Hiding a Second Appearance in a Physical Relief Surface

We present a novel information hiding process that couples geometrical modeling with automated 3D fabrication for creating hiddenappearance reliefs. Our relief surface produces a first grayscale appearance visible by simple direct illumination and a second grayscale appearance ensured to be visible when the relief is lit by a digital projector with a specifically designed pattern and from a particular direction. The two appearances/images can be different yet embedded in the same physical relief. Since the second appearance appears only on demand, it could be used to hide a second image, a company logo, or a watermark image, for instance. Our novel method calculates a relief surface that maintains the properties needed for producing a second (hidden) appearance while also ensuring the first appearance is visible under normal direct illumination. Our experiments show that our method robustly produces reliefs with two arbitrary desired appearances.