What is the difference between a frame got from a video device and image loaded after saving the frame

What is the difference between a frame got from a video device and image
loaded after saving the frame

In matlab I am not getting what should I get after working on a frame. But
I get if i write that frame in a file and read it back and it works fine.
I = step(vidDevice); % Acquire single frame
imwrite(I,'tmp.jpg');
im=imread('tmp.jpg');
out=skin_detect(im);
But I get only the black image if I work directly on the frame.
I = step(vidDevice); % Acquire single frame
out=skin_detect(im);