imagesc, subtraction of two images

good day .. my aim is it extract optic disc ..i have applied otsu threshold method ..i have 2 subtract two images but when i subtract i get a blank image . my process followed green component -->tophat using large and small structural element that gives me 2 outputs --->then i apply otsu threshold for both tophat(large) and tophat(small) ..after this i want to subtract these otsu images ..but i am not able to do it please help me

se = strel('disk',500,8);
im2 = imtophat(I,se);
se1 = strel('disk',50,8);
im3 = imtophat(I,se1);
IDX = otsu(im2,6);
figure;
subplot(2,2,1);
axis image off;
imagesc(IDX)
colormap(gray);
IDX1 = otsu(im3,6);
figure;
subplot(2,2,1);
axis image off;
imagesc(IDX1)
colormap(gray);

i tried IDX -IDX1 but i amnot getting any image please somebody tell me whr am i goin wrong

Answer 1

[Image Analyst]

You need to cast them to double before subtraction because otherwise it leaves them as uint8 and won't allow negative numbers and will clip to zero. Also, please pick better, more descriptive names, like

differenceImage = double(thresholdedImage1) - double(thresholdedImage2);
imshow(differenceImage, []);