基于DCT的数字水印matlab源码

function [J, W] = CoxWmk(I, alpha, N, W) %COXWMK Insert a random watermark. % [J, W] = COXWMK(I) watermarks the grayscale picture I using % Cox's scheme [2] with default parameters. J is the watermarked % picture and W is the watermark. % The watermark W is drawn from a Normal random process N(0,1) % if it is not given by the user as last parameter. % % [J, W] = COXWMK(I, Alpha, N) watermarks the grayscale picture I % using modifying the Nth largest DCT coefficients v_i in the % following way: v_i := v_i * (1 + Alpha * w_i) where w_i % are the components of the watermak. % % Use IMSHOW(mat2gray(J)) to display the output. % % See: COXDETECT % Fabien A.P. Petitcolas 26-11-97 % Copyright (c) 1997 by the University of Cambridge % $Revision: 0.6$ % References: % 1) Ross J. Anderson, editor. Information hiding: first % international workshop, volume 1174 of Lecture notes % in computer science. University of Cambridge, Isaac % Newton Institute, Springer Verlag, Berlin, Germany, % May 1996. % 2) Ingemar J. Cox, Joe Kilian, Tom Leighton, and Talal % Shamoon. A secure, robust watermark for multimedia. % In Anderson [1], pages 183-206 if (nargin == 1) N = 1000; alpha = 0.1; W = randn(1,N); end if (nargin == 2) N = 1000; W = randn(1,N); end if (nargin == 3) W = randn(1,N); end sI = size(I); if ((sI(1) * sI(2)) < N) error('Image too small or too many coefficients.'); end % Generate the random watermark %W = randn(1,N); % Compute the DCT of the image DCTI = dct2(I); % Find the N largest coefficients in the DCT matrix % Better if the extraction of the N largest was done % at the same time than the computation of the DCT... Index = FindNLargest(abs(DCTI), N); % Modify these coefficients for i = 1:N DCTI(Index(1,i),Index(2,i)) = DCTI(Index(1,i),Index(2,i)) * (1 + alpha * W(i)); end % Simply take the inverse DCT of the modifyied matrix J = idct2(DCTI);