Enhancing the specular effect of metallic color prints by reducing the use of yellow ink

T.Bugnon, A. Maesani, R.D. Hersch

The MetallicArt technique is used as a visual security feature to hide patterns in CMY images using an additional silver ink. The patterns are hidden under non-specular viewing conditions and visible under specular viewing conditions. By varying the amount of silver ink, we control the intensity of the specular reflection of the printed pattern. Using a spectral reflection prediction model, we can constrain the maximization of the silver ink in order to keep the color difference between the original CMY color and the fitted CMYS color under a given threshold, ensuring that the pattern is hidden under non-specular viewing conditions. When the printing device requires the inks to be UV cured, the leafing effect of the silver ink is reduced. This also reduces the specular reflection of the silver ink which becomes comparable to the specular reflection of the yellow ink. Since both specular reflections are similar, it is difficult to distinguish the embedded patterns under specular viewing conditions when both inks are present. We therefore propose a new algorithm to embed patterns in CMY images that not only maximizes silver, but also minimizes yellow. Experimental results show that the proposed algorithm enhances the visibility of the patterns under specular viewing conditions, allowing the use of MetallicArt when the printing device requires UV curing.

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