detecting solid white circles using opencv HoughCircles - opencv

Question

I am trying to detect solid circles using opencv. The example code from the opencv documentation seems like it cannot detect solid white. How would I modify that code to work for solid white circles? Can you explain why it does not work for solid white circles?
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>
using namespace cv;
/** #function main */
int main(int argc, char** argv)
{
Mat src, src_gray;
/// Read the image
src = imread( argv[1], 1 );
if( !src.data )
{ return -1; }
/// Convert it to gray
cvtColor( src, src_gray, CV_BGR2GRAY );
/// Reduce the noise so we avoid false circle detection
GaussianBlur( src_gray, src_gray, Size(9, 9), 2, 2 );
vector<Vec3f> circles;
/// Apply the Hough Transform to find the circles
HoughCircles( src_gray, circles, CV_HOUGH_GRADIENT, 1, src_gray.rows/8, 200, 100, 0, 0 );
/// Draw the circles detected
for( size_t i = 0; i < circles.size(); i++ )
{
Point center(cvRound(circles[i][0]), cvRound(circles[i][1]));
int radius = cvRound(circles[i][2]);
// circle center
circle( src, center, 3, Scalar(0,255,0), -1, 8, 0 );
// circle outline
circle( src, center, radius, Scalar(0,0,255), 3, 8, 0 );
}
/// Show your results
namedWindow( "Hough Circle Transform Demo", CV_WINDOW_AUTOSIZE );
imshow( "Hough Circle Transform Demo", src );
waitKey(0);
return 0;
}
I would post images, but I don't have enough stack overflow street cred yet. Sorry!

Answer 1

You should extract the edges first. This is what the Hough Transform detects. Add a cvCanny transform before HoughCircles.