python中圆检测为什么出现如下错误？

Question

import cv2
import numpy as np
cap = cv2.VideoCapture(0)
while(True):
    ret, frame=cap.read()
    gray_img = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
    img = cv2.medianBlur(gray_img, 5)
  

    circles = cv2.HoughCircles(img,cv2.cv.CV_HOUGH_GRADIENT,
                1,410,param1=100,param2=30,minRadius=15,maxRadius=25)

    circles = np.uint16(np.around(circles))
    
    for i in circles[0,:]:
   
        cv2.circle(frame,(i[0],i[1]),i[2],(0,255,0),2)
        
        cv2.circle(frame,(i[0],i[1]),2,(0,0,255),3)
    
    cv2.imshow("HoughCirlces", frame)
    if cv2.waitKey(97)  == ord('a'):
        break 
        
cv2.destroyAllWindows()
cap.release()

Answer 1

circles = cv2.HoughCircles(img,cv2.cv.CV_HOUGH_GRADIENT,
                1,410,param1=100,param2=30,minRadius=15,maxRadius=25)

中minRadius,与maxRadius限制太苛刻，一旦未捕获到圆，circles为空，后面运算会出错。
改为minRadius=0，maxRadius=0