import numpy as np
import cv2 as cv

img = cv.imread('lightning.png')
gray = cv.cvtColor(img, cv.COLOR_BGR2GRAY)
ret,thresh = cv.threshold(gray,127,255,0)
contours,hierarchy = cv.findContours(thresh,1,2)
cnt = contours[0]

# 等效直径计算
area = cv.contourArea(cnt)
equi_diameter = np.sqrt(4*area/np.pi)
# 输出结果
print('等效直径：',equi_diameter )