OpenCV C++ cv::convexityDefects error

Question

vector hull;
vector defects;
convexHull(Mat(largest),hull,false);
convexityDefects(largest,hull,defects);

*largest i

Answer 1

The second argument of convexityDefects has to be the type of vector<vector<int>, while yours is vector<Point>.

Answer 2

for hull you should use a vector of vectors like this:

vector<vector<Point>> hullsP( contours.size() );
vector<vector<int> > hullsI(contours.size());

and pass the "int" type to covexityDefects.like this :

vector<vector<Vec4i>> convdefect(contours.size());

for( int i = 0; i < contours.size(); i++ )
{ 
    convexHull( Mat(contours[i]), hullsP[i], false );
    convexHull( Mat(contours[i]), hullsI[i], false );       
    if(hullsI[i].size() > 3 )
        convexityDefects(contours[i],hullsI[i],convdefect[i]);
}