how to find mean for the values in an image

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Sharen H
Sharen H on 20 Jul 2013
Answered: Sujit Singh on 15 Mar 2017
i have to divide my image into n number of small 2X2 matrix and find the mean value and replace that 2x2 matrix with the mean value . example
[2 3 4 5
2 3 4 5
1 2 3 4
1 2 3 4]
i have to find the mean of
[2 3
2 3]
similarly for others and replace that concerned matrix with that mean value
the output should get is
[2 2 4 4
2 2 4 4
1 1 3 3
1 1 3 3]

Answers (3)

Wayne King
Wayne King on 20 Jul 2013
Edited: Wayne King on 20 Jul 2013
Do you have the Image Processing Toolbox? You can use blockproc(), but I'm not sure how you are getting your expected output above. You say you need the mean of 2x2 chunks submatrices like [2 3; 2 3]. Do you mean that you want the column means of those, which are 2 and 3, or you want the mean of all 4 elements which is 2.5? How are you taking the mean and ending up with a matrix the same size as the original?
I'll show you how to use blockproc() to return the column means for each 2x2 submatrix in x. That means for each 2x2 submatrix, you'll get back a 1x2 with the two column means. Accordingly, a 4x4 matrix reduces to a 2x4.
fun = @(block_struct) mean(;
Y = blockproc(x,[2 2],fun);
If you want the overall mean of each 2x2 submatrix, giving you a 2x2 matrix at the end:
fun = @(block_struct) mean(;
Y = blockproc(x,[2 2],fun);

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Andrei Bobrov
Andrei Bobrov on 20 Jul 2013
Edited: Andrei Bobrov on 23 Jul 2013
for youe case without blockproc (without Image Processing Toolbox):
kron(cellfun(@mean2,mat2cell(x,2*[1 1],2*[1 1])),[1 1; 1 1])
or use Peter J. Acklam' idea. Thanks to the cyclist for this link.
m = [2 2]; % size your small block
s = size(x);
n = s./m;
a = permute(reshape(x,m(1),n(1),n(2),[]),[1 3 2 4]);
out = kron(reshape(mean(reshape(a,[],prod(m))),n(1),[]),ones(n))
use function blkproc
sizeblock = [2 2];
Y = blkproc(x,sizeblock,@mean2);
out = kron(Y,ones(size(x)./sizeblock));
Sharen H
Sharen H on 23 Jul 2013
when i try for a [3,3] small block i am not able to process...

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Sujit Singh
Sujit Singh on 15 Mar 2017
We can assume given image size is 4X4 and you want to divide and calculate the mean of 2X2 block size and then get the mean processed image, so for this we can use blkproc in your case: 1.Img=imread('Img.tiff')% Read the image 2.meanFilterFunction = @(x)mean2(x(:))*ones(size(x),class(x)); % this is mean function 3. BS=[2 2] % block size 2X2 4.Mean_image = blkproc(Img, BS, meanFilterFunction); 5. imshow(Mean_image, []);
I hope this will help you , now you can change the size of image and size of block also .


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