Combining perspective and color visual cryptography for securing color image display

International audienceIn this work, we propose to extend the secure information display introduced by Yamamoto et al. [1] to full color images. Yamamoto's technique makes use of black and transparent mask as decoding shadow image of a visual cryptography scheme sharing 3 bits multi-color messages. By combining perspective setup together with a color visual cryptography (VC) scheme which does not use any mask, we can securely display color images. A satisfying color VC scheme is used which can be printed on a transparency film. When printed, colors act as filters [2] and allow a wider color gamut for the message which is not limited to saturated color as in [1] because of the black and transparent mask as decoding shadow image. In our implementation of the two-out-of-two visual cryptography scheme which shares a secret message into two color shadow images, the first one is projected onto a glass diffuser and the second one is printed on a transparency. A registration method is used in order to overcome the difficulty of shadow image alignment. As the two shadow images are superposed with an air layer, the message disappears when the angular position is not close to the ideal one. Examples with binary colored messages and with color images are provided to show the extension. By moving the detector (or the eyes) angularly around the right position, perspective effects can be perceived