参考：https://www.freesion.com/article/31311480188/
两种方法
一、dst=cv2.addWeighted(img,a,blank,b,g)
二、img2 = img_contrast_bright(img,a,b,g)
a是对比度调整值0~0.3
g是亮度调整值0~100

import cv2
import os
import numpy as np

def img_contrast_bright(img,a,b,g):
    h,w,c=img.shape
    blank=np.zeros([h,w,c],img.dtype)
    dst=cv2.addWeighted(img,a,blank,b,g)
    return dst


img = cv2.imread(file_dir+i)
cv2.imshow('aa',img)

a=1.2
b=1-a 
g=10
img2 = img_contrast_bright(img,a,b,g)
cv2.imshow('bb',img2)


img3 = np.uint8(np.clip((a * img + g), 0, 255))
cv2.imshow('cc',img3)
 cv2.waitKey(2000)