# How to find the variance of an image in a special windows? variance(in 3*3 windows)

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sara on 26 Jan 2015
Commented: Image Analyst on 2 Feb 2015
I obtain mean of image like this
if true
function [ I2] = mean3dar3( m )
h = fspecial('average', [3 3]);
I2 = imfilter(m,h);
end
I want to obtain variance of image in a special windows like mean but I dont know how can I make filter of this... thanks
end

Image Analyst on 26 Jan 2015
You could use the built-in function stdfilt() and square it.
subplot(1,3,1);
imshow(grayImage);
fontSize = 20;
title('Original Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'Outerposition', [0, 0, 1, 1]);
stdImage = stdfilt(grayImage, true(3));
subplot(1,3, 2);
imshow(stdImage, []);
title('Standard Deviation Image', 'FontSize', fontSize);
varianceImage = stdImage .^2;
subplot(1,3, 3);
imshow(varianceImage, []);
title('Variance Image', 'FontSize', fontSize);
Or you could also do it with blockproc() though it's not so straightforward. Let me know if you want a demo of that method.
##### 2 CommentsShowHide 1 older comment
Image Analyst on 31 Jan 2015
Here are my blocproc demos. I show it used in a variety of ways.

Alessandro Masullo on 26 Jan 2015
Edited: Alessandro Masullo on 26 Jan 2015
You can use the function conv2:
small_window = ones(3)/9;
A_sq = conv2(A.^2,small_window,'same');
A_m = conv2(A,small_window,'same');
Var = A_sq - A_m.^2;

sara on 30 Jan 2015
thanks IMage Analyst and Alessandro Masullo
Excuse me, Is there any way to do cross correlation in a special windows,too?like stdfilt command??
cross correlation(in 3*3 windows)...
Image Analyst on 2 Feb 2015
If you correlate a 3x3 window with another 3x3 window, yes, you will get a 5x5 window. You won't get a single number. And since with each 3x3 colored block you show in your image, there are 8 other 3x3 colored blocks surrounding it, you will have eight 5x5 matrices if you do what you said to do.

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