# Problem in extracting the watermark ! !

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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.

--------------------------------------------------------------------------------------------------------------------------------

*The code for embedding and extraction is as follows :*

______________________________________________________________

[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

_____________________________________________________________

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

-----------------------------------------------------------

Please help

Thank you.

##### 1 Comment

Walter Roberson
on 14 Dec 2011

### Accepted Answer

Eric Pahlke
on 15 Dec 2011

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 .* ?

##### 2 Comments

Walter Roberson
on 17 Mar 2012

### More Answers (4)

mcc mscelec
on 14 Apr 2012

##### 1 Comment

Pardeep Kumar
on 26 May 2013

sir ,there is error showing in the line store(k)=WM_detect(ext_wm,orig_wm);

ie.value assigned to variable store might be unused .

Lester
on 2 Mar 2013

sir ,

may i know what are the steps followed for embedding and extracting the image in DCT domain in the above code....???

##### 0 Comments

Alaa Eleyan
on 22 Nov 2013

the code is working properly..

You just need to add space between W2 and (D_w(....... in the for loop :

Wrong : W2=[W2(D_w(IND(k,1),IND(k,2))/D(IND(k,1),IND(k,2))-1)*10];

Correct : W2=[W2 (D_w(IND(k,1),IND(k,2))/D(IND(k,1),IND(k,2))-1)*10];

##### 0 Comments

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