Extraction of data from DWT subband - matlab

Question

I am attempting to extract data from a DWT subband. I am able to embed data correctly (I have followed it in the debugger),cal PSNR etc. PSNR rate seem very high 76.2?? however,I am having lot of trouble extracting data back!It is sometimes extracting the number 128?? Can anyone help or have any idea why this is? I would be very thankful.I have been working on this all day & having no luck!I am very curious to know??
Data Embedding:
coverImage = imread('lena.bmp');
message = importdata('minutiaTest.txt');
%message = 'Bifurcations:';
[LL,LH,HL,HH] = dwt2(coverImage,'haar');
if size(message) > size(coverImage,1) * size(coverImage,2)
error ('message too big to embed');
end
bit_count = 0;
steg_coeffs = [4, 4.75, 5.5, 6.25, 7];
for jj=1:size(message,2)+1
if jj > size(message,2)
charbits = [0,0,0,0,0,0,0,0];
else
charbits = dec2bin(message(jj),8)';
charbits = charbits(:)'-'0';
end
for ii=1:8
bit_count = bit_count + 1;
if charbits(ii) == 1
if HH(bit_count) <= 0
HH(bit_count) = steg_coeffs(randi(numel(steg_coeffs)));
end
else
if HH(bit_count) >= 0
HH(bit_count) = -1 * steg_coeffs(randi(numel(steg_coeffs)));
end
end
end
end
stego_image = idwt2(LL,LH,HL,HH,'haar');
imwrite(uint8(stego_image),'newStego.bmp');
Data Extraction:
new_Stego = imread('newStego.bmp');
[LL,LH,HL,HH] = dwt2(new_Stego,'haar');
message = '';
msgbits = '';
for ii = 1:size(HH,1)*size(HH,2)
if HH(ii) > 0
msgbits = strcat (msgbits, '1');
elseif HH(ii) < 0
msgbits = strcat (msgbits, '0');
else
return;
end
if mod(ii,8) == 0
msgChar = bin2dec(msgbits);
if msgChar == 0
break;
end
msgChar = char (msgChar);
message = [message msgChar];
msgbits = '';
end
end

Answer 1

The problem arises from reading your data with importdata.
This command will load the data to an array. Since you have 39 lines and 2 columns (skipping any empty lines), its size will be 39 2. However, the program assumes that your message will be a string. For example, 'i am a string' has a size 1 13. This expectation of the program compared to the data you actually give it creates all sorts of problems.
What you want is to read your data as a single string, where the number 230 is not one element, but 3 individual characters. Tabs and newlines will also be read in as well.
To read your file:
message = fileread('minutiaTest.txt');
After you extract your message, to save it to a file:
fid = fopen('myFilename.txt','w');
fprintf(fid,message);
fclose(fid);