Data Hiding by White Modulation in Color Direct Binary Search Halftones

Abstract

In this paper, we 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. Our 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. Our 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.

Halftone Modulation for Embedding UV Watermarks in Color Printed Images

“…In the CDBS-WM watermarking method [5], the UV watermark embedding is performed by modulating the white halftone pixels according to the binary visual watermark. This is achieved by extending the cost function that is minimized in CDBS with a binary watermark term that is dependent on the number of white halftone pixels.…”

“…An estimate of the UV response for all 2-by-2 cells is obtained using 7and stored in a LUT for use during the iterative watermark embedding that minimizes the cost function in (4). More details about the actual minimization of a cost function of type (4) can be found in [5].…”

“…However, achieving equal perceptibility between the two methods is a difficult task because the embedded watermark using CDBS-WM is somewhat perceptually non-uniform. In the original work [5], CDBS-WM uses the CIELAB space instead of YyCxCz to achieve higher perceptual uniformity of the watermark distortion, which comes at the price of grainier halftone texture and hence, lower image quality [5][6]. Figure 6 shows the CMYK watermarked images using both the CDBS-WM method (top) and the proposed method (bottom) captured under indoor office light.…”

“…Using only the white primary in CDBS-WM can be seen as a more aggressive embedding approach that may cause visible distortions in certain image parts. Another suspected reason for the poorer performance of the CDBS-WM method is the small inaccuracy of the proposed 2-by-2 printer model -it is computational and not fully measurement-based as in the original CMY-based CDBS-WM work [5]. The same images under office light with added UV light are shown in Figure 7.…”

“…In a recent work, we proposed the CDBS-WM method for UV-watermark embedding [5] that is based on the CDBS halftoning algorithm [6]. As in the previously published methods, the carrier of the hidden signal in CDBS-WM is the substrate fluorescence, which is modulated with the fractional coverage of white-paper area.…”