BPCS STEGANOGRAPHY - matlab

Question

i am doing BPCS STEGANOGRAPHY.. i have separate secret blocks (4096- 8*8 blocks) and conjugation block( 64 - 8*8 blocks) ready for embedding. each can be accessed separately by calling
secret_block(:,:,1),
secret_block(:,:,2),
secret_block(:,:,3)
etc and
conju_block(:,:,1),
conju_block(:,:,2),
conju_block(:,:,3)
etc
During bpcs embedding cover image is bitplane sliced and stored as c1( lsb plane ),c2,c3,c4 c5,c6,c7,c8 (msbplane). I need to select lsb bit plane to msb plane and dividing each bitplane into 8*8 block, selected_block(:,:,i)) and among them complex blocks are replaced with initially secret_block (:,:,i) and later on conju_block(:,:,i) etc until all the 4096+64 blocks reached..
Please anyone tell me how replace on particular block of selected_block(:,:,i) with another block (conju_block(:,:,i) or secret_block(:,:,i)) ? Is it possible to use replace_block() for the same ,then what does 'sys' parameter means ????

Answer 1

I would start by putting all bit planes in one cell,
C{1} = c1;
C{2} = c2;
etc
so you can go through the bit planes in an automated manner.
for i = 1:8
curr_plane = C{i};
end
There are many ways to split a 2D array into blocks. A convenient one is mat2cell. The web documentation gives you a visual example of how it works.
Note that since you want 8x8 blocks, both the rows and columns must be exactly divisible by 8. You have 3 options:
only allow images with number of rows and columns divisible by 8,
automatically rescale images to such a size, or
take the biggest slice of the image matrix with such a size, do your steganography and put that modified image back into your slice of the original image, from where it came from.
We continue with the assumption that you have an image with number of rows and columns divisible by 8. Let's call your initial cover image cover.
row = size(cover, 1)/8;
col = size(cover, 2)/8;
blocks = mat2cell(curr_plane, ones(1, row)*8, ones(1, col)*8);
blocks is now a cell which holds all the 8x8 blocks of your bit plane, curr_plane. For example, if your bit plane had a size of 16x48, you now have 2x6 blocks. You can iterate through them simply with
for bi = 1:size(blocks,1)
for bj = 1:size(blocks,2)
my_8x8_block = blocks{bi,bj};
end
end
You're almost there. To sum up what we've done, we go from bit plane to bit plane and split each one in 8x8 blocks. Now, for every my_8x8_block, you can feed it into a function which calculates whether it's complex enough or not. If it is, you can simply overwrite it with the kth secret or conjugate block.
blocks{bi,bj} = secret_block(:,:,k);
Finally, once you have gone through a whole bit plane, you can put back the blocks to construct the modified bit plane
C{i} = cell2mat(blocks);