OpenCV：为什么HoughCircles（）没有给出正确的结果？

Question

我正在尝试制作一个简单的程序来检测图像中的圆圈。我已经编写了这段代码，结果可以在附图中看到。有人可以告诉我，我做错了什么？

import time 
import cv2 
import imutils 
import numpy as np

img=cv2.imread('circle.jpg')
gray=cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
binary=cv2.threshold(gray,150,255,cv2.THRESH_BINARY)[1]

cv2.imshow('binary',binary)

circles=cv2.HoughCircles(binary,cv2.HOUGH_GRADIENT,1,50,50,30,5,100)

if  circles is not None:
    circles=np.round(circles[0,:]).astype("int")

    for i in circles:

        cv2.circle(img,(i[0],i[1]),i[2],(0,255,0),4)
        cv2.circle(img,(i[0],i[1]),2,(0,0,255),3)

else:

    print"no circle" 

cv2.imshow('circle',img)
k=cv2.waitKey(0)

if k==27:

    cv2.destroyAllWindows()

Image