Getting this error - Index exceeds the number of array elements (0). quant(keym​at(Nm*(ii-​1)+jj))=bi​tset(quant​(keymat(Nm​*(ii-1)+jj​)),1,str2d​ouble(temp​cipher(ii,​jj))); for DCT steganography

I am getting the following error for DCT steganography.

quant(keymat(Nm*(ii-1)+jj))=bitset(quant(keymat(Nm*(ii-1)+jj)),1,str2double(tempcipher(ii,jj)));

Index exceeds the number of array elements (0)

This is my complete code -

% ideal DCT with no change
% embed in lower frequency 
% less huffman length 
% ideal quantisation table

clear all; 
close all;

% p and q are 2 prime numbers 
% phi is totient of the product of p and q
% e is any number coprime to phi
% d, the modular multiplicative inverse of e (mod φ(n))

p = input('\nEnter value of p ( It should be prime number eg 31): ');
q = input('\nEnter value of q ( It should be prime number eg 19): ');

Phi=(p-1)*(q-1);
n=p*q;
x=2;e=1;
while x > 1
    e=e+1;
    x=gcd(Phi,e);%Search greatest common division
end

i=1;
d=0;
while i > 0
    d = d+1;
    x = e*d;
    x = mod(x,20);
    if x == 1
       i = 0; 
    end
end




M = input('\nEnter message: ','s');
x=length(M);
c=0;

for j= 1:x
    for i=0:122
        if strcmp(M(j),char(i))
            c(j)=i;
        end
    end
end

disp('ASCII Code of the entered Message:');
disp(c); 

% For Encryption

cipher= power(c,e);
cipher= mod(cipher,n);

disp('cipher: ');
disp(cipher);

tempcipher=dec2bin(cipher);

% determine the size of message object to embed
Mm=size(tempcipher,1);	        %Height
Nm=size(tempcipher,2);	        %Width

% Compression

jk=8;
block_size=jk;
mask_size=jk;
    
I1 = imread('Eren.jpg');
imwrite(I1,'Levi.jpg');
I1=im2double(I1); 
I1=I1*255;    
no1=(floor(size(I1,1)/(block_size)))*block_size;
no2=(floor(size(I1,2)/block_size))*block_size;
I1=imresize(I1,[no1,no2]);

    
I=I1;
Red= I(:,:,1);
T = dctmtx(block_size);
dct = @(block_struct) T.* block_struct.data*T';
  
B1 = blockproc(I,[8,8],dct);
   
transformed=cat(1,B1);
    
%% table generation
quantization_table = [
 16  11  10  16  24   40   51   61;
 12  12  14  19  26   58   60   55;
 14  13  16  24  40   57   69   56;
 14  17  22  29  51   87   80   62;
 18  22  37  56  68   109  103  77;
 24  35  55  64  81   104  113  92;
 49  64  78  87  103  121  120  101;
 72  92  95  98  112  100  103  99
    ];

%quantization_table = ones(8,8);
%quantization_table(1,1) = 16;quantization_table(1,2) = 11;quantization_table(1,3) = 10;quantization_table(1,4) = 16;
%quantization_table(2,1) = 12;quantization_table(2,2) = 12;quantization_table(2,3) = 14;
%quantization_table(3,1) = 14;quantization_table(3,2) = 13;
%quantization_table(4,1) = 14;
    
mask=zeros(block_size,block_size);
for i=1:mask_size
    mask(i,1:(mask_size-i+1))=1;
end
mask;
    
%% quantization
quant=int16(zeros(size(I1,1),size(I1,1),1));
recon=double(zeros(size(I1,1),size(I1,1),1));
for k=1:1
    for i=1:block_size:size(I1,1)
        for j=1:block_size:size(I1,2)
            for ii=1:block_size
                for jj=1:block_size
                    aa=transformed(i+ii-1,j+jj-1,k);
                    quant(i+ii-1,j+jj-1,k)=(aa);
                    quant(i+ii-1,j+jj-1,k)=(aa/quantization_table(ii,jj));
                    
                end
            end
        end
    end
end

keymat=find(quant>10,Mm*Nm);

for ii = 1:Mm
    for jj = 1:Nm
       quant(keymat(Nm*(ii-1)+jj))=bitset(quant(keymat(Nm*(ii-1)+jj)),1,str2num(tempcipher(ii,jj)));
    end
end

