#10a. Point Polygon Test: Part I

Point Polygon Test determines whether the point is inside a contour, outside, or lies on an edge (or coincides with a vertex). It returns positive (inside), negative (outside), or zero (on an edge) value, correspondingly. When flag is turned off , the return value is +1, -1, and 0, respectively. Otherwise, the return value is a signed distance between the point and the nearest contour edge.

I've divided the tutorial in two parts to make it easy to understand.

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#include<iostream>
#include<highgui.h>
#include<cv.h>
using namespace std;
using namespace cv;

Mat img_all_contours;
vector<vector<Point> > closed_contours;
vector<Vec4i> heirarchy;

vector<vector<Point> > make_contours_closed(vector<vector<Point> > contours){
vector<vector<Point> > closed_contours;
closed_contours.resize(contours.size());
for(int i=0;i<contours.size();i++)
approxPolyDP(contours[i],closed_contours[i],0.1,true);
return closed_contours;
}
int main()
{
Mat img=imread("images/circles3.jpg");
img_all_contours=img.clone();

Mat imgb;
cvtColor(img,imgb,CV_RGB2GRAY);

Mat edges;
Canny(imgb,edges,50,100);
vector<vector<Point> > contours;

findContours(edges,contours,heirarchy,CV_RETR_TREE,CV_CHAIN_APPROX_NONE);

closed_contours=make_contours_closed(contours);

drawContours(img_all_contours,closed_contours,-1,Scalar(0,255,0));
imshow("Contours",img_all_contours);

while(char(waitKey(1))!='q'){}
return 0;

}

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Here, the Main() function is similar to the Hierarchical Contour Detection code(#9a).

I've introduced a function 'make_contours_closed' which basically closes all the open contours(if any), so that whenever it is clicked, there's always a closed contour associated with the position of click.

approxPolyDP(contours[i],closed_contours[i],0.1,true);

contours[i]: Input Array

closed_contours[i]: Output Array

0.1: Parameter specifying the approximation accuracy. This is the maximum distance between the original curve and its approximation.

true: the approximated curve is closed (its first and last vertices are connected). If false, it is not closed

You may look at the documentation of 'approxPolyDP' here.
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Input Image:

Output Image: