Please check my code of image steganography using LSB technique on egdes. It is giving an error which i am unable to find.

a=imread('Lenna.png');

b=rgb2gray(a);

edge_b=edge(b,'sobel');

imshow(edge_b);

   message = 'hello lenna'

      message = strtrim(message);

      m = length(message) * 8;

      AsciiCode = uint8(message); 

      binaryString = transpose(dec2bin(AsciiCode,8));

      bst = binaryString(:);

      N = length(bst);

      z = zeros(N,1); 

      for k = 1:N

          if(bst(k) == '1')

              z(k) = 1;

          else

              z(k) = 0;

          end

      end

height = size(b,1);

      width = size(b,2);

k1=m;

while k1>0,

for i=1:height-1

for j=1:width-1

if edge_b(i,j)==1

    lsb=mod(b(i,j),2);

    if (lsb == z(k1))

        b(i,j) = b(i,j);

    else

         if(lsb == 1)

            b(i,j) = b(i,j) - 1;

         else

            b(i,j) = b(i,j) + 1;

         end

    end

    k1 = k1 - 1;

end

end

end

end

error: ??? Attempted to access z(0); index must be a positive integer or logical.

[HANIYAH FATIMA]

Did you try the decoding as well? Please provide code for it

Answer 1

[Arun Kumar]

there is no need for while loop.

for i=1:height-1

for j=1:width-1

if k1>0

if edge_b(i,j)==1

lsb=mod(b(i,j),2);

if (lsb == z(k1))

      b(i,j) = b(i,j);

else

       if(lsb == 1)

          b(i,j) = b(i,j) - 1;

       else

          b(i,j) = b(i,j) + 1;

       end

end

k1 = k1 - 1;

end

end

end

end

[HANIYAH FATIMA]

Please provide decoding also

Answer 2

[Ash Ch]

You are GOD. Thank you so much. The code finally ran after decades. But can you please tell me why 'while' didn't work?

[eng]

plz. where is the decoding code ? can you continue it from your kind ?

[Walter Roberson]

You initialize k1 as m, length of the message times 8. You decrement k1 each time you encounter an edge pixel and you access z(k1). If the number of edge pixels exceeds m then you decrement k1 right down to 0 in the for loops.

In the original code if the number of edge pixels is less than the message length then your k1 will be positive after the nested for loops and the while would trigger another pass of placing bits where they were already placed. This could happen several times. Eventually you hit a pass where there are fewer bits remaining to be placed than there are edge bits.

The original cide only works properly in the case where the number of edge bits happens to be exactly the same as the number of message bits.