How to detect heptagon object?

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I would like to detect a heptagon object (arrow).Im using Opencv for a simple shape detection to detect heptagon (7 vertices). With the code Im able to detect triangle, circle, rectangle but not a heptagon object(in the image it is an arrow). Here my code

#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <cmath>

 using namespace cv;
 using namespace std;

 Mat src; Mat src_gray;

  /**
      * Helper function to find a cosine of angle between vectors
      * from pt0->pt1 and pt0->pt2
      */
     static double angle(cv::Point pt1, cv::Point pt2, cv::Point pt0)
     {
        double dx1 = pt1.x - pt0.x;
        double dy1 = pt1.y - pt0.y;
        double dx2 = pt2.x - pt0.x;
        double dy2 = pt2.y - pt0.y;
        return (dx1*dx2 + dy1*dy2)/sqrt((dx1*dx1 + dy1*dy1)*(dx2*dx2 + dy2*dy2) + 1e-10);
     }

     /**
      * Helper function to display text in the center of a contour
      */
     void setLabel(cv::Mat& im, const std::string label, std::vector<cv::Point>& contour)
     {
        int fontface = cv::FONT_HERSHEY_SIMPLEX;
        double scale = 0.4;
        int thickness = 1;
        int baseline = 0;

        cv::Size text = cv::getTextSize(label, fontface, scale, thickness, &baseline);
        cv::Rect r = cv::boundingRect(contour);

        cv::Point pt(r.x + ((r.width - text.width) / 2), r.y + ((r.height + text.height) / 2));
        cv::rectangle(im, pt + cv::Point(0, baseline), pt + cv::Point(text.width, -text.height), CV_RGB(255,255,255), CV_FILLED);
        cv::putText(im, label, pt, fontface, scale, CV_RGB(0,0,0), thickness, 8);
     }

     int main()
     {

        cv::Mat src = cv::imread("shapes.png");
        if (src.empty())
            return -1;

        // Convert to grayscale
        cv::Mat gray;
        cv::cvtColor(src, gray, CV_BGR2GRAY);

        // Use Canny instead of threshold to catch squares with gradient shading
        cv::Mat bw;
        cv::Canny(gray, bw, 0, 50, 5);

        // Find contours
        std::vector<std::vector<cv::Point> > contours;
        cv::findContours(bw.clone(), contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);

        std::vector<cv::Point> approx;
        cv::Mat dst = src.clone();

        for (int i = 0; i < contours.size(); i++)
        {
            // Approximate contour with accuracy proportional
            // to the contour perimeter
            cv::approxPolyDP(cv::Mat(contours[i]), approx, cv::arcLength(cv::Mat(contours[i]), true)*0.02, true);

            // Skip small or non-convex objects
            if (std::fabs(cv::contourArea(contours[i])) < 100 || !cv::isContourConvex(approx))
                continue;

            if (approx.size() == 3)
            {
                setLabel(dst, "TRI", contours[i]);    // Triangles
            }
            else if (approx.size() >= 4 && approx.size() <= 6)
            {
                // Number of vertices of polygonal curve
                int vtc = approx.size();

                // Get the cosines of all corners
                std::vector<double> cos;
                for (int j = 2; j < vtc+1; j++)
                    cos.push_back(angle(approx[j%vtc], approx[j-2], approx[j-1]));

                // Sort ascending the cosine values
                std::sort(cos.begin(), cos.end());

                // Get the lowest and the highest cosine
                double mincos = cos.front();
                double maxcos = cos.back();

                // Use the degrees obtained above and the number of vertices
                // to determine the shape of the contour
                if (vtc == 4 && mincos >= -0.1 && maxcos <= 0.3)
                    setLabel(dst, "RECT", contours[i]);
                else if (vtc == 5 && mincos >= -0.34 && maxcos <= -0.27)
                    setLabel(dst, "PENTA", contours[i ...