Hough Circle Transform

Goal

In this tutorial you will learn how to:

• Use the OpenCV function HoughCircles to detect circles in an image.

Theory

Hough Circle Transform

• The Hough Circle Transform works in a roughly analogous way to the Hough Line Transform explained in the previous tutorial.

• In the line detection case, a line was defined by two parameters . In the circle case, we need three parameters to define a circle:

  

  where define the center position (gree point) and is the radius, which allows us to completely define a circle, as it can be seen below:

  

• For sake of efficiency, OpenCV implements a detection method slightly trickier than the standard Hough Transform: The Hough gradient method. For more details, please check the book Learning OpenCV or your favorite Computer Vision bibliography

Code

1. What does this program do?
   • Loads an image and blur it to reduce the noise
   • Applies the Hough Circle Transform to the blurred image .
   • Display the detected circle in a window.
2. The sample code that we will explain can be downloaded from here. A slightly fancier version (which shows both Hough standard and probabilistic with trackbars for changing the threshold values) can be found here.

#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;
}

Explanation

1. Load an image

   src = imread( argv[1], 1 );
   
   if( !src.data )
     { return -1; }
   

2. Convert it to grayscale:

   cvtColor( src, src_gray, CV_BGR2GRAY );
   

3. Apply a Gaussian blur to reduce noise and avoid false circle detection:

   GaussianBlur( src_gray, src_gray, Size(9, 9), 2, 2 );
   

4. Proceed to apply Hough Circle Transform:

   vector<Vec3f> circles;
   
   HoughCircles( src_gray, circles, CV_HOUGH_GRADIENT, 1, src_gray.rows/8, 200, 100, 0, 0 );
   

   with the arguments:

   • src_gray: Input image (grayscale)
   • circles: A vector that stores sets of 3 values: for each detected circle.
   • CV_HOUGH_GRADIENT: Define the detection method. Currently this is the only one available in OpenCV
   • dp = 1: The inverse ratio of resolution
   • min_dist = src_gray.rows/8: Minimum distance between detected centers
   • param_1 = 200: Upper threshold for the internal Canny edge detector
   • param_2 = 100*: Threshold for center detection.
   • min_radius = 0: Minimum radio to be detected. If unknown, put zero as default.
   • max_radius = 0: Maximum radius to be detected. If unknown, put zero as default

5. Draw the detected circles:

   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 );
    }
   

   You can see that we will draw the circle(s) on red and the center(s) with a small green dot

6. Display the detected circle(s):

   namedWindow( "Hough Circle Transform Demo", CV_WINDOW_AUTOSIZE );
   imshow( "Hough Circle Transform Demo", src );
   

7. Wait for the user to exit the program

   waitKey(0);
   

Result

The result of running the code above with a test image is shown below: