how to get this video background removal worked?

hello everyone,

I cannot get this code working and it gives me black screen as output!

clc

clear

close all

obj = vision.VideoFileReader('1.mov');

frames= 550;

threshold=25;

for f=1:frames

I=step(obj);

%figure(1);imshow(I);title('Input Video Sequence');

if f==1

Background=I;

Fg=abs(I-Background);

else

Fg=abs(I-Background);

end

Fg(Fg>threshold) = 255;

Fg(Fg<=threshold) = 0;

figure(2);imshow(Fg);

drawnow;

pause(0.2)

end