Reduce brigthness in real time OpenCv Python

Question

I am looking around how to reduce the brightness of the webcam image. Can you tell me how to do it?

I couldn't find anything ...

To increase the value I use this function (I increase the brightness with a trackbar):

def change_brightness(frame, value):
    hsv = cv2.cvtColor(frame, 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 is the full code:

cv2.namedWindow("VIDEO")
#cv2.createTrackbar("alpha", "VIDEO", 0, 4, nothing)
cv2.createTrackbar("gamma", "VIDEO", 50, 100, nothing)
while True:
        k = cv2.waitKey(1)
        _, frame = video.read()
        frame = cv2.resize(frame, FRAME_DIM)  # frame contiene l'immagine della webcam
        frame = cv2.flip(frame, 1) # giro orizzontalmente (parametro = 1) l'immagine

        #test = cv2.getTrackbarPos("alpha", "VIDEO")
        test = cv2.getTrackbarPos("gamma", "VIDEO")
        
        gamma = cv2.getTrackbarPos("gamma", "VIDEO")
        frame = change_brightness(frame, gamma)
        if (k % 256 == ord("q")):  # PATH, folder_name, full_path, IMG_LABELquando premo "q" si chiude la webcam e la relativa finestra
            cv2.destroyAllWindows()
            break
        camera = cv2.imshow("VIDEO", frame)

Can you post a simple working version of the code, so it can be tested? — asylumax, Feb 06 '22 at 19:59
What happens if you pass a negative `value` to `change_brightness`? — mkrieger1, Feb 06 '22 at 20:04
The conversion from BGR to HSV and back again is costing a lot of time you don't need to spend. Everything can be done in RGB. — Mark Ransom, Feb 06 '22 at 20:08
I see you have these three lines with `lim` in them. What are they for? — mkrieger1, Feb 06 '22 at 20:13
I am here, I omitted it, I took it for granted ................... @asylumax — bellapetezio, Feb 06 '22 at 20:17
Well, if you can, edit your post to show that. Also, do you know what OS, version of Python and OpenCV you are running? Adding that can be sometimes useful. — asylumax, Feb 06 '22 at 20:18
what prevents you from using the same principle as in `change_brightness` (to raise brightness) but for reducing brightness? — Christoph Rackwitz, Feb 06 '22 at 20:28
you copied that code from an answer under https://stackoverflow.com/questions/32609098/how-to-fast-change-image-brightness-with-python-opencv — Christoph Rackwitz, Feb 06 '22 at 20:29

asylumax · Accepted Answer · 2022-02-08T02:23:23.897

This seems to work; the essence is that the slider goes from 0 to 510, so you subtract an offset to get values to add or subtract to the 'v' value of HSV. The original code used a neat trick to easily go through all elements of the image to add brightness; the trick is repeated for subtracting off values. For example, if you subtract off -10 in brightness, anything lower than 10 is zero; otherwise, subtract off.

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Created on Sun Feb  6 18:06:51 2022

https://stackoverflow.com/questions/71011111/reduce-brigthness-in-real-time-opencv-python#comment125531455_71011111
"""
import cv2


def change_brightness(frame, value):
    hsv = cv2.cvtColor(frame, cv2.COLOR_BGR2HSV)
    h, s, v = cv2.split(hsv)

    # increase brightness; input value is 1 to 255
    if(value>0):
        lim_upper = 255 - value
        # If v>lim, then set to 255
        v[v > lim_upper] = 255
        # if v<lim, then add
        v[v <= lim_upper] += value
    # decrease brightness; input value from 0 to -255
    else:
        lim_lower = -value
        v[v < lim_lower] = 0
        v[v >= lim_lower] -= -value
    
    final_hsv = cv2.merge((h, s, v))
    img = cv2.cvtColor(final_hsv, cv2.COLOR_HSV2BGR)
    return img

# Read from video camera
video_capture_device_index = 0
webcam = cv2.VideoCapture(video_capture_device_index)
cv2.namedWindow("VIDEO")
cv2.createTrackbar("gamma", "VIDEO", 255, 510,(lambda a: None))

while True: 
        ret, frame = webcam.read()
        k = cv2.waitKey(1)
        # flip horizontally (mirror)
        frame = cv2.flip(frame, 1) # giro orizzontalmente (parametro = 1) l'immagine

        
        # varies from 0-510, so subtract so range is -255 to 255
        gamma = cv2.getTrackbarPos("gamma", "VIDEO")
        gamma = gamma - 255

        
        frame = change_brightness(frame, gamma)
        # Use q to quit
        if (k == ord("q")):  # PATH, folder_name, full_path, IMG_LABELquando premo "q" si chiude la webcam e la relativa finestra
            cv2.destroyAllWindows()
            break
        
        camera = cv2.imshow("VIDEO", frame)

webcam.release()
print("end video.")

Thanks for your answer! Can you add an explanation to your code? Remember that we are potentially archiving these posts and they should be as as clear as possible for future users (this includes you). — stateMachine, Feb 07 '22 at 01:33
and you can drop the `% 256` too because that's already done in `waitKey` — Christoph Rackwitz, Feb 07 '22 at 01:34

Reduce brigthness in real time OpenCv Python

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