Watermarking of Color Images based on spectral modulation of viewing illuminant

In this paper we present a new method of watermarking devoted to color images. Watermarking technics generally sign images by introducing changes that are imperceptible to the human eye, but are easily recoverable by a computer program. The locations in the image where the signature is embedded are determined by a secret key in order to prevent possible attacks by pirates or alterations due to compression and coding transformations, or geometrical transformations. Meanwhile a lot of methods have been proposed to watermark grey level images, only few methods have been devoted to color images. In [1], Kutter proposed a new method to watermark a color image based on amplitude modulation of the blue channel, depending on the value of the bit, and proportional to the luminance. Next, Kim et al. proposed in [2] an another method based on magnitude increase of the saturation component with the contrainst that the resulting color difference is acceptable to the human visual system. The basic idea of this paper is to use the spectral radiance distribution of a secret illuminant to watermark the image. To reach this aim the secret illuminant used in place of the viewing illuminant is itself embedded, in some notches locations, by a watermark signal proportional to the spectral distribution of the studied image. The computation of notches locations is based on the analysis of the magnitude of the spectral histogram of the original image. In order to obtain a good tradeoff between robustness and invisibility, the amplitude of the watermark signal is controled by the amplitude of color differences between the original image and the watermarked one. Besides, it appears that, even for large variation of the specified histogram, the transformed images look visually undistrorted. The proposed process do not require to know neither the original image, neither the original viewing illuminant, to extract the watermark signal.