How to divide gray scale image into 8x8 blocks and access each block separately to apply dct2 on the block

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Ari M Salih 2019-4-29

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编辑： Image Analyst 2021-10-23

I want to divide the grayscale image which in my case is the 'cameraman.tif' into 8x8 blocks then run dct2 on each block to

later hide a binary watermark image into each block.

So far I tried blockproc and many other methods but none of them satisfy my needs.

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dharaneesh govindaraj 2021-10-23

I want to do same process. I dont know how to convert the image and watermark. Please help me by providing code

Image Analyst 2021-10-23

编辑：Image Analyst 2021-10-23

@dharaneesh govindaraj, start with running and understanding my attached blockproc() demos. Then adapt them to make the function it runs fft2() instead of whatever I used. But this is essentially what Stephane does below.

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Answer 1

Stephane Dauvillier 2019-4-29

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https://ww2.mathworks.cn/matlabcentral/answers/459169-how-to-divide-gray-scale-image-into-8x8-blocks-and-access-each-block-separately-to-apply-dct2-on-the#answer_372808

编辑：Stephane Dauvillier 2019-4-29

在 MATLAB Online 中打开

file = 'cameraman.tif';
im=imread(file);
imshow(im);

In order to use blockproc you have to use a function that accpts blocksstructure, that's why you need to use something like this

imModified = blockproc(im,[8 8],@(blkStruct) dct2(blkStruct.data));
imshow(imModified)

I hope this answers your question

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Ari M Salih 2019-4-29

But how do I access each block to add the watermarked image to it?

This works but I still need to access them

Stephane Dauvillier 2019-4-29

在 MATLAB Online 中打开

You have a function to add your watermak ?

Let's just say you have your function has followed

function imageWithWatermak = myFunction(imageWithoutWatermark)
...
end

Then you will do

blockproc(im,[8 8],@(blkStruct) myFunction(blkStruct.data));

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Answer 2

KALYAN ACHARJYA 2019-4-29

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/459169-how-to-divide-gray-scale-image-into-8x8-blocks-and-access-each-block-separately-to-apply-dct2-on-the#answer_372807

编辑：KALYAN ACHARJYA 2019-4-29

在 MATLAB Online 中打开

image1=rgb2gray(imread('test.jpg'));
[rows colm]=size(image1);
% pading for make 8 disible, im my case rows 96 and colm 200
% 96 alread divisible by 8, no need modification: 12 
% colm 200 no need modification: 25
% Otherwise do zero pading, where required rows or colm, or both
result=dct2(image1,[12 25]);
% This size [12 25] divides the image in 8x8 blocks, each block size is 12x25
imshow(result);

There may more simpler way also.

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