[WM]: On Attack by wmking wmked images(I.J.Cox 97')

["=3D?GB2312?B?uN/V/c6w?=3D" <gaozhengwei@gmail.com>]

hi,all:
i got some trouble when implementing the watemarking system proposed in
I.J.Cox 's paper:


 I. Cox, J. Kilian, F. T. Leighton, and T. Shamoon, *"Secure*

*spread spectrum watermarking for multimedia,"* *IEEE Trans.*

*Image Processing, *vol. 6, pp. 1673=A8C1687, Dec. 1997.



On Attack by watermarking watermarked images,i cannt get proper results =
as
the paper claimed.

the watermark detector response show exactly one peak.



my matlab code section is as follow:

-------------------------------------------------------------------------=
------

*%Multi-watermarking*
disp('Attack by watermarking watermarked image...');

n=3D1000;
alpha=3D.1;
cnt=3D5;

for k=3D4:6
i=3Dimread(file{k});
i=3Dim2double(i);

i2=3Di;
for index=3D1:cnt
    w{index}=3Drandn(1,n);
    i2=3Dcox_embedwmk(i2,w{index},alpha);
end

w2=3Dcox_extractwmk(i,i2,n,alpha);

*%SNR-PSNR-SIM Calculation*
if ndims(i)>2
    i_YIQ=3Drgb2ntsc(i);
    i2_YIQ=3Drgbn*
if ndims(i)>2ntsc(i2);
    snr=3DSNR(i_YIQ(:,:,1),i2_YIQ(:,:,1));
    psnr=3DPSNR(i_YIQ(:,:,1),i2_YIQ(:,:,1));
else
    snr=3DSNR(i,i2);
    psnr=3DPSNR(i,i2);
end

disp(sprintf('SNR=3D%4.2f,PSNR=3D%4.2f',...
    snr,psnr));

figure;
subplot(121),imshow(i),title('original image');
subplot(122),imshow(i2),title('multi-wmked image(5 times)');

*%uniqueness of watermark*
j=3D1;
for index=3D1:1000
    if mod(index,100)=3D=3D0 && j<=3Dcnt
       w_original=3Dw{j};
       j=3Dj+1;
    else
        w_original=3Drandn(1,n);
    end
    sim_test2(1,index)=3Dcox_sim(w_original,w2);
end
figure,stem(sim_test2,'marker','none');
title('Uniqueness of watermark');

end
-------------------------------------------------------------------------=
-------------------------------------------
 can anyone tell me what is wrong there?
thanks in advance ^_^

--
Gao Zhengwei
State Key Lab of CAD/CG,ZheJiang University
gaozhengwei@google.com


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