Find circle centre

circle_center.py

import cv2 as cv2
import numpy as np

fn = '200px-Traffic_lights_dark_red-yellow.svg.png'
# OpenCV reads image with BGR format
img = cv2.imread(fn)
# Convert to HSV format
img_hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)

# lower mask (0-10)
lower_red = np.array([0, 50, 50])
upper_red = np.array([10, 255, 255])
mask = cv2.inRange(img_hsv, lower_red, upper_red)

# Bitwise-AND mask and original image
masked_red = cv2.bitwise_and(img, img, mask=mask)

# Check for circles using HoughCircles on opencv
circles = cv2.HoughCircles(mask, cv2.cv.CV_HOUGH_GRADIENT, 1, 20, param1=30, param2=15, minRadius=0, maxRadius=0)
print 'Radius ' + 'x = ' + str(circles[0][0][0]) + ' y = ' + str(circles[0][0][1])