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.. _Hough_Circles:
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Hough Circle Transform
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**************************
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Goal
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=====
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In this chapter,
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* We will learn to use Hough Transform to find circles in an image.
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* We will see these functions: **cv2.HoughCircles()**
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Theory
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========
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A circle is represented mathematically as :math:`(x-x_{center})^2 + (y - y_{center})^2 = r^2` where :math:`(x_{center},y_{center})` is the center of the circle, and :math:`r` is the radius of the circle. From equation, we can see we have 3 parameters, so we need a 3D accumulator for hough transform, which would be highly ineffective. So OpenCV uses more trickier method, **Hough Gradient Method** which uses the gradient information of edges.
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The function we use here is **cv2.HoughCircles()**. It has plenty of arguments which are well explained in the documentation. So we directly go to the code.
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::
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import cv2
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import numpy as np
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img = cv2.imread('opencv_logo.png',0)
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img = cv2.medianBlur(img,5)
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cimg = cv2.cvtColor(img,cv2.COLOR_GRAY2BGR)
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circles = cv2.HoughCircles(img,cv2.HOUGH_GRADIENT,1,20,
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param1=50,param2=30,minRadius=0,maxRadius=0)
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circles = np.uint16(np.around(circles))
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for i in circles[0,:]:
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# draw the outer circle
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cv2.circle(cimg,(i[0],i[1]),i[2],(0,255,0),2)
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# draw the center of the circle
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cv2.circle(cimg,(i[0],i[1]),2,(0,0,255),3)
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cv2.imshow('detected circles',cimg)
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cv2.waitKey(0)
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cv2.destroyAllWindows()
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Result is shown below:
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.. image:: images/houghcircles2.jpg
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:alt: Hough Circles
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:align: center
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Additional Resources
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=====================
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Exercises
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===========
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