reordering a matrix

hai every body i am doing a project for image hiding

where i am dividing the images both host and secret into blocks and sort the blocks based on the average standard deviation value of the blocks

for sorting the blocks i have used the following syntax

[value,index]=sort(avgtar) %avgtar- average standard deviation of all the blocks in the host image

     22.9373   30.8103   37.9581   37.4260
     25.2341   34.7779   40.5795   52.2461
     32.9759   49.8769   42.8880   56.2237
     50.3640   53.9688   52.8147   59.7235

index

    2   1  1  1
    1   2  2  3
    3   3  3  4
    4   4  4  2

[values,indexs]=sort(avgsec) %avgsec- average standard deviation of all the blocks in the secret image

values

   19.5623     22.4814    18.9131   15.8497
   25.1652     29.2180    26.4766   22.5056
   25.6204     32.9959    30.0659   23.5101
   42.7828     34.7110    32.5074   28.6728

indexs

     3  4  4  1
     4  3  3  4 
     1  2  2  3 
     2  1  1  2

now i have to map the blocks in host image to the blocks in secret image in one to one manner

and then reorder the mapping according to the indices of the secret image blocks

i don't know how to map and reorder please help me

Answer 1

[Laurens Bakker]

Hi Ramya,

First, you re-order the known image according to the sorted order column by column:

for i=1:size(avgtar,2)
    avgtar(:,i) = avgtar( index(:,i) , i );
end

now you need to reverse the ordering, but according to the sorted order of the secret image. Again do this column by column:

for i=1:size(avgsec,2)
    avgtar( indexs(:,i), i ) = avgtar(:,i);
end

The tricky bit is the un-doing of the sort of the secret image, which is done in the second for loop I described.

Of course you can easily merge the two for loops:

for i=1:size(avgtar,2)
    avgtar( indexs(:,i), i ) = avgtar( index(:,i) , i );
end

But I thought it would be easier to explain the process step by step.

Cheers,

Laurens