Обнаружение линии + полигона

Как определить линию и многоугольник, когда линия находится в многоугольнике?
Я использую обнаружение Канни + окпПолиДП, но вместо линии + полигона я получаю
два семиугольника.
Пример, здесь:

Мой код:

Mat PolygonsDetection(Mat src)
{
Mat gray;
cvtColor(src, gray, CV_BGR2GRAY);

Mat bw;
Canny(src, bw, 40, 120, 3, true);
imshow("canny", bw);

morphologyEx(bw, bw, MORPH_CLOSE, cv::noArray(),cv::Point(-1,-1),1);

imshow("morph", bw);

vector<vector<Point>> countours;
findContours(bw.clone(), countours, CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE);

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

for(int i = 0; i < countours.size(); i++)
{
approxPolyDP(Mat(countours[i]), approx, arcLength(Mat(countours[i]), true) * 0.020, true);
int vtc = approx.size();

if ((vtc == 2) || (vtc == 4) || (vtc == 7))
{
if (vtc == 7)
{
cv::line(dst, approx.at(0), approx.at(1), cvScalar(0,255,0),4);
cv::line(dst, approx.at(1), approx.at(2), cvScalar(0,255,0),4);
cv::line(dst, approx.at(2), approx.at(3), cvScalar(0,255,0),4);
cv::line(dst, approx.at(3), approx.at(4), cvScalar(0,255,0),4);
cv::line(dst, approx.at(4), approx.at(5), cvScalar(0,255,0),4);
cv::line(dst, approx.at(5), approx.at(6), cvScalar(0,255,0),4);
cv::line(dst, approx.at(6), approx.at(0), cvScalar(0,255,0),4);

SetLabel(dst, std::to_string((long long)cellCounter), countours[i]);
}
else if (vtc == 4)
{

line(dst, approx.at(0), approx.at(1), cvScalar(255,0,0),4);
line(dst, approx.at(1), approx.at(2), cvScalar(255,0,0),4);
line(dst, approx.at(2), approx.at(3), cvScalar(255,0,0),4);
line(dst, approx.at(3), approx.at(0), cvScalar(255,0,0),4);

SetLabel(dst, std::to_string((long long)cellCounter), countours[i]);
}
eslse if (vtc == 2)
{
// Never not comes here
}
}
}
return dst;
}