How to fast change image brightness with python + OpenCV?

The other answers suggest doing the saturation "by hand" using all kinds of numpy magic, but you can also use cv2.add() and let OpenCV handle that for you:

import cv2
import numpy as np

image = cv2.read('image.png')
hsv = cv2.cvtColor(image, cv2.COLOR_BGR2HSV)
value = 42 #whatever value you want to add
cv2.add(hsv[:,:,2], value, hsv[:,:,2])
image = cv2.cvtColor(hsv, cv2.COLOR_HSV2BGR)
cv2.imwrite('out.png', image)

Slice to select just the third channel and then modify those elements -

hsv[:,:,2] += value

I know this question is a bit old, but I thought I might post the complete solution that worked for me (takes care of the overflow situation by saturating at 255):

def increase_brightness(img, value=30):
    hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)
    h, s, v = cv2.split(hsv)

    lim = 255 - value
    v[v > lim] = 255
    v[v <= lim] += value

    final_hsv = cv2.merge((h, s, v))
    img = cv2.cvtColor(final_hsv, cv2.COLOR_HSV2BGR)
    return img

This can be used as follows:

frame = increase_brightness(frame, value=20)