Error using ==> Plus Matrix Dimensions must agree

Asked by shahosj on 25 Jul 2012

Latest activity Closed by Walter Roberson on 22 Aug 2012

This question is closed and may reopen in the future if edited.

Here's the source code:

    % Add a watermark to process
  Q=input(' Enter to put the value of the shrinkage factor (recommended less than 1)：Q＝');
  subplot(3,3,1)
  RGB=imread('g11','jpg');
  imshow(RGB);
  title('The original image');
  subplot(3,3,2)
  imshow(RGB(:,:,3));
  title('B component');
  subplot(3,3,3)
  N=dct2(RGB(:,:,3));
  imshow(log(abs(N)),[]),colorbar;
  title('B component of the energy distribution');
  subplot(3,3,4)
  I=imread('g12','jpg');
  imshow(I);
  title(' Grayscale watermark image ');
  subplot(3,3,5)
  M=dct2(I);
  imshow(log(abs(M)),[]),colorbar;
  title('Watermark energy distribution');
  subplot(3,3,7)
  J=M(1:128,1:128);
  J(128:464,128:364)=0;
  J=rot90(J);
  J=rot90(J);
  J(465:800,365:600)=0;
  J=rot90(J);
  J=rot90(J);
  N=N+Q*J;
  K=idct2(N);
  RGB(:,:,3)=K;
  imshow(RGB);
  title(' Add to watermark images ');
  subplot(3,3,8)
  I=imcrop(RGB,[1 1 598 798]);
  imshow(I);
  subplot(3,3,9)
  % Watermark extraction process
  subplot(3,3,6)
  RGB1=imread('g11','jpg');
  J=RGB1(:,:,3);
  X=J(1:799,1:599);
  N=dct2(I(:,:,3));
  M=dct2(X);
  M=(N-M)/Q;
  B=idct2(M(337:464,237:364));
  Y=mat2gray(B);
  imshow(Y);
  title('Zoom extracted watermark image')

After i run it, i got

??? Error using ==> plus
Matrix dimensions must agree.

Error in ==> ecp3 at 30
N=N+Q*J;

Can someone help me? Ive been doing this all night and seem cant find the solution, i appreciate any help. Thank u.

1 Comment

Walter Roberson on 29 Jul 2012

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2 Answers

Answer by Walter Roberson on 25 Jul 2012

N is constructed from the RGB image you read in, but J is built to a fixed size (a subset of the image that is then extended by assigning 0 beyond those boundaries). There is no reason to expect that N and J will be the same size.

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shahosj on 26 Jul 2012

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Is there any way to make J to be as same size as N? Let say im using image from this http://www.petitcolas.net/fabien/watermarking/image_database/. Maybe u can help elaborate more on this. Thank u. If i have to make extra from J=M(1:128,1:128); to J=M(1:128,1:128,1:128); it will say it exceeds the matrix dimension.

Walter Roberson on 29 Jul 2012

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if size(M,3) < 128; M(1,1,128) = 0; end

and then you can access M(1:128,1:128,1:128) for whatever good that will do you. Usually the third dimension of an image is length 3, M(:,:,1) being the Red pane, M(:,:,2) being the Green pane, and M(:,:,3) being the Blue pane.

But that does not really apply here, as your M is not an image: your M is dct2() applied to a jpeg image. If the original image "g12" does not happen to be a grayscale image, it is not clear to me that the dct2() of it will be defined, as the dct2() documentation strongly implies a 2 dimensional output.

There must be any number of ways to make your J the same size as your M, and I could suggest several of them; the problem is to make the resulting J meaningful. For example, if you were to imresize() J to be the same size as M, would that be even slightly useful or would it be just GIGO

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Answer by Walter Roberson on 10 Aug 2012

Please read the guide to Tags and retag this Question: http://www.mathworks.co.uk/matlabcentral/answers/43073-a-guide-to-tags

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