Block-Based Visual Cryptography For Color Image Without Pixel Expansion

In recent years,with the development of the new generation of information technology,visual cryptography has been paid more and more attention,and many research results have been achieved,but there are also some problems.For the traditional visual cryptography,many algorithms are for binary image or gray-scale image research,the research on color image is less.But in today's information age,color image and even 3D image have entered people's life.The original visual cryptography technology is only designed for binary image.When applied to gray image and color image,the quality of reconstructed image is relatively poor.In addition,it also brings the problem of pixel expansion,which not only increases the size of the sub storage image,but also increases the processing time and storage space of the computer.Therefore,in order to solve these problems,we need to design a color image visual cryptography scheme without pixel expansion.This thesis mainly designs a block-based visual cryptography scheme for color images.In order to keep the size of the restored secret image unchanged,we choose the pixel block as the unit of visual encryption.In the color image,we divide the image into three channels:R,G and B,we can get the halftone image of three channels by halftone processing.Then the halftone image is divided into 2 by 2 pixel-block,take the average color of each pixel-block,and then convert it to HSV color space.According to the difference of H value,we do color mapping,and then encrypted according to the encryption rules.Different colors are respectively expanded into a 2 by 2 pixel-block on two share images,and filled with red,green,blue and white respectively.Then we can generate a variety of colors with different permutations and combinations for encryption.Finally,by superimposing the two share images,we can recover the secret image.In order to verify the feasibility of the algorithm,the algorithm proposed in this paper is implemented and tested by MATLAB.The algorithm is evaluated from four aspects:feasibility,security,visual quality and computational complexity,and compared with other existing algorithms.The experimental results show that our algorithm can solve the problem of pixel expansion and improve the quality of the restored image,and the visual encryption algorithm is applied to the color image,which makes the application scope wider.