In a image segmentation process I want save the output in a variable

I want to use the output to find the contour of the image. It shows correct out while i use 'sm[]' rather 'sm'

I used the code below ----------------

img = imread('o17.jpg');

gfrgb = imfilter(img, fspecial('gaussian', 3, 3), 'symmetric', 'conv');

cform = makecform('srgb2lab', 'whitepoint', whitepoint('d65'));

lab = applycform(gfrgb,cform);

l = double(lab(:,:,1)); lm = mean(mean(l));

a = double(lab(:,:,2)); am = mean(mean(a));

b = double(lab(:,:,3)); bm = mean(mean(b));

sm = (l-lm).^2 + (a-am).^2 + (b-bm).^2;

figure;imshow(sm,[]);