image stretching and squeezing code not working for big matrices

This is an image scrambling code that stretches the x and y coordinate by a factor of 2 and squeezes the z coordinate by 4. But this doesn't seem to be doing the job. Initial image image3d(x,y,z) pixel will map to NEWI(i,j,k) uniquely. But this code is not giving unique results for matrix greater than 8 x 8.

QUARTILE=n/4    %n=64 
   MID=n/2
   UQUARTILE=3*(n/4)
  % NEWI=zeros(n,n,n);
   
   for i= 1:n
        for j= 1:n
            for k=1:n
                
                if        ( QUARTILE<k ) && (k<=MID ) ||    (  UQUARTILE<k)       
                    x= mod(2*(i-1),n)+1+1;
                else 
                    x=mod(2*(i-1),n)+1;
                end
                    if k>MID
                    y= mod(2*(j-1),n)+1+1;
                else
                    y=mod(2*(j-1),n)+1;
                    end
            
               if i<=MID && j<=MID              
                     z=ceil(k/4);
               elseif i>MID && j<=MID
                    z=ceil(k/4)+QUARTILE;
               elseif i<=MID && j>MID
                    z=ceil(k/4)+MID;
               elseif i>MID && j>MID
                    z=ceil(k/4)+UQUARTILE;
                end
               NEWI(i,j,k)=image3d(x,y,z);
            end
        end
   end