Asked by Andrew
on 17 Feb 2013

I have a code where i take every 8*8 block of a 512*512 image and find its average, and subtract it away, storing the result into a new matrix(64*64).

n = 1; for i = 1:bs:row; m = 1; for j = 1:bs:col; ave = mean(mean(I(i:i+bs-1, j:j+bs-1))); A(i:i+bs-1, j:j+bs-1) = I(i:i+bs-1, j:j+bs-1) - ave; DC(n,m) = ave; m = m + 1; end n = n + 1; end

But i would like to apply DCT transform to each of the 8*8 blocks, without applying it to the entire image at once, and then store my results in a new matrix of the same size as the original image matrix. I am not sure how to continue from here. Can someone please guide me through this?

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Answer by Matt J
on 18 Feb 2013

Accepted answer

I'll assume you have a dct function of some kind and also MAT2TILES from the File Exchange

Icell = mat2tiles(I,[8,8]); ave=Icell;

for i=1:numel(Icell)

ave{i}=mean(Icell{i}(:)); Icell{i}=dct(Icell{i}-ave{i});

end

DC=cell2mat(ave); result=cell2mat(Icell);

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Walter Roberson
on 18 Feb 2013

Read the function you are using! It starts

%% LOSSY COMPRESSION-DECOMPRESSION USNIG DISCRETE COSINE TRANSFORM TECHNIQUE. function[]=mydct(filename,n,m) % "filename" is the string of characters including Image name and its % extension. % "n" denotes the number of bits per pixel. % "m" denotes the number of most significant bits (MSB) of DCT Coefficients.

This documents that you must pass exactly 3 parameters to mydct(), the first of which must be a file name, and the other two must be scalars. But instead your code is passing in an array of data values as the first parameter, and you are not passing in the second or third parameter at all.

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