# Number of vertices of a circle

Im struggling with the shape detection using OpenCV for C++. The edged figures such as triangle and rectangular are detected trouble-free. But when it comes to circle it estimates number of vertices up to 6-8. Could somebody help me?

``````void getContours(Mat video){
Mat grayscale, canny_output;
cvtColor(video, grayscale,COLOR_RGB2GRAY);//converting image to grayscale
GaussianBlur(grayscale, grayscale, Size(9, 9), 2, 2 );
threshold(grayscale, grayscale,60,255,THRESH_BINARY);
vector <vector<Point>> contours, output_contour;
vector <Vec4i> hierarchy;
findContours( grayscale, contours, hierarchy, RETR_TREE,CHAIN_APPROX_SIMPLE );
Mat drawing = Mat::zeros( grayscale.size(), CV_8UC3 );
vector<Point> c;
for (size_t i = 0; i<contours.size(); i++){
c = contours[i];
Rect crect = boundingRect(c);
// compute the center of the contour, then detect the name of the
// shape using only the contour
Moments M = moments(c);
int cX, cY;
cX = static_cast<int>(M.m10/M.m00);
cY = static_cast<int>(M.m01/M.m00);
string shape = detect(Mat(c));
drawContours( drawing, contours, (int)i, Scalar(0, 255, 0), 2);
Point pt(cX,cY);
putText(drawing,shape,pt, FONT_HERSHEY_SIMPLEX, 0.5, Scalar(255, 255, 255), 2);
imshow("contour", drawing);

}
}
``````
``````string detect(const Mat &curve){
string shape = "unidentified";
double peri = arcLength(curve, true);
Mat approx;
approxPolyDP(curve, approx, 0.04 * peri, true); // 0.01~0.05
const int num_of_vertices = approx.rows;
if(num_of_vertices == 0){
shape = "circle";

}
if(num_of_vertices==2){
shape = "line";
}
cout<<"\n"<<num_of_vertices;