OpenCv Convexity Defect - c++

Question

I am using OpenCV 2.4.6.
I am trying to get all the convexity defect depth_points of the biggest contour. But i am getting the bellow exception
Assertion failed (mtype == type0 || (CV_MAT_CN(mtype) == CV_MAT_CN(type0) && ((1 << type0) & fixedDepthMask) != 0)) in unknown function
the current code is pasted bellow
Thank you for any help
vector<vector<Point> >hulls( 1 );
vector<Point> hull;
std::vector<Vec4i> defects;
if(contours.size()>1){
convexHull( Mat(contours[largest_contour_index]), hulls[0], false );
convexityDefects(contours[largest_contour_index], hulls[0], defects);
drawContours(sourceVideo,contours,largest_contour_index,Scalar(255, 0, 0));
drawContours(sourceVideo,hulls,0,Scalar(0, 255, 0));
}
the error is in this line
convexityDefects(contours[largest_contour_index], hulls[0], defects);

Answer 1

Try this it works for me:
#include <stdlib.h>
#include <stdio.h>
#include <iostream>
#include <ctype.h>
#include <time.h>
#include <opencv2/opencv.hpp>
using namespace cv;
using namespace std;
String window_name = "Hand_HSV";
Mat frame,copyFrame;
// Detect Skin from YCrCb
Mat DetectYCrCb(Mat img,Scalar min, Scalar max){
Mat skin;
cvtColor(img, skin, cv::COLOR_RGB2YCrCb);
inRange(skin, min, max, skin);
Mat rect_12 = getStructuringElement(cv::MORPH_RECT, Size(12,12) , Point(6,6));
erode(skin, skin, rect_12,Point(),1);
Mat rect_6 = getStructuringElement(cv::MORPH_RECT, Size(6,6) , Point(3,3));
dilate(skin,skin,rect_6,Point(),2);
return skin;
}
void DetectContour(Mat img){
Mat drawing = Mat::zeros( img.size(), CV_8UC3 );
vector<vector<Point> > contours;
vector<vector<Point> > bigContours;
vector<Vec4i> hierarchy;
findContours(img,contours, hierarchy, cv::RETR_LIST, cv::CHAIN_APPROX_SIMPLE, Point());
if(contours.size()>0)
{
vector<vector<int> >hull( contours.size() );
vector<vector<Vec4i>> convDef(contours.size() );
vector<vector<Point>> hull_points(contours.size());
vector<vector<Point>> defect_points(contours.size());
for( int i = 0; i < contours.size(); i++ )
{
if(contourArea(contours[i])>5000)
{
convexHull( contours[i], hull[i], false );
convexityDefects( contours[i],hull[i], convDef[i]);
// start_index, end_index, farthest_pt_index, fixpt_depth
for(int k=0;k<hull[i].size();k++)
{
int ind=hull[i][k];
hull_points[i].push_back(contours[i][ind]);
}
for(int k=0;k<convDef[i].size();k++)
{
if(convDef[i][k][3]>20*256)
{
int ind_0=convDef[i][k][0];
int ind_1=convDef[i][k][1];
int ind_2=convDef[i][k][2];
defect_points[i].push_back(contours[i][ind_2]);
cv::circle(drawing,contours[i][ind_0],5,Scalar(0,255,0),-1);
cv::circle(drawing,contours[i][ind_1],5,Scalar(0,255,0),-1);
cv::circle(drawing,contours[i][ind_2],5,Scalar(0,0,255),-1);
cv::line(drawing,contours[i][ind_2],contours[i][ind_0],Scalar(0,0,255),1);
cv::line(drawing,contours[i][ind_2],contours[i][ind_1],Scalar(0,0,255),1);
}
}
drawContours( drawing, contours, i, Scalar(0,255,0), 1, 8, vector<Vec4i>(), 0, Point() );
drawContours( drawing, hull_points, i, Scalar(255,0,0), 1, 8, vector<Vec4i>(), 0, Point() );
}
}
}
namedWindow( "Hull demo",cv::WINDOW_AUTOSIZE );
imshow( "Hull demo", drawing );
}
int main( int argc, char** argv )
{
VideoCapture capture(0);
//VideoCapture capture("Video_Hand.MPG");
namedWindow( window_name, cv::WINDOW_AUTOSIZE );
if (capture.isOpened()){
while(true)
{
capture >> frame;
imshow( window_name, frame);
Mat skinYCrCb = DetectYCrCb(frame,Scalar(0, 100, 80), Scalar(255, 185, 135));
imshow("Result",skinYCrCb);
DetectContour(skinYCrCb);
int c = waitKey(10);
if( (char)c == 27 )
{
break;
}
}
}
return 0;
}

Answer 2

Are you sure vector<vector<Point> >hulls is correct? The documentation (docs.opencv.org) mentions, for the 2nd parameter of convexityDefects:
convexhull – Convex hull obtained using convexHull() that should contain indices of the contour points that make the hull.
So i think it should be rather vector<vector<int> >hulls