C++ Vector Subscript out of range with opencv error [closed] - c++

Question

Closed. This question needs to be more focused. It is not currently accepting answers.
Want to improve this question? Update the question so it focuses on one problem only by editing this post.
Closed 19 days ago.
Improve this question
Any thoughts where I might be going wrong. PS: New to coding and StackOverFlow.
#include<iostream>
#include<opencv2/opencv.hpp>
#include<opencv2/highgui.hpp>
#include<opencv2/imgcodecs.hpp>
#include<opencv2/imgproc.hpp>
//Declare the image variables
cv::Mat img, imgGray, imgBlur, imgCanny, imgDil;
void GetContours(cv::Mat dilatedImg, cv::Mat originalImg);
int main(int argc, char** argv)
{
std::string path="E://Trial//Resources//Resources//shapes.png";
img= cv::imread(path);
//pre=processing
cv::cvtColor(img,imgGray,cv::COLOR_BGR2GRAY);
cv::GaussianBlur(imgGray, imgBlur,cv::Size(3,3),3,0);
cv::Canny(imgBlur,imgCanny,25,75);
cv::Mat kernel= cv::getStructuringElement(cv::MORPH_RECT, cv::Size(3,3)) ;
cv::dilate(imgCanny,imgDil,kernel);
//Processing
GetContours(imgDil, img);
//Display contours
cv::imshow("Image",img);
cv::waitKey(0);
return 0;
}
void GetContours(cv::Mat dilatedImg, cv::Mat originalImg)
{
std::vector<std::vector<cv::Point>> contours;
std::vector<cv::Vec4i> hierarchy;
std::vector<std::vector<cv::Point>> conPoly(contours.size());
double area=0;
//finds the contours in the shapes
cv::findContours(dilatedImg, contours, hierarchy, cv::RETR_EXTERNAL, cv::CHAIN_APPROX_SIMPLE);
for(int i=0; i<contours.size(); i++)
{
area = cv::contourArea(contours[i]);
std::cout<<area<<std::endl;
if(area>1000)
{
//Draw contours around shapes
cv::drawContours(originalImg,contours, i,cv::Scalar(255,0,255),2);
// create a bounding box around the shapes
cv::approxPolyDP(cv::Mat(contours[i]), conPoly[i], 3, true);
//draw contours using the contour points
cv::drawContours(originalImg,conPoly, i,cv::Scalar(255,255,0),2);
}
}
}
ApproxPollyDP is where I think the code is failing. I am getting an Assertion Failed error with vector out of range. I think I am doing some silly mistake but I have not been able to debug the issue.

Answer 1

The vector conPoly declared as shown below
std::vector<std::vector<cv::Point>> contours;
std::vector<cv::Vec4i> hierarchy;
std::vector<std::vector<cv::Point>> conPoly(contours.size());
is an empty vector because initially contours.size() is equal tp 0 because the vector contours in turn is declared as empty.
So using the subscript operator with the vector
cv::approxPolyDP(cv::Mat(contours[i]), conPoly[i], 3, true);
invokes undefined behavior.

Answer 2

So this works:
std::vector<std::vector<cv::Point>> contours;
std::vector<cv::Vec4i> hierarchy;
double area=0;
//finds the contours in the shapes
cv::findContours(dilatedImg, contours, hierarchy, cv::RETR_EXTERNAL, cv::CHAIN_APPROX_SIMPLE);
std::vector<std::vector<cv::Point>> conPoly(contours.size());