??? Index exceeds matrix dimensions.

I think I have a problem with my matrices but I can't figure out where to fix it. The code that I have written is:

??? Index exceeds matrix dimensions.

Error in ==> dctwatermark at 75 dct_block=dct2(cover_object(y:y+blocksize-1,x:x+blocksize-1));

[Azzi Abdelmalek]

Where is your code?

[manjusha tikariha]

start_time=cputime;

k=20;                           % set gain factor for embeding
blocksize=8;                    % set the dct blocksize

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='P.jpg';
b=double(imread(file_name));
cover_object=rgb2gray(b);

% 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='Desert.jpg';
c=double(imread(file_name));
message=rgb2gray(c);
Mm=size(message,1);                  %Height
Nm=size(message,2);                  %Width

% reshape the message to a vector
message=round(reshape(message,Mm*Nm,1)./512);

% 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='water.jpg';
key=double(imread(file_name))./256;

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

% generate PN sequence
pn_sequence_zero=round(2*(rand(1,sum(sum(midband)))-0.5));

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

      % 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)=dwt_block(jj,ii)+k*pn_sequence_zero(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.bmp','bmp');

% display processing time
elapsed_time=cputime-start_time,

% display psnr of watermarked image
p=PSNR(cover_object,watermarked_image_int);

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

Answer 1

[Walter Roberson]

I speculate that your cover_object is not a multiple of blocksize in one (or both) directions, and your code only testing whether the x or y starts in the object instead of testing whether the end would also be in the object.

[Walter Roberson]

At the MATLAB command line give the command

dbstop if error

and run the program. When it stops, examine x and y and blocksize and size(cover_obj)