This paper discusses a method to neutralize the influence of printed colors using the additive color mixing model. The method is developed for Picture Illusion by Overlap, an image hiding method for printed materials which requires no computation in revealing the hidden image. In Picture Illusion by Overlap, a color image is hidden into two different color images. When the two images are printed onto slides and stacked together, the hidden image appears to the viewers' eyes. The color neutralization enables the image hiding operation by reducing the influence of the hidden pixels within the two images. Some experiments are made to verify the effect of the color neutralization method and the illusion scheme.
Extended visual cryptography [Ateniese et al., Theor. Comput. Sci. 250, 143–161 (2001)] is a method that encodes a number of images so that when the images are superimposed, a hidden image appears while the original images disappear. The decryption is done directly by human eyes without cryptographic calculations. Our proposed system takes three natural images as input and generates two images that are modifications of two of the input pictures. The third picture is viewed by superimposing the two output images. A trade-off exists between the number of gray levels and the difficulty in stacking the two sheets. Our new approach enhances the registration tolerance to obtain the third image and reduces the difficulty of superimposing the image while allowing a variety of gray levels. It is done by extending dot-clustered subpixel arrangements and enabling continuous gray-scale subpixel values. The system has considerably enhanced tolerance to the registration error. We show this by superimposing the output by computer simulation and calculating the peak SNRs with the original images.
Extended Visual Cryptography is a method which
encodes a number of images so that when the images are superimposed, the hidden image appears without a trace of original images. The decryption is done directly by human eyes without cryptographic calculations. The proposing system takes three pictures as input and generates two images which correspond to two of the input pictures. The third picture is perceived by superimposing the two output images.
Previous methods are based on halftoning and Boolean operations.
Transparency values must be quantized before encryption, and a pixel is halftoned by a fixed numbers of completely transparent and opaque
subpixels. Then a transparency of the superimposed pixel is controlled by changing the subpixel arrangements of the two output pixels. Since the subpixel arrangement is basically random, a tradeoff exists that to express the more graylevels, each subpixel must become the smaller, making it the more difficult to superimpose by hand.
Our new approach tolerates registration error for the third image and
eases the difficulty, by adopting concentric-circular subpixel
arrangement and continuous grayscale subpixel values. The system
becomes considerably robust to the registration error. Also, it
achieves quality improvement for all three images, by explicitly dealing with continuous graylevels.