A DCT Domain Visible Watermarking Technique for Images Saraju P. Mohanty * Dept. of Comp. Science & Engg. University of South Florida Tampa, FL 33620, USA smohanty@csee.usf.edu K.R. Ramakrishnan Dept. of Electrical Engg. Indian Institute of Science Bangalore 560 012, India krr@ee.iisc.ernet.in Mohan S Kankanhalli School of Computing National University of Singapore Kent Ridge, Singapore 119260 mohan@comp.nus.edu.sg ABSTRACT The growth of computer networks has boosted the growth of the information technology sector to a greater extent. There is a trend to move from conventional libraries to digital libraries. In the digital libraries images and texts are made available through the internet for scholarly research. At the same time care is taken to prevent the unauthorized use of the images commercially. In some cases the observer is encouraged to patronize the institution that owns the material. To satisfy both these needs simultaneously the owner needs to use visible watermarking. Visible watermarking is a type of digital watermarking used for protection of publicly available images. In this paper, we describe a visible watermarking scheme that is applied into the host image in the DCT domain. A mathematical model has been developed for that purpose. We have also proposed a modification of the algorithm to make the watermark more robust. 1. INTRODUCTION Digital watermarking is defined as a process of embedding data (watermark) into a multimedia object to help to protect the owner's right to that object. The embedded data (watermark) may be either visible or invisible. In visible watermarking of images, a secondary image (the watermark) is embedded in a primary (host) image such that watermark is intentionally perceptible to a human observer whereas in the case of invisible watermarking the embedded data is not perceptible, but may be extracted/detected by a computer program. Some of the desired characteristics of visible watermarks are listed below [1][2]. • A visible watermark should be obvious in both color and monochrome images. • The watermark should be spread in a large or important area of the image in order to prevent its deletion by clipping. • The watermark should be visible yet must not significantly obscure the image details beneath it. • The watermark must be difficult to remove; removing a watermark should be more costly and labor intensive than purchasing the image from the owner. • The watermark should be applied automatically with little human intervention and labor. There are very few visible watermarking techniques available in current literature. The IBM digital library organization has used a visible watermarking technique to mark the digitized pages of manuscript from the Vatican archive [3][9]. Kankanhalli et al.[4] have proposed a visible watermarking technique in DCT domain. They divide the image into different blocks, classify the blocks by perceptual methods proposed in [5] and modify the DCT coefficients of host image as follows. c ' ij (n) = α n c ij (n) + β n w ij (n) n = 1,2… (1) The α n and β n coefficients are for block n. The c ij (n) are the DCT coefficients of the host image block and w ij (n) the DCT coefficients of the watermark image block. In this paper, we propose a visible watermarking technique that modifies the DCT coefficients of the host image using eqn.(1). But, the α n and β n values are found out using a mathematical model developed by exploiting the texture sensitivity of the human visual system (HVS). This ensures that the perceptual quality of the image is better preserved. We call α n the scaling factor and β n as the embedding factor. We have also proposed a modification to make the watermark more robust. 2. FINDING THE SCALING AND EMBEDDING FACTORS While finding the scaling factors (α n ) and embedding factors (β n ), the following are taken into consideration [4][5][6][7] so that the quality of the watermarked image is not degraded. • The edge blocks should be least altered to avoid significant distortion of the image. So one can add only small amount of watermark gray value in the edge block of host image. This means that scaling factor α n should be close to α max , (the maximum value of the scaling factor) and embedding factor β n should be close to β min , (the minimum value of the embedding factor). • The distortion visibility is low when the background has strong texture. In a highly textured block, energy tends to be more evenly distributed among the different AC DCT coefficients. That means AC DCT coefficients of highly textured blocks have small variances and we can add more to those blocks. So for convenience, we assume α n to be directly proportional to variance (σ n ) and β n to be inversely proportional to variance (σ n ).