[WM]: Image distorsioned inserting a watermark usign Cox algorithm: MORE DETAILS

[". tpj" <tpj@ifrance.com>]

[WM]: Image distorsioned inserting a watermark usign Cox et = al.
algorithm

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Hi,
   I am trying to insert a watermark into a image using the = Cox et al.
algorithm described into "Secure Spread Spectrum watermarkin= g for multimedia", but I have some problems.

   I apply the dct on entire image, only for luminance compo= nent (using a dct matrix version, with a reasonable cost for little
images) and then I find the N greatest dct coefficients to i= nsert the watermark(generated randomly with a N(0,1) distribution) by
= the equation vi'=vi*(1+ alpha*xi). Then, my problem is that I have obse= rved the original image is lightly and visible
modified.
   
    For more details:
      * I use N=1000 (number of dct coefficients to be marked)
      * I use alpha=0.1
      * I select the N greatest coefficients from the AC coe= fficients. I don't use the DC coefficient.

    Anybody can give me some idea about my problem?  
    To select the N largest coefficients, I must consider th= e sign or the value without sign?

    Has anybody obtained the same results or similar that th= ose described into the Cox's paper?? I'll be grateful for any aportation.

Thanks for advanced.
tpj


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