In this article, the authors propose a method for hiding a visual watermark in color printed images with arbitrary, natural content. The embedded watermark is imperceptible under normal illumination, but it is revealed under UV illumination. Their method is using the white-paper fluorescence as a source of the watermark signal. The binary visual watermark is embedded by controlling the amount of fluorescence during the halftoning process, which is achieved by modulating the fractional white area of exposed paper substrate. The method is based on the iterative Color Direct Binary Search halftoning, which ensures high quality of the printed images, and uses a suitable error metric to control the perceived distortion due to the watermark embedding. Results show that the proposed method achieves low perceptibility of the embedded watermark under normal illumination, while the watermark is easily detectable using a portable UV flashlight even in bright daylight conditions.
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