Hiding a message in an image?

Can someone please look at this document I attached, and also my code. Im not sure if my code is correct or not. I do get the two pictures with very minimal change at the end. I just want some insight into whether my code looks good or not, it feels like im missing something.

pic=imread('Cat.png')
imshow(pic) 
title('original picture')
[nrow ncol]=size(pic)
cpic=pic
Mi=input('Input message:','s')
M=double(Mi)
bin_m=dec2bin(8)
bin_m=bin_m'
bin_m=bin_m(:)
if length(M)>nrow*ncol
    error('The message is too long!')
end
k=1
for r=1:nrow
    for c=1:ncol
        if strcmpi(bin_m(k),'0');
            cpic(r,c)=cpic(r,c);
        elseif strcmpi(bin_m(k),'1') && cpic(r,c)==255;
            cpic(r,c)=cpic(r,c)-1;
        else
            cpic(r,c)=cpic(r,c)+1;
        end
    end
end
imshow(pic)
figure;
imshow(cpic)

[Tommy Halim]

Do you have code to extract hidden message from your code?

Answer 1

[Walter Roberson]

bin_m=dec2bin(8) is almost certainly wrong. Perhaps you meant

bin_m=dec2bin(M, 8);

[Brian Tiffman]

Now it looks better, Thanks man. Overall, do you think my code looks good?

[Image Analyst]

No. Many reasons, or ways to improve it.

For example, not having a semicolon after imread() will cause the whole image to spew out into the command window.

Another reason is the lack of comments.

Also you have variable names that are not very descriptive. This and the previous problem would make your code hard to maintain should another individual inherit it.

Next, you're using size() wrong for an image. Why? See this: http://blogs.mathworks.com/steve/2011/03/22/too-much-information-about-the-size-function/ To fix:

[rows, columns, numberOfColorChannels] = size(pic);

Next, you might learn how to use subplot() so that your pictures show up on the same figure instead of stacking up a bunch of overlapping windows.

Next, k does not change in your nested for loop so bin_m(k) is always bin_m(1). What's the point of that?

And the error message could be more informative and friendlier. Instead of this:

if length(M)>nrow*ncol
    error('The message is too long!')
end

try it like this:

if length(M) > rows * columns
    errorMessage = sprintf('Error: your message is %d characters long\nbut it cannot be longer than %d characters long,\nwhich is the number of pixels in the image', length(M), rows*columns);
    uiwait(warndlg(errorMessage));
    return;
end

See http://www.mathworks.com/matlabcentral/answers/8026-best-way-s-to-master-matlab and http://blogs.mathworks.com/videos/2012/07/03/debugging-in-matlab/

[Brian Tiffman]

I've tried incrementing k by using k=k+1, but I keep getting an error message. I'm not sure whats going on. The link you posted at the end of your last comment did help, and is a good read for me, so thanks.

[Image Analyst]

After you read that last link, you'll know what to do if you need more help.