ret, dst = cv2.threshold(src, thresh, maxval, type)

src：輸入圖，只能輸入單通道圖像，通常來說為灰度圖
dst：輸出圖
thresh：閾值
maxval：當像素值超過了閾值（或者小于閾值，根據type來決定），所賦予的值
type：二值化操作的類型，就是怎么處理閾值，包含以下5種類型：cv2.THRESH_BINARY；cv2.THRESH_BINARY_INV；cv2.THRESH_TRUNC；cv2.THRESH_TOZERO；cv2.THRESH_TOZERO_INV；

cv2.THRESH_BINARY 超過閾值部分取maxval（最大值），否則取0

cv2.THRESH_BINARY_INV THRESH_BINARY的反轉

cv2.THRESH_TRUNC 大于閾值部分設為閾值，否則不變

cv2.THRESH_TOZERO 大于閾值部分不改變，否則設為0

cv2.THRESH_TOZERO_INV THRESH_TOZERO的反轉

cv2.THRESH_OTSU   方法能夠根據當前圖像生成最佳的類間分割閾值，原理是遍歷所有可能的閾值，從而找到最合適的閾值，thresh：閾值要設置為0，適合雙峰型閥值，其他情況不適用

import cv2
import matplotlib.pyplot as plt
img = cv2.imread('cat.jpg')
img_gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
cv2.imshow('img_gray)', img_gray)
ret1, thresh1 = cv2.threshold(img_gray, 127, 255, cv2.THRESH_BINARY)
ret2, thresh2 = cv2.threshold(img_gray, 127, 255, cv2.THRESH_BINARY_INV)
ret3, thresh3 = cv2.threshold(img_gray, 127, 255, cv2.THRESH_TRUNC)
ret4, thresh4 = cv2.threshold(img_gray, 127, 255, cv2.THRESH_TOZERO)
ret5, thresh5 = cv2.threshold(img_gray, 127, 255, cv2.THRESH_TOZERO_INV)