Visibility enhancements of gray and color images with size invariant visual cryptography

Abstract

ABSTRACTIn a visual cryptography scheme (VCS), a secret image (SI) is divided into n shares, where every pixel in the SI may be represented by m pixels in the shares. Stacking exactly n shares reveals the SI with pixel expansion m. However, the size of the shares and revealed SI is the size of the SI times m, and the transmission of large shares requires additional time and space, which is the main disadvantage of pixel expansion. Moreover, the representation of gray and color images in VCS requires even more pixel expansion. Furthermore, if the two dimensions of m are not equal, the aspect ratio will be variant and distort the revealed SI. In this article, we adopt a simple solution for the above problems: pixel expansion and a variant aspect ratio. We also offer a promising novel solution that ensures that the size of the shares is kept smaller than that of the SI by resizing the SI, and by using interpolation, erode, smart sharpen, and an unsharp mask.