how to make it short codes?

Question

Tia 2013-7-25

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hi all.. if i have value of each block 8x8 pixels. i'm looking for how to random each block? any idea for it? but it's not effective. here's my codes.. could you fix it?

B=imread('filename');
a = reshape(B(1:128, 1:128), 8, 16, 8, 16);
a = permute(a, [1,3,2,4]);
%example for scramble positions
a(:,:,1,1)=a(:,:,16,2);
a(:,:,1,2)=a(:,:,2,1);
a(:,:,1,3)=a(:,:,4,1);
a(:,:,1,4)=a(:,:,6,1);
a(:,:,1,5)=a(:,:,7,4);
a(:,:,1,6)=a(:,:,9,16);
a(:,:,1,7)=a(:,:,5,10);
a(:,:,1,8)=a(:,:,7,1);
a(:,:,1,9)=a(:,:,5,1);
a(:,:,1,10)=a(:,:,5,1);
a(:,:,1,11)=a(:,:,4,4);
a(:,:,1,12)=a(:,:,3,1);
a(:,:,1,13)=a(:,:,2,3);
a(:,:,1,14)=a(:,:,2,5);
a(:,:,1,15)=a(:,:,5,4);
a(:,:,1,16)=a(:,:,15,2);
a(:,:,2,1)=...
a(:,:,..,..)=...

and so on until a(:,:,16,16). so we have 256 blocks to random without any change in the value of each block.

how to scramble it? i hope it'll easier

thanks..

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Answer 1

Andrei Bobrov 2013-7-25

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编辑：Andrei Bobrov 2013-7-25

在 MATLAB Online 中打开

Use tricks by Peter Acklam

B = randi(300,128,128); % Let your data
b = [8 8];              % size of small block
s = size(B);
m = s./b;
B1 = permute(reshape(B,b(1),m(1),m(2),b(2)),[1 3 2 4]);
B2 = reshape(B1,[b prod(m)]);
idx3 = randperm(size(B2,3));
B3 = B2(:,:,idx3);
a = reshape(permute(reshape(B3,b(1),s(2),[]),[1 3 2]),s(1),[]);

OR with use of cell arrays

B = randi(300,128,128); % Let your data
b = [8 8];              % size of small block
m = size(B)./b;
C0 = mat2cell(B,b(1)*ones(m(1),1),b(2)*ones(m(2),1));
ii = randperm(numel(C0));
a = cell2mat(reshape(C0(ii),m));

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Tia 2013-7-25

wow,it's great. thank you very much

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Answer 2

Roger Stafford 2013-7-25

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If I have understood your question correctly, you can keep the first three lines:

 B = imread('filename');
 a = reshape(B(1:128, 1:128), 8, 16, 8, 16);
 a = permute(a, [1,3,2,4]);

Then do this:

 p = bsxfun(@plus,64*randperm(256),(-63:0)'); % Calculate 16384 linear indices
 a = reshape(a(p),8,8,16,16); % Permute the various 8 x 8 blocks in 'a'

(Note: Your method of operating on 'a' one 8 x 8 block at a time has the possibility of changing a block and then erroneously using those changed values to change another block instead of using its original values. To avoid this, you would have had to use a new variable temporarily and when finished, copy it back into 'a'. In any case, writing 256 lines of code would be rather tedious.)