# which way is better to calculate var of image with blockproc?

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sara on 31 May 2015
Edited: Walter Roberson on 29 Jan 2018
hi everyone I have an image and I want to use var,mean,skewnes,... of this image in a 3*3 block.I thought this code is correct:
function [ varianceImage ] = var3dar3( Img )
blockSize = [3 3];
varFilterFunction = @(theBlockStructure)var(double(theBlockStructure.data(:)));
blockyImagevar = blockproc(Img, blockSize, varFilterFunction);
varianceImage=blockyImagevar (:);
end
but when I want to test this code I decide to compare var with this code:
function [varianceImage] = GetvarianceGL(Img)
blockSize = [3 3];
varFilterFunction = @(theBlockStructure) varimg(double(theBlockStructure.data(:)));
blockyImagevar = blockproc(Img, blockSize, varFilterFunction);
varianceImage=blockyImagevar(:);
end
that in this code varimage is this:
function [varianceGL] = varimg(Img)
[pixelCounts,GLs] = imhist(Img); %Gray Levels (the intensity value) and the count of pixels in the image having the gray level.
imshow(imhist(Img))
% Get the number of pixels in the histogram.
numberOfPixels = sum(pixelCounts)
% Get the mean gray lavel.
meanGL = sum(GLs .* pixelCounts) / numberOfPixels
% Get the variance, which is the second central moment.
varianceGL = sum((GLs - meanGL) .^ 2 .* pixelCounts) / (numberOfPixels-1);
end
which one is correct var3dar3 or GetvarianceGL? why the result of var is diffrent between manual way and ready way in matlab toolbox? I need manual way for my future works. Your help would be appreciated "

Walter Roberson on 31 May 2015
In your first code you have
StDevFilterFunction = @(theBlockStructure) var(double(theBlockStructure.data(:)));
but notice you are taking var() there not std(). The rest of your code also thinks it is dealing with std, such as squaring the result to get the variance.
sara on 31 May 2015
Excuse me...I change the std to var but the results of both code are different. Thanks for your attention

Image Analyst on 31 May 2015
The only reason I can see is if Img is not uint8, but maybe double or something. In that case, imhist() will not use 256 bins when creating the histogram and so the stats you get from varimg will be less accurate. Set a breakpoint there and check what class it is - uint8 or double.
sara on 4 Jun 2015

Edited: Walter Roberson on 29 Jan 2018
This code calculates the variance of a RGB image but it's not standard variance, see below:
%------------------------------------------
close all;
clear all;
clc;
blockSize = [6 6];
varFilterFunction = @(theBlockStructure) var(double(theBlockStructure.data(:)));
blockyImagevar = blockproc(I, blockSize, varFilterFunction);
varianceImage=blockyImagevar(:);
display(varianceImage);
%---------------------------------------
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