 # 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;

}
`````` you could try to filter the curve by circularity, like here:

(instead of looking at the polygon approximation, which isn’t useful for circles)

you use approxPolyDP. of course it results in approximations that have very few corners. it can’t tell you “zero corners” for a circle.

use a tighter epsilon for your approxPolyDP() call

besides that, your circles look lumpy.