"Muhammad Awais" <innocent_awais222@yahoo.com> wrote in message <i8cn61$ma2$1@fred.mathworks.com>...
> Thank you very much Wayne.
> Well I am trying to reproduce the algorithm in
> Xiang Gen Xia, Charles G., 'Wavelet transform based watermark for digital images', OPTICS EXPRESS, Vol. 3, No. 12,7 December 1998.
> it says:
> "In the encoding part, we first decompose an image into several bands with a pyramid
> structure as shown in Figs. 34 and then add a pseudorandom sequence (Gaussian
> noise) to the largest coefficients which are not located in the lowest resolution, i.e., the corner at the left and top as follows. Let y[m; n] denote the DWT coecients, which are not located at the lowest frequency band, of an image x[n;m]. We add a Gaussian noise sequence N[m; n] with mean 0 and variance 1 to y[m; n]:
> ~y[m; n] = y[m; n] + y2[m; n]N[m; n];"
> "We do not change the DWT coecients at the lowest resolution. Then, we take the two dimensional IDWT of the modified DWT coefficients ~y and the unchanged DWT coefficients at the lowest resolution"
> This means whatever level of decomposition I select, I need to add noise as watermark (or any other watermark) to all the details i.e. all subbands accept those at the upper left corner (approx. coeff.)
> In the paper it never says something that different noise or watermark should be added to different details (h,v,d).
> A quite similar concept is found in
> http://www.mathworks.com/matlabcentral/fx_files/3508/1/digital%20watermarking.pdf
> Thank you,
> regards
> Awais.
> > Hi Muhammad, There are many digital watermarking techniques with wavelets so I'm not sure which one you are trying to reproduce. I would caution you to look at a few things:
> >
> > 1.) Make sure your result is not perceptible.
> > 2.) Are you sure you want to add noise to all the details? and also do you want to add the same noise (and not independent sources) to all details?
> > 3.) You don't give us anyway of ascertaining how you determined the variance of the noise to add, or why you are only doing a level1 decomposition and adding noise at that level. Again, if you are reproducing somebody's algorithm then point us to that algorithm.
> > 4.) I believe you are correct to not mess with the approximation coefficients.
> >
> > As far as your code goes, the only thing I see is that you want to use reconstruction (synthesis) filters in waverec2, not the analysis filters as you have used.
> >
> > Wayne
Hi Muhammed, I looked at the paper at the mathworks.com link you included and I agree that you are implementing it as presented there. Just remember to use the synthesis filters at the reconstruction.
Wayne
