MATLAB: Problem in extracting the watermark ! !

dctextracting the watermarkwatermarking

Dear Sir,
I have an issue in extracting the watermark..
PROPOSED APPROACH :
1.Bring the image to frequency domain by applying the DCT
2.generate a watermark signal
3.Use the thousand largest coefficients of the original image to embed a watermark sequence of length 1000.
4.Coefficients are modified according to the stream bits of the message using the equation below,
 CAW = CA (1 + α Wi)
5.Extraction process – simply subtracting the original DCT coefficients from the watermarked image coefficients.
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*The code for embedding and extraction is as follows :*
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[fname pthname]=uigetfile('*.jpg;*.png;*.tif;*bmp','Select the Asset Image'); %select image 
        I=imread([pthname fname]);
      wmsz=1000; %watermark size
       I=I(:,:,1);%get the first color in case of RGB image 
      [r,c]=size(I); 
      D=dct2(I);%get DCT of the Asset 
      D_vec=reshape(D,1,r*c);%putting all DCT values in a vector 
      [D_vec_srt,Idx]=sort(abs(D_vec),'descend');%re-ordering all the absolute values 
      W=randn(1,wmsz);%generate a Gaussian spread spectrum noise to use as watermark signal 
      Idx2=Idx(2:wmsz+1);%choosing 1000 biggest values other than the DC value 
      %finding associated row-column order for vector values 
      IND=zeros(wmsz,2); 
      for k=1:wmsz 
      x=floor(Idx2(k)/r)+1;%associated culomn in the image 
      y=mod(Idx2(k),r);%associated row in the image 
      IND(k,1)=y; 
      IND(k,2)=x; 
      end 
      D_w=D; 
      for k=1:wmsz 
      %insert the WM signal into the DCT values 
      D_w(IND(k,1),IND(k,2))=D_w(IND(k,1),IND(k,2))+.1*D_w(IND(k,1),IND(k,2)).*W(k); 
      end 
      I2=idct2(D_w);%inverse DCT to produce the watermarked asset
      %The extraction process is simply subtracting the original DCT %coefficients from the 
      %watermarked image ones. The code can be written like below: 
      W2=[];%will contain watermark signal extracted from the image 
      for k=1:wmsz 
      W2=[W2(D_w(IND(k,1),IND(k,2))/D(IND(k,1),IND(k,2))-1)*10];%watermark extraction 
      end
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The ERROR appears as follows in the 25th line :
??? Attempted to access W2(-0.0432565); index must be a positive integer or logical.
Error in ==> pooya at 29 
W2=[W2(D_w(IND(k,1),IND(k,2))/D(IND(k,1),IND(k,2))-1)*10];%watermark extraction
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Please help
Thank you.

Best Answer

There's a lot of code to sort through to figure out exactly what you need to fix, but what stands out is this line:
D_w(IND(k,1),IND(k,2))=D_w(IND(k,1),IND(k,2))+.1*D_w(IND(k,1),IND(k,2)).*W(k);
You're using D_w as an index on line 25(where you get the error), which means it has to be an integer.
However in the line above you're assigning fractional values to D_w:
    (D_w(stuff) = D_w(stuff) + .1*D_w(stuff)
Was the .1* intended to be a .* ?
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