added houghcircles sample, improved circle detection (ticket #951)

samples/cpp/houghcircles.cpp

@@ -0,0 +1,48 @@
+#include "opencv2/highgui/highgui.hpp"
+#include "opencv2/imgproc/imgproc.hpp"
+
+#include <iostream>
+
+using namespace cv;
+using namespace std;
+
+void help()
+{
+    cout << "\nThis program demonstrates circle finding with the Hough transform.\n"
+            "Usage:\n"
+            "./houghcircles <image_name>, Default is pic1.png\n" << endl;
+}
+
+int main(int argc, char** argv)
+{
+    const char* filename = argc >= 2 ? argv[1] : "board.jpg";
+
+    Mat img = imread(filename, 0);
+    if(img.empty())
+    {
+        help();
+        cout << "can not open " << filename << endl;
+        return -1;
+    }
+
+    Mat cimg;
+    medianBlur(img, img, 5);
+    cvtColor(img, cimg, CV_GRAY2BGR);
+    
+    vector<Vec3f> circles;
+    HoughCircles(img, circles, CV_HOUGH_GRADIENT, 1, 10,
+                 100, 30, 1, 30 // change the last two parameters 
+                                // (min_radius & max_radius) to detect larger circles
+                 );
+    for( size_t i = 0; i < circles.size(); i++ )
+    {
+        Vec3i c = circles[i];
+        circle( cimg, Point(c[0], c[1]), c[2], Scalar(0,0,255), 3, CV_AA);
+        circle( cimg, Point(c[0], c[1]), 2, Scalar(0,255,0), 3, CV_AA);
+    }
+
+    imshow("detected circles", cimg);
+    waitKey();
+
+    return 0;
+}