A few lines of code to make your picture will be clear

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# coding=utf-8
import cv2
import numpy as np

img = cv2.imread('shuiyin.png')
retval, thre = cv2.threshold(img, 12, 255, cv2.THRESH_BINARY)

grayscal = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
retval1, thre1 = cv2.threshold(grayscal, 12, 255, cv2.THRESH_BINARY)
gaus = cv2.adaptiveThreshold(grayscal, 255, cv2.ADAPTIVE_THRESH_GAUSSIAN_C, cv2.THRESH_BINARY, 155, 1)
cv2.imshow('re1', img)
cv2.imshow('re2', thre)
cv2.imshow('re3', thre1)
cv2.imshow('re4', gaus)
cv2.waitKey(0)
cv2.destroyAllWindows()