how can I read perform some operation on an image byte by byte???

plz guide me. I m performing Image steganography in MATLAB. Need help Need to read images byte by byte and perform some operation byte wise

Answer 1

[Walter Roberson]

ImageData = imread('ImageFileName.JPG');

ImageAsBytes = typecast(ImageData(:), 'uint8');

fid = fopen('tempimg.dat', 'w');
fwrite(fid, ImageAsBytes');
fclose(fid);

fid = fopen('tempimg.dat', 'r');

for K = 1 : numel(ImageAsBytes)
  thisbyte = fread(fid, '*uint8', 1);
  .... perform the byte-wise operation here
end

fclose(fid);

Note that this code has been constructed to suit your requirement to "read images byte by byte", which is seldom necessary (or desirable) for steganography purposes.

[Sana Shaikh]

Thankyou Sir!

Kindly help me in performing:

Step-1: Read one by one byte from the secret message file and convert each byte to 8-bits. Then we apply 1 bit right shift operation on the entire file so that each byte will be modified accordingly.

• Step-2: We read 8 bits at a time and divide into two blocks 4 bits each and then perform the XOR operations with 4-bits on the left side with 4 bits on the right side and substitute the new bits in right 4-bit positions. The same thing repeated for all bytes in the file.

• Step-3: Repeat step-1 by performing 2 bits right shift for all bytes in the secret message file.

[Walter Roberson]

secretfid = fopen('secretmessage.txt','r');
while true
  [thissecretbyte, secretcount] = fread(secretfid, '*uint8', 1);
  if secretcount == 0; break;
   .... use the byte
end
fclose(secretfid);

It is not possible to right shift entire files by a bit; the most you can do is shift each byte after you read it. You need to figure out what you want to do with the bits that would be right-shifted "off the end".

See also bitset() and bitget() and xor()

[Sana Shaikh]

Thankyou for guiding.

Will u answer a few questions please?

1. What does "" while true "" means over here???? means what is this condition indicating??

2. If secretcount == 0; break; does this statement means that for storing the secret msg bits you r skipping the first byte???

3. Can you please tell me that if I store data in a temporary file using tempname ; so How can I display those contents of the temp file??

[Sana Shaikh]

""

clc; ImageData = imread('hide.bmp');

ImageAsBytes = typecast(ImageData(:), 'uint8') // for byte by byte as you guided :)

fid = fopen(tempname , 'w+');

fwrite(fid, ImageAsBytes);

for K = 1 : numel(ImageAsBytes)

 thisbyte = fread(fid,inf, 'uint8');
end

fclose(fid);

In the code given above the statement " thisbyte = fread(fid,inf, 'uint8'); " is displaying a long list of these:

thisbyte =

     []

thisbyte =

     []

thisbyte =

     []

thisbyte =

     []
...............

so what does that mean ? Nothing stored in that temp file???

[Image Analyst]

Look at ImageAsBytes in the variable editor and see what it is. If you don't know what I just said, then look at this: http://blogs.mathworks.com/videos/2012/07/03/debugging-in-matlab/

Answer 2

[Image Analyst]

See my demo code here: http://www.mathworks.com/matlabcentral/answers/67941#comment_137874

[Sana Shaikh]

Thank you Sir!