Aspect ratio invariant visual secret sharing schemes with minimum pixel expansion

Visual secret sharing (VSS) scheme is a perfect secure method to hide a secret image by breaking it into shadow images and one can decode it easily by the human visual system. We use m, the pixel expansion, subpixels to represent a pixel. Suppose the secret image is a circle and m is not a square value, i.e., the aspect ratio is changed. After performing the VSS technique, the circle will be changed to an ellipse and consequently lead to the loss of information. To avoid distorting the image, the dummy subpixels are added to keep the aspect ratio unchanged. In this paper, we propose a novel method to dramatically reduce the number of extra subpixels to construct the aspect ratio invariant VSS schemes.