Re: [WM]: a question about cox's algorithm

[yanmiaobest <yanmiaobest@163.com>]

Hi,
   thank you very much for you reply!

   my method is this:
              
            divid the original image into 8*8 subimages, and apply dct transform to the green channel of each block. And chose some coefficient to inject the signature.
            but I encounter a problem.  Injecting into the low frequency area is perceptable, injecting into the high frequency area is subjected to attack, is that right? but wharever area I inject the signature, it is more or less perceptible. My formula is:   x = x * ( 1 + a * p ), where  a = -1 when signature is 0, a = 1 when signature is 1. and p = 0.3. x is original image tranformed by dct. I could use smaller p value to avoid this, but that makes the extraction very difficult. Smaller p value means chaning the x very little. My extaction method is: sig = 1 when Xw > Xb, otherwise sig = 0. Xw is the watermarked image transformed by dct, and Xb is the original image transormed by dct( all applied on the green channel). when p is too small, this method sucks, because after injected the watermark, I have to transformed it into spatial domain using inverse dct, but many value is the same as the original image after inverse dct, so I can't extract the watermark.
           please help me
best
 regards

 

 

 

> What do you mean by variance of cox's algorithm?=20
>   =20
>   The answer to your question is : It will depend upon the gray level of =
> your image. For ex., if the image is represented by 8bpp
> (256 color) then according to Cox, the 63 coefficients of every block =
> will be sorted in descending order, barring DC coefficient
> (0,0), select first few largest coefficients and insert the random =
> number according to formula given in paper. The random numbers
> will be real numbers with mean 0 and variance 1. After embedding them =
> the pixels which were integers initially will be converted to
> real. They will be rounded off to its nearest values. This does not =
> result in much difference in perceptible quality and SIM or PSNR
> values.
>   =20
>   Good Luck
>   =20
>   Anurag Mishra
>
> =D1=D6=ED=B5 <yanmiao06@mails.gucas.ac.cn> wrote:
>   hi all
> I am new in this field. And I am about to impelment a variance of cox's =
> algorithm. The image is divided in 8 * 8 block, and dct
> transform is applied to each block, then insert watermark into each =
> block. My questions is this:
> 1. Dose each pixel in a 8 * 8 block can only be inserted one bit, which =
> means a block can at most inject 64 bits infomation?
> 2. when trasformed using dct, the values are real numbers, but after =
> inject the watermark, I have to round real numbers into
> integer, because image gray-level or rgb value is represented by =
> integer. Dose this affect my final result?
>
> best regards