% zigzag coding

outputas=[];
for i=1:block_size:size(I1,1)
    for j=1:block_size:size(I1,2)
        outputas=cat(2,outputas,zigzag(quant(i:i+7,j:j+7)));
    end
end

% hufman coding 
tb = tabulate(outputas);
pval = tb(:,3);
symbols=tb(:,1);
[dict,avglen] = huffmandict(symbols,(pval/100));
comp = huffmanenco(outputas,dict);

% hufman decoding
outputas = huffmandeco(comp,dict);


% inverse zigzag
tempout=[size(I1,1),size(I1,2)];
tempi=1;
tempj=1;
for i=1:64:size(outputas,2)
    tempout(tempi:tempi+7,tempj:tempj+7)=izigzag(outputas(i:i+63),8,8);
    tempj=tempj+8;
    if tempj > size(I1,2)
        tempi =tempi+8;
        tempj =1;
    end
    
end

quant=tempout;

%% dequantisation
for k=1:1
    for i=1:block_size:size(I1,1)
        for j=1:block_size:size(I1,2)
            for ii=1:block_size
                for jj=1:block_size
                    
                    recon(i+ii-1,j+jj-1,k)=(quant(i+ii-1,j+jj-1,k)*quantization_table(ii,jj));
                end
            end
        end
    end
end
    

%% reconstruction    
B1=(recon(:,:,1));
invdct = @(block_struct) T' * block_struct.data * T;
RE1 = blockproc(B1,[block_size block_size],invdct);

imwrite(RE1,'341.jpg');

%% decipher
re1 = blockproc(RE1,[8,8],dct);
quant1=int16(zeros(size(I1,1),size(I1,1),1));
for k=1:1
    for i=1:block_size:size(I1,1)
        for j=1:block_size:size(I1,2)
            for ii=1:block_size
                for jj=1:block_size
                    aa=re1(i+ii-1,j+jj-1,k);
                    quant1(i+ii-1,j+jj-1,k)=(aa);
                    quant1(i+ii-1,j+jj-1,k)=(aa)/quantization_table(ii,jj);
                end
            end
        end
    end
end

for ii = 1:Mm
    for jj = 1:Nm
        temp1cipher(ii,jj)=bitget(quant1(keymat(Nm*(ii-1)+jj)),1);
    end
end

I2=cat(1,RE1);
    
%% error
I1 = I1/255;
I2 = I2/255;
result1=0;
for i=1:size(I1,1)
    for j=1:size(I1,2)
        for k=size(I1,3)
            diff1=(I1(i,j,k)-I2(i,j,k));
            result1=result1+diff1*diff1;                
        end
     end
end
  
sizee=size(I2,1);    
MSE1 = result1/sizee;
PSNR1 = 10*log10(255*255/MSE1);    
PSNR1

temp1cipher=num2str(temp1cipher);
cipher = bin2dec(temp1cipher);
for j= 1:x
   message(j)= crypto(cipher(j),Pk,d); 
end

disp('Decrypted ASCII of Message:');
disp(message);
disp(['Decrypted Message is: ' message]);
    

Answer 1

[Divija Aleti]

Hi Talha,

As I do not have the images you have used, I am unable to run your code. But I can tell you the meaning of the error.

Let's say you have an array 'x' which has 3 elements. So while indexing, x(1), x(2), and x(3) are all valid because they correspond to the first, second and third elements of the array 'x' respectively. However, if your index is greater than 3, for example x(6), it gives the error - "Index exceeds the number of array elements" - because 'x' has only 3 elements and there is no 6th element for it to call.

In your case, error has occured in the line

quant(keymat(Nm*(ii-1)+jj))=bitset(quant(keymat(Nm*(ii-1)+jj)),1,str2double(tempcipher(ii,jj)));

Now here, you have many arrays like 'quant', 'keymat' and 'tempcipher'. I suggest you to check the number of elements in each of these arrays. Then check all your indexing operations and try to find which one is trying to access an element which does not exist. Since you are looping over the index variables, do check for all the values that the variables might take.

Hope this helps!

Regards,

Divija