this program shows an error plz help me......,,,,,Error using dct2_embed_mod (line 47) Message too large to fit in Cover Object

%Name:    Chris Shoemaker
%Course:  EER-280 - Digital Watermarking
%Project:   Comparison-Based Correlation in DCT mid-band
%           Uses two PN sequences; one for a "0" and another for a "1"
%           Watermark Embeding

clear all;

% save start time
start_time=cputime;

k=15;                           % set gain factor for embeding
blocksize=8;                    % set the dct blocksize
pn_sequence_search='T';         % perform search to find highly uncorrelated pn sequences {T,F}

midband=[   0,0,0,1,1,1,1,0;    % defines the mid-band frequencies of an 8x8 dct
            0,0,1,1,1,1,0,0;
            0,1,1,1,1,0,0,0;
            1,1,1,1,0,0,0,0;
            1,1,1,0,0,0,0,0;
            1,1,0,0,0,0,0,0;
            1,0,0,0,0,0,0,0;
            0,0,0,0,0,0,0,0 ];

% read in the cover object
file_name='_lena_std_bw.bmp';
cover_object=double(imread(file_name));

% determine size of cover image
Mc=size(cover_object,1);          %Height
Nc=size(cover_object,2);          %Width

% determine maximum message size based on cover object, and blocksize
max_message=Mc*Nc/(blocksize^2);

% read in the message image
file_name='_copyright.bmp';
message=double(imread(file_name));
Mm=size(message,1);                  %Height
Nm=size(message,2);                  %Width

% reshape the message to a vector
message=round(reshape(message,Mm*Nm,1)./256);

% check that the message isn't too large for cover
if (length(message) > max_message)
    error('Message too large to fit in Cover Object')
end

% pad the message out to the maximum message size with ones's
message_vector=ones(1,max_message);
message_vector(1:length(message))=message;

% generate shell of watermarked image
watermarked_image=cover_object;

% read in key for PN generator
file_name='_key.bmp';
key=double(imread(file_name))./256;

% reset MATLAB's PN generator to state "key"
rand('state',key);

% generate PN sequences for "1" and "0"
pn_sequence_one=round(2*(rand(1,sum(sum(midband)))-0.5));
pn_sequence_zero=round(2*(rand(1,sum(sum(midband)))-0.5));

% find two highly un-correlated PN sequences
if (pn_sequence_search=='T')
    while (corr2(pn_sequence_one,pn_sequence_zero) > -0.55)
        pn_sequence_one=round(2*(rand(1,sum(sum(midband)))-0.5));
        pn_sequence_zero=round(2*(rand(1,sum(sum(midband)))-0.5));
    end
end

% process the image in blocks
x=1;
y=1;
for (kk = 1:length(message_vector))

      % transform block using DCT
      dct_block=dct2(cover_object(y:y+blocksize-1,x:x+blocksize-1));

      % if message bit contains zero then embed pn_sequence_zero into the mid-band
      % componants of the dct_block
      ll=1;
      if (message_vector(kk)==0)
          for ii=1:blocksize
              for jj=1:blocksize
                  if (midband(jj,ii)==1)
                      dct_block(jj,ii)=dct_block(jj,ii)+k*pn_sequence_zero(ll);
                      ll=ll+1;
                  end
              end
          end

      % otherwise, embed pn_sequence_one into the mid-band componants of dct_block    
      else
          for ii=1:blocksize
              for jj=1:blocksize
                  if (midband(jj,ii)==1)
                      dct_block(jj,ii)=dct_block(jj,ii)+k*pn_sequence_one(ll);
                      ll=ll+1;
                  end
              end
          end               
      end

      % transform block back into spatial domain
      watermarked_image(y:y+blocksize-1,x:x+blocksize-1)=idct2(dct_block);    

      % move on to next block. At and of row move to next row
      if (x+blocksize) >= Nc
          x=1;
          y=y+blocksize;
      else
          x=x+blocksize;
      end
  end

% convert to uint8 and write the watermarked image out to a file
watermarked_image_int=uint8(watermarked_image);
imwrite(watermarked_image_int,'dct2_watermarked_mod.bmp','bmp');

% display processing time
elapsed_time=cputime-start_time,

% display psnr of watermarked image
psnr=psnr(cover_object,watermarked_image,Nc,Mc),

% display watermarked image
figure(1)
imshow(watermarked_image,[])
title('Watermarked Image')

Answer 1

[Walter Roberson]

Steganography alters only a small number of bits per location, so the maximum number of bits you can hide is (bits_hidden_per_location * number_of_hiding_locations) . Each byte of message to hide requires 8 bits, so you need length(message)*8 locations available to hide it all. If numel(cover_image) * bits_hidden_per_location < length(message)*8 you cannot possibly hide the message in that image. Furthermore some embedding schemes will not hide a bit in every location; for example some systems only hide one or two bits for every 8 x 8 block, or some only hide in one color plane.

The message is telling you that your cover image is not large enough to hide the entire message using the particular encoding scheme you chose.