hello guys...my problem is how can i do background subtraction from the function rgb2gray?

this is my code

trafficObj = VideoReader('left.avi'); get(trafficObj) implay('left.avi');

A = trafficObj.NumberOfFrames; for i = 1:A firstVideo=rgb2gray(trafficObj.read(i)); imshow(firstVideo); title('rgb2gray');

     secondVideo=imsubtract(trafficObj.read(1),trafficObj.read(i));
      imshow(secondVideo)
      title('subtraction');

      drawnow;
 end