The output is Convex hull must represented as a sequence of indices or sequence of pointers in function cvConvexityDefects. Really I don't know how to do conversion in the right way, I've ben searching on the web and tried to adapt/copy/understand some pieces of code, but it is always with the C syntax.

I hope I was clear. Thank you in advance!

Answer 1

I raised this question because I wasn't able to figure out a solution (it is not only today that I was dealing with the matter hehe), but after all I was able to manage the problem!

I had to change the way I calculated the convex hull, using the index array form. So now we have a vector< int > instead a vector< Point >.

This is the code I used (it works I painted the points over an image):

void HandDetection::findConvexityDefects(vector<Point>& contour, vector<int>& hull, vector<Point>& convexDefects){
    if(hull.size() > 0 && contour.size() > 0){
    CvSeq* contourPoints;
    CvSeq* defects;
    CvMemStorage* storage;
    CvMemStorage* strDefects;
    CvMemStorage* contourStr;
    CvConvexityDefect *defectArray = 0;

    strDefects = cvCreateMemStorage();
    defects = cvCreateSeq( CV_SEQ_KIND_GENERIC|CV_32SC2, sizeof(CvSeq),sizeof(CvPoint), strDefects );

    //We transform our vector<Point> into a CvSeq* object of CvPoint.
    contourStr = cvCreateMemStorage();
    contourPoints = cvCreateSeq(CV_SEQ_KIND_GENERIC|CV_32SC2, sizeof(CvSeq), sizeof(CvPoint), contourStr);
    for(int i=0; i<(int)contour.size(); i++) {
        CvPoint cp = {contour[i].x,  contour[i].y};
        cvSeqPush(contourPoints, &cp);
    }

    //Now, we do the same thing with the hull index
    int count = (int)hull.size();
    //int hullK[count];
    int* hullK = (int*)malloc(count*sizeof(int));
    for(int i=0; i<count; i++){hullK[i] = hull.at(i);}
    CvMat hullMat = cvMat(1, count, CV_32SC1, hullK);

    //We calculate convexity defects
    storage = cvCreateMemStorage(0);
    defects = cvConvexityDefects(contourPoints, &hullMat, storage);
    defectArray = (CvConvexityDefect*)malloc(sizeof(CvConvexityDefect)*defects->total);
    cvCvtSeqToArray(defects, defectArray, CV_WHOLE_SEQ);
    //printf("DefectArray %i %i\n",defectArray->end->x, defectArray->end->y);

    //We store defects points in the convexDefects parameter.
    for(int i = 0; i<defects->total; i++){
        CvPoint ptf;
        ptf.x = defectArray[i].depth_point->x;
        ptf.y = defectArray[i].depth_point->y;
        convexDefects.push_back(ptf);
    }

    //We release memory
    cvReleaseMemStorage(contourStr);
    cvReleaseMemStorage(strDefects);
    cvReleaseMemStorage(storage);
    }
}

This worked for me. If you see something wrong or another way to manage it, please tell me!