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

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

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 insert the waterma= rk xi (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 modified when it is marked and
it = is visible. 

   
    Anybody can give me some idea about my problem?

Thanks for advanced.
tpj



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