import cv2
input_imgfn="tobrighten.jpg"
output_imgfn="brightened.jpg"
def change_brightness(img, value=30):
    hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)
    h, s, v = cv2.split(hsv)
    v = cv2.add(v,value)
    v[v > 255] = 255
    v[v < 0] = 0
    final_hsv = cv2.merge((h, s, v))
    img = cv2.cvtColor(final_hsv, cv2.COLOR_HSV2BGR)
    return img
img = cv2.imread(input_imgfn) #load rgb image
img = change_brightness(img, value=90) #increases
#img = change_brightness(img, value=-30) #decreases
cv2.imwrite(output_imgfn, img)