Subtracting Background From Image using Opencv in Python

Question

The following program displays 'foreground' completely black and not 'frame'. I also checked that all the values in 'frame' is equal to the values in 'foreground'. They have same channels,data type etc. I am using python 2.7.6 and OpenCV version 2.4.8

    import cv2
    import numpy as np

    def subtractBackground(frame,background):
        foreground = np.absolute(frame - background)
        foreground = foreground >= 0
        foreground = foreground.astype(int)
        foreground = foreground * frame
        cv2.imshow("foreground",foreground)
        return foreground

    def main():
        cap = cv2.VideoCapture(0)
        dump,background = cap.read()

        while cap.isOpened():
            dump,frame = cap.read()
            frameCopy = subtractBackground(frame,background)
            cv2.imshow('Live',frame)
            k = cv2.waitKey(10)
            if k == 32:
                break

    if __name__ == '__main__':
        main()

maxy · Accepted Answer · 2015-10-10 20:03:53Z

3

Because you are telling OpenCV to display a 64bpc image. You cast .astype(int) which means 'int64' or 'int32' depending on your architecture. Cast .astype('uint8') instead. Your maximum brigthness of 255 looks black compared to the full 64bit range.

Related problem:

foreground = np.absolute(frame - background)

Integer underflow. The expression frame - background does not automatically convert to a signed data type. You need a different data type for such calculations (try float if performance doesn't matter), or find an OpenCV function that handles the whole thing for you.

foreground = foreground >= 0

Because foreground is of type 'uint8', which can never be negative, the result is all ones. Simply insert some print repr(foreground) statements to debug such problems.

edited Oct 10, 2015 at 20:03

answered Oct 10, 2015 at 19:57

maxy

5,5671 gold badge25 silver badges30 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

shubh diwase · Accepted Answer · 2018-09-20 11:56:24Z

0

You can use background sub tractor provided by opencv itself. you can find the tutorial here. for example look at the code

import numpy as np

import cv2
cap = cv2.VideoCapture(0)
fgbg = cv2.createBackgroundSubtractorMOG2()
while(1):
    ret, frame = cap.read()
    fgmask = fgbg.apply(frame)
    cv2.imshow('frame',fgmask)
    k = cv2.waitKey(30) & 0xff
    if k == 27:
        break
cap.release()
cv2.destroyAllWindows()

answered Sep 20, 2018 at 11:56

shubh diwase

211 bronze badge

Comments

YScharf · Accepted Answer · 2021-04-27 18:16:31Z

0

replace foreground = np.absolute(frame - background)

with foreground = cv2.absdiff(frame, background)

answered Apr 27, 2021 at 18:16

YScharf

2,16022 silver badges27 bronze badges

Collectives™ on Stack Overflow

Subtracting Background From Image using Opencv in Python

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related