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

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sara 2015-5-31

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编辑： Walter Roberson 2018-1-29

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 "

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回答（3 个）

Walter Roberson 2015-5-31

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在 MATLAB Online 中打开

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.

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sara 2015-5-31

Excuse me...I change the std to var but the results of both code are different. Thanks for your attention

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Image Analyst 2015-5-31

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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.

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sara 2015-6-3

I'm sorry.I thought that the file was attached.

sara 2015-6-4

Can you guide me, please?

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Morteza Hajitabar Firuzjaei 2018-1-29

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编辑：Walter Roberson 2018-1-29

在 MATLAB Online 中打开

This code calculates the variance of a RGB image but it's not standard variance, see below:

%------------------------------------------
close all; 
clear all;
clc;
I = imread('b.png');
blockSize = [6 6];
varFilterFunction = @(theBlockStructure) var(double(theBlockStructure.data(:)));
blockyImagevar = blockproc(I, blockSize, varFilterFunction);
varianceImage=blockyImagevar(:);
display(varianceImage);
%---------------------------------------