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

[yanmiaobest <yanmiaobest@163.com>]

hi,
  my watermark image is a binary image,  if I want to hide 0, substract a small value from the original one,
  and if I want to hide 1, add a small value to the original one.
  So 'a'  is -1 when  signature value is 0, and 1 when signature is 1.

  I followed your advice, but it did't work either. I wonder if somthing wrong with my process. Here is my process:

  1. divide the original image into 8*8 blocks
  2. perform dct on each block
  3. inject watermark into each block to a fixed position. In my example, I inject just one bit per block into the right buttom coeffienct.
  4. inverse dct and write them to file
 extraction:
  1. divide original image and watermarked image into 8*8 block
  2. perform dct on each block.
  3. compare the injected position and extract watermark.

Here is a block of the  original image:

19 11 9 15 18 15 13 16
17 10 8 14 17 15 14 17
13 8 7 11 14 14 15 17
10 6 7 7 11 12 16 17
8 6 8 4 8 10 16 16
9 10 9 5 4 10 16 17
9 12 11 4 2 8 14 15
11 15 13 4 2 9 15 15

after dct transform, it becomes:
91.00 -14.40 16.21 1.24 0.50 1.28 0.59 1.85
13.20 -0.98 -10.15 10.14 14.12 -0.41 0.08 -0.54
6.75 8.94 0.18 -0.24 0.14 -0.90 -0.07 -2.34
-0.85 0.18 0.32 -0.34 0.31 0.31 0.14 0.11
-0.50 -0.01 -0.14 0.47 -0.50 -0.06 0.33 1.84
-1.15 0.19 -0.06 -0.44 0.11 0.09 0.07 -0.32
1.26 -0.12 0.43 0.13 -0.33 0.15 -0.18 -0.87
-1.47 0.21 -0.65 -0.09 -0.16 -0.16 0.11 0.73
                                        ------ inject into here
after injection, 0.73 becomes 0.80
and after inverse dct, the values are:
19 11 9 15 18 15 13 16
17 10 8 14 17 15 14 17
13 8 7 11 14 14 15 17
10 6 7 7 11 12 16 17
8 6 8 4 8 10 16 16
9 10 9 5 4 10 16 17
9 12 11 4 2 8 14 15
11 15 13 4 2 9 15 15
here is the problem:   nothing changes!   So I can't extact watermark.
I tried many positions to inject, but the problem remains, and many times, the inverse dct value is out range, which means the value > 255.