We present a new approach for the reproduction of color images on a metallic substrate that look bright and colorful under specular reflection observation conditions and also look good under nonspecular reflection observation conditions. We fit amounts of both the white ink and the classical cyan, magenta and yellow inks according to a formula optimizing the reproduction of colors simultaneously under specular and non-specular observation conditions. In addition, we can hide patterns such as text or graphical symbols in one viewing mode, specular or non-specular, and reveal them in the other viewing mode. We rely on the tradeoff between amounts of white diffuse ink and amounts of cyan, magenta and yellow inks to control lightness in specular and in non-specular observation conditions. Further effects are grayscale images that alternate from a first image to a second independent image when tilting the print from specular to non-specular reflection observation conditions. Applications comprise art and entertainment, publicity, posters, as well as document security.
IntroductionDirect prints on a pure metallic substrate provide bright and brilliant colors when seen under specular reflection, but they look dark and dull when viewed under non-specular reflection [Pjanic and Hersch 2013]. Such prints are becoming commercially available for advertisement purposes and for the decoration of private homes.In the present contribution, we aim at producing partly specular reflecting and partly diffusely reflecting prints that look bright and colorful under specular and also look nice under non-specular observation angles. We rely on the ability to print on top of the metallic substrate diffusely reflecting white ink halftones that reduce the specular reflection component and increase the diffuse reflection component of the print. In addition, we offer the possibility of hiding patterns within a grayscale or color image when seen under specular viewing angles and make them appear when viewed under non-specular viewing angles or vice versa. _______________________We rely on the fact that increasing the amount of white ink darkens the image in specular viewing mode but increases the lightness of the image in the non-specular viewing mode and that the increase of similar amounts of cyan, magenta and yellow inks darkens the image both in specular and non-specular viewing modes.Thanks to the trade-off between the amounts of the white and of the colored inks, the lightness can be kept constant in one viewing mode and can be varied in the second viewing node. This also enables us to produce a metallic print that shows a first grayscale image in one viewing mode, and then, by tilting it to the other viewing mode, shows a second independent grayscale image. We had to meet a number of challenges. The first challenge consisted in creating a color reproduction workflow that enables finding surface coverages of inks yielding colors similar to the original colors under both specular and non-specular observation conditions. This is...