In order to solve the contradiction between watermark transparency and robustness, this paper combines Non-subsampled Contourlet Transformation (NSCT), Discrete Wavelet Transformation (DWT), Singular Value Decomposition (SVD), and Human Visual System (HVS) to improve a watermarking algorithm. Firstly, this algorithm performs a first-level NSCT decomposition on the carrier image to obtain its low-frequency part, and then performs a second-level DWT on the low-frequency part, extracts its second-level low-frequency sub bands and block these sub bands to sub-blocks according to their size. Secondly, SVD is performed for each sub-block and the best embedding position of the watermark according to the sensitivity of HVS for the average brightness and the modified cosine similarity measurement method is calculated. Finally, the watermark scrambled and encrypted by Arnold is embedded into the selected singular value. This algorithm can realize the blind extraction of watermark. Experimental results show that the algorithm can not only satisfy the transparency but the robustness of the watermark at the same time as well.
K E Y W O R D Sblind watermarking, digital watermarking, discrete wavelet transform, NSCT, singular value decomposition
INTRODUCTIONNowadays, multimedia documents like images, audio, and video are easy to store and disseminate with the help of digital communication through internet. The document, however, can be modified by intruders during transmission or storage for illegal use. This illegal alteration is becoming a problem in many human-centric applications. Therefore, the effective protection and content certification of digital products have become a real problem and one can incorporate digital watermarking scheme to deal with such issues. Digital watermarking technology uses the principle of information hiding to embed the logo information into the carrier data in a perceptible or imperceptible way, without affecting the use value of the original carrier, protecting copyright, private communication, content authentication, and tracking infringements. 1 Digital watermarking can be applied to images, videos, audio, or texts. 2 The embedded information should be protected and extracted successfully to verify the image. Digital image watermarking can be of two types: visible and invisible. For invisible watermarking, there should not be any perceptual change in the host image. In visible watermarking, the watermark should can be seen so that the watermark can be easily detected. 3 Watermarking can be classified into blind and non-blind watermarking depending