Extracting 8 bits of binary at a time (to convert to ASCII) from a long row using a for loop?

I have this script that analyses an encoded picture which has a message in it and looks for the difference in the two pictures. :

clear;
close all;
pic=imread('Cat.png');
pic1=imread('CodedCat.png');
origpic=double(pic);
cpic=double(pic1);
[rows, columns]=size(origpic);
Difference = abs(origpic - cpic);
bin_message = zeros(1, rows*columns);
n=0;
for row = 1 : rows
    for col = 1 : columns
        if Difference(row, columns) == 1
            bin_message(n) = 1;

          end
          n = n + 1;
      end
  end

I understand that I now have a 1d array with 1s and 0s and need to grab 8 bits at a time and convert each set of 8 bits back to the original message character.

I was given this hint(this is homework): message(1) = char(bin2dec(bin_message(1:8))) message(2) = char(bin2dec(bin_message(9:16))) and I have to use a for loop. I am so stuck. I don't understand how I can pull it out 8 columns at a time and then go to ASCII?? Can anyone shed some light?

[Guillaume]

See http://www.mathworks.com/matlabcentral/answers/256672-extracting-a-message-from-a-picture

Answer 1

[Image Analyst]

Here's a snippet from my text-into-image steganography demo:

% Now extract the string.
retrievedBits = bitget(stegoImage(1:numPixelsNeededForString), 1)
numPixelsNeededForString/bitsPerLetter])
letterCount = 1;
for k = 1 : bitsPerLetter : numPixelsNeededForString
  % Get the binary bits for this one character.
  thisString = retrievedBits(k:(k+bitsPerLetter-1));
  % Turn it from a binary string into an ASCII number (integer) and then finally into a character/letter.
  thisChar = char(bin2dec(num2str(thisString)));
  % Store this letter as we build up the recovered string.
  recoveredString(letterCount) = thisChar;
  letterCount = letterCount + 1;
end
message = sprintf('The recovered string = \n%s\n', recoveredString);
fprintf('%s\n', message); % Also print to command window.
% Display a popup message to the user with the recovered string.
uiwait(helpdlg(message));

Adapt as needed.