Open cv finding convex hull - c++

Question

The following code in OpenCV is for me to detect a yellow color ball and draw its convex hull.The code although doesnt give any compilation errors,gives the following error in the output window.
I used the largest area function to avoid smaller unwanted contours.
The error is
Assertion failed <0 <= contourIdx && contourIdx< last> in cv::drawContours ,file C:Buildsmasters..(some path),line 2299****
#include <opencv\cv.h>
#include <opencv2\highgui\highgui.hpp>
#include<opencv\cvaux.h>
#include<opencv\cxcore.h>
#include <opencv2\imgproc\imgproc.hpp>
#include <iostream>
#include<conio.h>
#include <stdlib.h>
using namespace cv;
using namespace std;
int main(){
Mat img, frame, img2, img3;
double maxarea = 0;
int lrgctridx; //largest contour index
VideoCapture cam(0);
while (true){
cam.read(frame);
cvtColor(frame, img, CV_BGR2HSV);
//thresholding
inRange(img, Scalar(0, 143, 86), Scalar(39, 255, 241), img2);
//finding contours
vector<vector<Point>> Contours;
vector<Vec4i> hier;
//morphological transformations
erode(img2, img2, getStructuringElement(MORPH_RECT, Size(3, 3)));
erode(img2, img2, getStructuringElement(MORPH_RECT, Size(3, 3)));
dilate(img2, img2, getStructuringElement(MORPH_RECT, Size(8, 8)));
dilate(img2, img2, getStructuringElement(MORPH_RECT, Size(8, 8)));
//finding the contours required
findContours(img2, Contours, hier, CV_RETR_CCOMP, CV_CHAIN_APPROX_NONE, Point(0, 0));
//finding the contour of largest area and storing its index
for (int i = 0; i < Contours.size(); i++)
{
double a=contourArea(Contours[i]);
if (a> maxarea)
{
maxarea = a;
lrgctridx=i;
}
}
//convex hulls
vector<vector<Point> >hull(Contours.size());
for (int i = 0; i < Contours.size(); i++)
{
convexHull(Contours[i], hull[i], false);
}
//REQUIRED contour is detected,then draw a convex hull
if (maxarea!=0)
drawContours(frame, hull, lrgctridx, Scalar(255, 255, 255), 1, 8, vector<Vec4i>(), 0, Point());
imshow("output", frame);
char key = waitKey(33);
if (key == 27) break;
}
}
Any help would be much appreciated.Thaanks in advance!

Answer 1

You should reset lrgctridx at every iteration.
Say you find a countour at time "t", and you set lrgctridx = 1;. At time "t+1" you don't find any contours, so Contours and hull size will be 0, but you're trying to access position 1.
Just put lrgctridx = 0 before the for loop. Same for maxarea.
lrgctridx = 0;
maxarea = 0;
for (int i = 0; i < Contours.size(); i++)
{
....
Now your condition to draw contours is ok, but you'd better replace it with
if(!Contours.empty()) {
drawContours(...);
....