A HIGH SECURE REVERSIBLE VISIBLE WATERMARKING SCHEME

1. AHIGHSECUREREVERSIBLEVISIBLEWATERMARKINGSCHEME Source: IEEE International Conference on Multimedia and Expo, 2007,Pages 2106~2109Author: Han-Min Tsai, Long-Wen Chang Reporter: Peng-Yuan Chen

2. OUTLINE • Introduction • The Proposed Scheme • Experiment Result • Conclusion

3. INTRODUCTION • A novel reversible visible watermarking algorithm is proposed. It can fully remove the watermark from the visible watermarked image such that the original image can be restored. • Pixel values of original image beneath the watermark are mapped to a small range [α, α+127] to generate a visible watermarked image.

4. NOTATION • :M x M original gray-scale image • W : N x N bi-level watermark image with black background pixels denoted as 0 and white logo pixels denoted as 1, and N ≤ M. • : W overlaps Iand the overlapped area in I, and the remaining area of I is called . • w : be the current pixel value of W to be embedded in the pixel g in . g W w embed Watermark W Original image I

5. THE PROPOSED SCHEME (1) • We then map g to reveal the visible watermark by (1) W W g’ g w embed Watermark W Original image I watermarked image

6. THE PROPOSED SCHEME (2) • The equation (1) maps the original pixel value in [0, 255] to the small range [α, α + 127].

7. THE PROPOSED SCHEME (3) • Let m be the average intensity of . The parameter α is determined by • If m ≤ 128, α is large to form a light watermark. • If m > 128, α is small to form a dark watermark. (2)

8. THE PROPOSED SCHEME (4) • The goal is to losslessly restore the original image, first approximate the original pixel values in by the inverse mapping function, as given by, • The approximated pixel values form the image . • and form the approximated image . (3) g’ w W W g embed Inverse map Watermark W Approximate image Original image I watermarked image

9. THE PROPOSED SCHEME (5) • The difference value must be either 0 or 1. • Therefore, the difference image D of subtracting from must be a bi-level image. • D is then lossless compressed as C(D) by the lossless compression algorithm JBIG (Joint Bi-level Image Experts Group). g’ w W W g embed Inverse map Watermark W (1) Approximate image Original image I Difference image watermarked image

10. THE PROPOSED SCHEME (6) • The secret key K is a discrete random variable with approximate normal distribution. • Let be the possible values taken by K, where n is the number of pixels in corrsponding to w = 1. Those pixel values g corresponding to w = 1 are further disordered by • At present, we have generated the N ×N image . • and forms the M ×M image . (4)

11. THE PROPOSED SCHEME (7) • In the equation (4), may be greater 255 or less 0. Hence, those pixels values g which may cause overflow or underflow will not add . • To record the location of those pixels, a N ×N location image L is formed by that those possible overflow or underflow pixels denoted as 1 and the remaining pixels denoted as 0. • L is then lossless compressed as C(L) by JBIG. (4)

12. THE PROPOSED SCHEME (8) • Finally, we use Reversible Data Embedding Algorithm to embed the side information C(L) and C(D) into to form .

13. REVERSIBLE DATA EMBEDDING ALGORITHM (BRIEFLY) Pixel A Pixel B d=3 d=3 Pixel value b=103 0 a’=94 a=97 M=100 b’=107 255

14. RESTORE • The receiver can use the Reversible Data Embedding Algorithm to extract C(L) and C(D) from and to restore . • C(L) and C(D) are then lossless decompressed to L and D, respectively. • Users with the correct key K can restore pixel values by in the overlapping area. • With L, the decoder know which pixels didn’t add during the embedding process. Thus, these pixels will not subtract during the extracting process. With the restored g, we generate by using equation (3). and form the approximate image , and the original image I can be restored by + D.

15. THE SECRET KEY • The secret key K is a discrete random variable with approximate normal distribution for the compromise between transparency and robustness. • The same seed can generate the same random number sequence in the same interval of normal distribution . Only the authorized user has the correct seed and correct interval a to generate the correct key. probability Interval [-a,a]={[-1.5,1.5],[-3,3],[-5,5]}

16. EXPERIMENT RESULT(1) • If is smaller, more values of K would be near 0 such that g’’ is similar to g’. Thus, the watermark is more transparent. • If is bigger, more values of K would be far from 0. Thus, the watermark would be less transparent. • However, the less transparent watermark would be more robust because the details of the watermarked area is corrupted by K.

17. EXPERIMENT RESULT(2) The NTHU watermark

18. EXPERIMENT RESULT(3) • In addition, applying the proposed algorithm to each RGB plane of color images, and the results are shown in Fig.7. • The variance for each color plane was set 156.

19. EXPERIMENT RESULT(4)

20. CONCLUSION • In this paper, we proposed a novel reversible visible watermarking scheme which meets the three major requirements of visibility, transparency, and robustness for visible watermarking. • It additionally provides the capability that only the user with correct key can lossless restore the original image from the visible watermarked image. • The variance of the key compromises between the transparency of visible watermarked image and robustness. • In the experimental results, the transparent degree of visible watermark are affected by the secret key. • Users with the wrong key can not successfully remove the visible watermark.

21. THE PROPOSED SCHEME Original Image I W Approximate restore Secret key K Randomize Location Image L Difference image D Record the overflow or underflow position C(L) Lossless compress C(D) Reversible Data Embedding Algorithm

22. RESTORE Reversible Data Embedding Algorithm Image C(D) Secret key K C(L) Lossless Decompress Location Image L Difference image D + Original Image I (3)