Color reproduction, pattern hiding and image alternations with halftone prints on metal

We propose new methods for creating color or achromatic images that are printed with classical cyan, magenta, yellow and a white diffusing ink on a metallic substrate. We optimize the surface coverages of the cyan, magenta, yellow and white inks in order to create color prints on a metallic surface that look bright and colorful under specular reflection and also look good under non-specular observation conditions. We also provide new means for the prevention of counterfeits. A first effect enables viewing on the same metallic print one achromatic image in specular viewing mode and a second independent achromatic image in non-specular viewing mode. The second effect enables hiding a pattern such as text, graphics or a grayscale image within the printed color image in one viewing mode, specular or non-specular and showing that pattern in the second viewing mode, non-specular or specular, respectively.