## I would like to apply DCT to each element in a matrix

on 17 Feb 2013

### Matt J (view profile)

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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### Matt J (view profile)

on 18 Feb 2013

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);```

Matt J

### Matt J (view profile)

on 18 Feb 2013

As the error message says, in imread(filename), the 'filename' variable must be a string. Did you check that it is? If so, how?

Andrew

### Andrew (view profile)

on 18 Feb 2013

I am not sure how to do that.

Walter Roberson

### Walter Roberson (view profile)

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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