function dct_det(attacked)
Q_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
        ];

ZT=attacked;
[m n]=size(ZT);
chuangai=zeros(m,n);
quality=50;
%塊dct變換
for i=1:(m/8);
    for j=1:(n/8);
        width=(i-1)*8+1;
        height=(j-1)*8+1;
        imdct(width:(width+7),height:(height+7))=dct2(ZT(width:(width+7),height:(height+7)));
    end
end
%開辟要比較的數據空間
Q=zeros(1,m*n/8/8*6);
watermark=zeros(1,m*n/8/8*6);
%比較zig-zag前6個對應系數
k=1;
for i=1:(m/8/2);
    for j=1:(n/8);
        width=(i-1)*16+1;
        height=(j-1)*8+1;
        l=(k-1)*6+1;
        if imdct(width,height)-imdct(width+8,height)<0
            Q(l)=0;
        else
            Q(l)=1;
        end
        if imdct(width+1,height)-imdct(width+9,height)<0
            Q(l+1)=0;
        else
            Q(l+1)=1;
        end
        if imdct(width,height+1)-imdct(width+8,height+1)<0
            Q(l+2)=0;
        else
            Q(l+2)=1;
        end
        if imdct(width,height+2)-imdct(width+8,height+2)<0
            Q(l+3)=0;
        else
            Q(l+3)=1;
        end
        if imdct(width+1,height+1)-imdct(width+9,height+1)<0
            Q(l+4)=0;
        else
            Q(l+4)=1;
        end
        if imdct(width+2,height)-imdct(width+10,height)<0
            Q(l+5)=0;
        else
            Q(l+5)=1;   
        end
         k=k+1;
    end
end
% imwrite(uint8(J2),'lena256.jpg','quality',quality);
% yy=imread('lena256.jpg');

em_cof=[1 4;2 3;3 2;4 1;4 2;3 3];
NN=size(em_cof,1);
    
k=1;
for i=1:(m/8/2)
    for j=1:(n/8)
        width=(i-1)*16;
        height=(j-1)*8;
        l=(k-1)*6+1;
     
         for kk=1:3  
         qstep=Q_table(em_cof(kk,1),em_cof(kk,2));
         orcof=imdct(width+em_cof(kk,1),height+em_cof(kk,2));
                  
       watermark(l)=mod(round(imdct(width+em_cof(kk,1),height+em_cof(kk,2))/qstep),2);
       if watermarked(l)~=Q(l)
                       
       end
     
         l=l+1;
         end
         
        width=(i-1)*16;
        height=(j-1)*8+8;
         for kk=4:NN  
         qstep=Q_table(em_cof(kk,1),em_cof(kk,2));
         orcof=imdct(width+em_cof(kk,1),height+em_cof(kk,2));
         imdct(width+em_cof(kk,1),height+em_cof(kk,2))=round(orcof/qstep)*qstep;
            if mod(round(imdct(width+em_cof(kk,1),height+em_cof(kk,2))/qstep),2)~=Q(l)
            err=err+1;
            if orcof>imdct(width+em_cof(kk,1),height+em_cof(kk,2))         
               imdct(width+em_cof(kk,1),height+em_cof(kk,2))=imdct(width+em_cof(kk,1),height+em_cof(kk,2))-qstep;
            else
              imdct(width+em_cof(kk,1),height+em_cof(kk,2))=imdct(width+em_cof(kk,1),height+em_cof(kk,2))+qstep;  
            end       
         end
         l=l+1;
         end
         
         k=k+1;
   end
end