.. _Hough_Circles: Hough Circle Transform ************************** Goal ===== In this chapter, * We will learn to use Hough Transform to find circles in an image. * We will see these functions: **cv2.HoughCircles()** Theory ======== 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. 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. :: import cv2 import numpy as np img = cv2.imread('opencv_logo.png',0) img = cv2.medianBlur(img,5) cimg = cv2.cvtColor(img,cv2.COLOR_GRAY2BGR) circles = cv2.HoughCircles(img,cv2.HOUGH_GRADIENT,1,20, param1=50,param2=30,minRadius=0,maxRadius=0) circles = np.uint16(np.around(circles)) for i in circles[0,:]: # draw the outer circle cv2.circle(cimg,(i[0],i[1]),i[2],(0,255,0),2) # draw the center of the circle cv2.circle(cimg,(i[0],i[1]),2,(0,0,255),3) cv2.imshow('detected circles',cimg) cv2.waitKey(0) cv2.destroyAllWindows() Result is shown below: .. image:: images/houghcircles2.jpg :alt: Hough Circles :align: center Additional Resources ===================== Exercises ===========