- 學位論文
灰階視覺密碼數位影像之像素擴展問題與對比強化之研究
A Pixel Non-Expansion Visual Cryptography Scheme with Contrast Enhancement under Gray-level Images
摘要
在本論文中，我們提出一個方法以改善新的視覺密碼的建構演算法中，灰階視覺密碼數位影像之像素擴展與對比損失的問題。提出的方法中，我們結合了一個同時實現像素不擴展與對比損失補償的方法來改善該演算法的缺點。該方法中運用了兩個概念，分別是最佳化的統計觀念和區塊對區塊的編碼方式，有別以往傳統造成像素擴展結果的點對序列或是點對區塊的概念。藉由這些基本觀念達到改善的目的，這也正是我們研究的目標。 然後我們將透過該方法達到像素不擴展目的，以解決影像失真、解密檔案過大與不便攜帶的問題。另外則是以最佳化線性規劃求得達到理想對比補償的編碼機率分配，去提升解密後的重疊影像的對比品質，目的在於避免機密訊息解讀錯誤或辨識不清的問題。此外，我們所提出的方法仍繼續保有傳統視覺密碼的優良特性。 最後將由實驗結果可知，我們提出的方法確實可以讓新的視覺密碼的建構演算法架構與提出的方法做結合以達到我們改善的目的。 在本論文中，我們主要的貢獻如下： (1)分享影像大小保持和機密影像相同，除減輕網路檔案傳輸負擔外，亦節省儲存空間。 (2)解密所得到的疊合影像，具有良好的對比效果，降低機密訊息辨識不清或是誤判的機會。 (3)仍保有視覺密碼的安全性、不用複雜數學運算的特性。 (4)仍保有擴展視覺密碼架構的優良特性，即疊合影像不具有一般經有序抖動法處理後的規律的網格在影像上。 我們相信本論文對擴展視覺密碼架構做更進一步改良的研究成果，對未來視覺密碼學的相關的研究架構建立與應用上，會有相當的助益。
並列摘要
In this thesis, we propose a pixel non-expansion visual cryptography scheme with contrast enhancement under gray-level images. The scheme utilizes a method to achieve our research goal. The method uses the concept of probability and considers both security and contrast to construct a optimization model. To solve the optimization problems, we employ a linear programming model. In addition, the method integrates a block encryption procedure for gray-level images to implement pixel non-expansion. We utilize the method to improve the shortcomings of a new construction algorithm for visual cryptography. With block encryption procedure, we solves the problems such as the problem of image distortion, the requirement of more storage space, and the difficulty in carrying shares by using pixel expansion scheme. With the thresholding technique for the improvement of contrast loss, we could produce a good contrast in stacked image so that we could indentifies secret information clear. At the same time, our proposed scheme still preserves all advantages of visual cryptography. Experimental results will show that, the proposed scheme has a better improvement for both problems of pixel expansion and contrast loss in stacked image, respectively. The contributions of our research are as follows: (1) The size of the stacked image is the same as that of the secret image. Besides, we improve the heavy loading for the limited network bandwidth and the storage space. (2) The stacked image through decrypting, has a good contrast effect. Thus, it prevents us from making a misjudgment or identifying unclear for secret information. (3) The proposed scheme inherits the advantages of visual cryptography, such as recover the secret information without any complex decryption algorithms and its natural security condition. (4) The proposed scheme reserves part good properties of extended visual cryptography model, like that, removes artifacts from stacked image such as regular patterns of dots, which usually occur in ordered dither. We believe this study results will be probably practical and efficient to establish a useful visual cryptography scheme with above advantages in application.