imshow() black image

%% Extraction Step
Ex_watermark=zeros(8);
Watermarked_B=Watermarked_image(:,:,3);
c=0;
for i=1:64:64*64-63
    c=c+1;
    block_index=key(i:i+63);
    blockB=double(reshape(B(block_index),[8,8]));
    % discrete cosine transform:
    dct_blockB=dct2(blockB);
    % Singular Value Decomposition:
    [Ub,Sb,Vb]=svd(dct_blockB(dct_idx));
    Sbw=reference{i};
    sigmaB=Sb(1); % Biggest Singular Value - Host
    sigmaBW=Sbw(1); % Biggest Singular Value - Watermarked
    % Extracting watermark image:
    if sigmaBW>sigmaB
        Ex_watermark(c)=1;
    end
end
subplot(1,4,4),
imshow(Ex_watermark)
xlabel('Extracted Watermark');

[KSSV]

Try

imshow(uint8(Ex_watermark))

[Mostfa Abd El-Aziz]

Dear,KSSV I have already used this suggestion but the problem still not solved

[KALYAN ACHARJYA]

Try

imshow(Ex_watermark,[]);

If it doesnot work, then share the complete code (If possible) with test images.

[KSSV]

Check the class of dct_blockb and use the same class for Ex_watermark....if nor working share the code and data.

[Mostfa Abd El-Aziz]

Dear, KSSV what you mean by class of dct_blockb ??

[KALYAN ACHARJYA]

See the data types of dct_blockb and Ex_watermark, same??

[Image Analyst]

Mostfa, did you overlook where he said "if not working share the code and data." Not doing that is delaying a solution to your problem. If using [] in imshow() did not solve it, then your image is all zero.

[Mostfa Abd El-Aziz]

clc
close all
clear
%% Input images
I=imread('gl1.jpg');
I=imresize(I,[512,512]);
logo=imread('watermark.jpg');
logo=imresize(logo,[64,64]);
figure
subplot(1,4,1)
imshow(I)
title('Host Image')
subplot(1,4,2)
B=I(:,:,3); % Blue Channel
imshow(logo)
xlabel('Logo (Watermark Image)')
gf=100; % Watermark Strength 
n_dct=60; % <=64
zigzag=[1 9 2 3 10 17 25 18 11 4 5 12 19 26 33 41 34 27 20 13 6 7 14 21 28 35 42 49 57 ...
    50 43 36 29 22 15 8 16 23 30 37 44 51 58 59 52 45 38 31 24 32 39 46 53 60 61 54 47 40 48 55 62 63 56 64];
dct_idx=zigzag(1:n_dct);
%% key generation:
key=randperm(512*512,64*64);
%% Block Selection (Embedding Step):
c=0;
Watermarked_B=B;
for i=1:64:64*64-63
    c=c+1;
    block_index=key(i:i+63);
    blockB=double(reshape(B(block_index),[8,8]));
    % discrete cosine transform:
    dct_blockB=dct2(blockB);
    % Singular Value Decomposition:
    [U,S,V]=svd(dct_blockB(dct_idx));
    sigmaB=S(1); % Biggest Singular Value
    % embedding watermark image:
    if logo(c)==1
        S(1)=sigmaB+gf;
    else
        S(1)=sigmaB-gf;
    end
    reference{i}=S;
    coeffs=U*S*V; % inverse svd
    rec_dct_block=zeros(8);
    rec_dct_block(dct_idx)=coeffs;
    rec_block=idct2(rec_dct_block); % inverse dct
    row_block=reshape(rec_block,1,64);
    Watermarked_B(block_index)=row_block;
end
Watermarked_image=I;
Watermarked_image(:,:,3)=Watermarked_B;
subplot(1,4,3),
imshow(Watermarked_image);
title('Watermarked Image')
%% Extraction Step
Ex_watermark=zeros(8);
Watermarked_B=Watermarked_image(:,:,3);
c=0;
for i=1:64:64*64-63
    c=c+1;
    block_index=key(i:i+63);
    blockB=double(reshape(B(block_index),[8,8]));
    % discrete cosine transform:
    dct_blockB=dct2(blockB);
    % Singular Value Decomposition:
    [Ub,Sb,Vb]=svd(dct_blockB(dct_idx));
    Sbw=reference{i};
    sigmaB=Sb(1); % Biggest Singular Value - Host
    sigmaBW=Sbw(1); % Biggest Singular Value - Watermarked
    % Extracting watermark image:
    if sigmaBW>sigmaB
        Ex_watermark(c)=1;
    end
end
subplot(1,4,4),
imshow(double(Ex_watermark))
xlabel('Extracted Watermark');