dct水印嵌入算法的困惑

xpchen

嵌入算法：
function [J, W,Index,DCTI] = CoxWmk111(I, alpha, N, W)

imagew=imread('dmg2.tif');%读取水印图像 dmg2.tif'为一个二值图像
imagew=double(imagew)./256;
s=size(imagew);
W1=round(reshape(imagew,1,s(1)*s(2)));


if (nargin == 1)
   N = s(1)*s(2);
%  alpha = 0.1;
   alpha = 0.01;
   % We modify the value of alpha to achieve better visual quality
   W = W1;
end
if (nargin == 2)
   N = s(1)*s(2);
   W = W1;
end
if (nargin == 3)
   W = W1;
end

I=imread(I);
subplot(2,2,1);
imshow(I);
title('载体图像');
sI = size(I);
if ((sI(1) * sI(2)) < N)
   error('Image too small or too many coefficients.');
end


if (isrgb(I))%载体图像转化为灰度图像
   I=rgb2gray(I);
end

% 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);
IconMarca = idct2(DCTI);
% We save the inverse DCT results in an auxiliar matrix


% We change the values above 255 to 255 in order to avoid posible
% distortions caused by the double representation of MatLab
J=IconMarca;
%J=abs(J);
J=uint8(J);
subplot(2,2,2);
imshow(J,[])
title('嵌入后图像');
imwrite(J,'watermark.jpg')


提取算法：
%CoxExtract.m
function X= CoxExtract111(I, J, alpha, N)


imagew=imread('dmg2.tif');%读取水印图像目的只是为了求维数，由提取出的一维水印变化到二维，
s=size(imagew);

if (nargin == 2)
    N = s(1)*s(2);

   alpha = 0.01;
end
if (nargin == 3)
   N = s(1)*s(2);
end


I=imread(I);
J=imread(J);
subplot(1,2,1);imshow(I);title('原始图像');

if (isrgb(I))
  I=rgb2gray(I);
end
sI = size(I);
if ((sI(1) * sI(2)) < N)
   error('Image too small or too many coefficients');
end
sJ = size(J);
if ((sI(1) ~= sJ(1)) || (sI(2) ~= sJ(2)))
   error('The images have different size.');
end

% Compute the DCT of the images

DCTI = dct2(I);
DCTJ = dct2(double(J));

subplot(1,2,2);imshow(J);title('水印图像');

% Find the N largest coefficients in the DCT matrix
Index = FindNLargest(abs(DCTI), N);
% Extract the watermark
for i = 1:N
   X(i) = (DCTJ(Index(1, i), Index(2, i)) / DCTI(Index(1, i), Index(2, i)) - 1) / alpha;
end

message=reshape(X,s(1),s(2)).*256;
figure;
imshow(message);
title('Recovered Message')

为什么提取不出水印？请教高手我算法的问题出在哪里，非常感谢。

xpchen

%FindNLargest.m
function INDEX = FindNLargest(X, N)
%FINDNLARGEST Find largest values in a matrix or a vector
%   INDEX = FINDNLARGEST(X, N) find the N largest values of X
%   and returns their indices in X.

%   This is a modified version of Fabien Petitcolas' implementation
%   of Cox's technique by Gabriela Delfino and Fabian Martinez
%
%   Fabien Petitcolas' website: http://www.cl.cam.ac.uk/~fapp2

% Put all the elements of the matrix in a vector
n=size(X);
% We first fix the size of the matrix to make
% the process faster
tmp=zeros(1,n(1)*n(2));
for i=1:n(1)
   for j=1:n(2)
      tmp(j+(i-1)*n(2)) = X(i,j);
   end
end

% The DC coefficient must not be considered into the N largest because % it must not be changed, so we set its value to 0.
tmp(1,1)=0;
% Sort the element of the vector
[tmp, index] = sort(tmp);
l = length(index);

% Take the N largest
for k=1:N
   j = mod(index(l+1-k),n(2));
   if j==0
      j = n(2);
   end
   i = floor((index(l+1-k) - 1) / n(2)) + 1;
   INDEX(1,k) = i;
   INDEX(2,k) = j;
end