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Hough Circle Transform {#tutorial_py_houghcircles}
======================
Goal
----
In this chapter,
- We will learn to use Hough Transform to find circles in an image.
- We will see these functions: **cv.HoughCircles()**
Theory
------
A circle is represented mathematically as \f$(x-x_{center})^2 + (y - y_{center})^2 = r^2\f$ where
\f$(x_{center},y_{center})\f$ is the center of the circle, and \f$r\f$ 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 **cv.HoughCircles()**. It has plenty of arguments which are well
explained in the documentation. So we directly go to the code.
@code{.py}
import numpy as np
import cv2 as cv
img = cv.imread('opencv-logo-white.png',0)
img = cv.medianBlur(img,5)
cimg = cv.cvtColor(img,cv.COLOR_GRAY2BGR)
circles = cv.HoughCircles(img,cv.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
cv.circle(cimg,(i[0],i[1]),i[2],(0,255,0),2)
# draw the center of the circle
cv.circle(cimg,(i[0],i[1]),2,(0,0,255),3)
cv.imshow('detected circles',cimg)
cv.waitKey(0)
cv.destroyAllWindows()
@endcode
Result is shown below:
Additional Resources
--------------------
Exercises
---------
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