Why is my image not being inverted?

So I am trying to process a RGB image, by converting it first to Greyscale and then carrying out further processing.

Here is my piece of code:

I=imread('E:\SAHIL\RW\matlab basics\image processing\soil analysis\images to process\1.jpg');
I=rgb2gray(I);
background=imopen(I,strel('disk',15));
surf(double(background(1:8:end,1:8:end))),zlim([0,255]);
set(gca,'ydir','reverse');
I2=I-background;
imshow(I2)
I3 = imadjust(I2);
imshow(I3);

my surfing is completely black plus the entire image turns out to be black when inverted whereas I have three elements in my image and my background is white. When the same thing is done with respect to rice.png which is a by default image present in matlab toolbox, all the results come completely accurate.

I want to know where am I going wrong and why my output occurs as completely unexpected.