how to do watermarking using dct.

Respected sir,

I have made the following code for watermarking in dct domain.

org_img= imread('lena.jpg');
img4x4 = mat2cell( org_img, size(org_img,1)/2 * ones(1,2), size(org_img,2)/2 * ones(1,2), size(org_img,3) );

% upper left

red_1= img4x4{1,1}(:,:,1);
green_1= img4x4{1,1}(:,:,2);
blue_1= img4x4{1,1}(:,:,3);

%dct to upper left

img1_r= dct2(red_1);
img1_g= dct2(green_1);
img1_b= dct2(blue_1);

%watermark one

w1= imread('cousins.jpg');
wtr_img1= imresize(w1, 1/2);

red1= wtr_img1(:,:,1);
green1= wtr_img1(:,:,2);
blue1= wtr_img1(:,:,3);

%dct to w1

wtr1_r= dct2(red1);
wtr1_g= dct2(green1);
wtr1_b= dct2(blue1);

%embedding of watermark

fimgr1= img1_r+ 0.01*wtr1_r;
fimgg1= img1_g+ 0.01*wtr1_g;
fimgb1= img1_b+ 0.01*wtr1_b;

%combining first block

wimg1= cat(3, fimgr1, fimgg1, fimgb1);

%first output block

rgb1= idct2(wimg1);
imwrite(uint8(rgb1),'DCTWatermarked1.jpg');
figure;imshow(uint8(rgb1));title('Watermarked Image 1');

It shows the error:

Error using  .* 
Matrix dimensions must agree.

Error in idct (line 74)
  y = ifft(W.*bb);

Error in idct2 (line 53)
    a = idct(idct(arg1).').';

kindly guide me how to correct it.

[Karan Chandra]

yashi can you send correct solution? please

Answer 1

[Walter Roberson]

dct2() and idct2() apply strictly to 2 dimensional matrices. Your wimg1 is 3 dimensional (RGB) so you cannot apply idct2 to it.

[yashi gautam]

thank you sir. so the solution would be to seperately apply idct to each and then combine them?

[Walter Roberson]

Yes.

[yashi gautam]

thanks for the help sir. :)

[Ragini Gaikwad]

Can you please help on this, how to apply idct and then combine