Brassil J T, Low S, Maxemchuk N F. Copyright protection for the electronic distribution of text document[J]. Proceedings of the IEEE, 1999,87(7) : 1181-1196.
[5]
Ganzalez R C, Wintz P. Digital Image Processing. (2nd Edition) [M]. New York: Addison-Wesley Publishing Company, 1987, 34-38.