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Hough Circle Transform {#tutorial_hough_circle}
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In this tutorial you will learn how to:
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- Use the OpenCV function @ref cv::HoughCircles to detect circles in an image.
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### Hough Circle Transform
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- The Hough Circle Transform works in a *roughly* analogous way to the Hough Line Transform
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explained in the previous tutorial.
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- In the line detection case, a line was defined by two parameters \f$(r, \theta)\f$. In the circle
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case, we need three parameters to define a circle:
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\f[C : ( x_{center}, y_{center}, r )\f]
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where \f$(x_{center}, y_{center})\f$ define the center position (green point) and \f$r\f$ is the radius,
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which allows us to completely define a circle, as it can be seen below:
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![](images/Hough_Circle_Tutorial_Theory_0.jpg)
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- For sake of efficiency, OpenCV implements a detection method slightly trickier than the standard
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Hough Transform: *The Hough gradient method*, which is made up of two main stages. The first
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stage involves edge detection and finding the possible circle centers and the second stage finds
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the best radius for each candidate center. For more details, please check the book *Learning
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OpenCV* or your favorite Computer Vision bibliography
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-# **What does this program do?**
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- Loads an image and blur it to reduce the noise
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- Applies the *Hough Circle Transform* to the blurred image .
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- Display the detected circle in a window.
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-# The sample code that we will explain can be downloaded from [here](https://github.com/Itseez/opencv/tree/master/samples/cpp/houghcircles.cpp).
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A slightly fancier version (which shows trackbars for
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changing the threshold values) can be found [here](https://github.com/Itseez/opencv/tree/master/samples/cpp/tutorial_code/ImgTrans/HoughCircle_Demo.cpp).
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@include samples/cpp/houghcircles.cpp
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src = imread( argv[1], 1 );
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-# Convert it to grayscale:
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cvtColor( src, src_gray, COLOR_BGR2GRAY );
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-# Apply a Gaussian blur to reduce noise and avoid false circle detection:
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GaussianBlur( src_gray, src_gray, Size(9, 9), 2, 2 );
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-# Proceed to apply Hough Circle Transform:
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vector<Vec3f> circles;
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HoughCircles( src_gray, circles, HOUGH_GRADIENT, 1, src_gray.rows/8, 200, 100, 0, 0 );
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- *src_gray*: Input image (grayscale).
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- *circles*: A vector that stores sets of 3 values: \f$x_{c}, y_{c}, r\f$ for each detected
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- *HOUGH_GRADIENT*: Define the detection method. Currently this is the only one available in
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- *dp = 1*: The inverse ratio of resolution.
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- *min_dist = src_gray.rows/8*: Minimum distance between detected centers.
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- *param_1 = 200*: Upper threshold for the internal Canny edge detector.
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- *param_2* = 100\*: Threshold for center detection.
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- *min_radius = 0*: Minimum radio to be detected. If unknown, put zero as default.
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- *max_radius = 0*: Maximum radius to be detected. If unknown, put zero as default.
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-# Draw the detected circles:
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for( size_t i = 0; i < circles.size(); i++ )
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Point center(cvRound(circles[i][0]), cvRound(circles[i][1]));
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int radius = cvRound(circles[i][2]);
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circle( src, center, 3, Scalar(0,255,0), -1, 8, 0 );
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circle( src, center, radius, Scalar(0,0,255), 3, 8, 0 );
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You can see that we will draw the circle(s) on red and the center(s) with a small green dot
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-# Display the detected circle(s):
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namedWindow( "Hough Circle Transform Demo", WINDOW_AUTOSIZE );
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imshow( "Hough Circle Transform Demo", src );
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-# Wait for the user to exit the program
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The result of running the code above with a test image is shown below:
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![](images/Hough_Circle_Tutorial_Result.jpg)