There are a number of ways to this that are roughly equivalent. By default the GETDATA command returns N frames where N is equal to the value of the FramesPerTrigger property when GETDATA is called. You could do something like:
data = getdata(vid);
diffFrame = data(:,:,:,2) - data(:,:,:,1);
You can also specify the number of frames that GETDATA should return:
image1 = getdata(vid, 1); % Here 1 is the number of frames to return
image2 = getdata(vid, 1);
diffFrame = image2 - image1;
Of course, you could also do:
data = getdata(vid); % Get both frames
image1 = data(:,:,:,1); % The first frame
image2 = data(:,:,:,2); % The second frame
diffFrame = image2 - image1;
