jpeg2im.m

function x = jpeg2im(y)
%JPEG2IM Decodes an im2jpeg compressed image.
% x = jpeg2im(y) decodes compressed image y , generating reconstructed
% approximation x.y is a structure generated by im2jpeg
% 
% See also im2jpeg.
narginchk(1,1);              %Check input arguments

m = [16 11 10 16 24 40 51 61       %JPEG normalizing array and zig-zag 
    12 12 14 19 26 58 60 55        %redordering pattern
    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];

order = [1 9 2 3 10 17 25 18 11 4 5 12 19 26 33  ...
    41 34 27 20 13 6 7 14 21 28 35 42 49 57 50   ...
    43 36 29 22 15 8 16 23 30 37 44 51 58 59 52  ...
    45 38 31 24 32 39 46 53 60 61 54 47 40 48 55 ...
    62 63 56 64];
rev = order;                        %Compute inverse ordering
for k = 1:length(order)
    rev(k) = find(order == k);
end

m = double(y.quality)/100*m;         %Get encoding quality
xb = double(y.numblocks);            %Get x blocks.
sz = double(y.size);
xn = sz(2);                          %Get x columns
xm = sz(1);                          %Get x rows.
x = huff2mat(y.huffman);             %Huffman decode.
eob = max(x(:));                     %Get end-of-block symbol

z = zeros(64,xb);  k = 1;            %Form block columns by copying
for j = 1:xb                         %successive values from x into 
    for i = 1:64                     %columns of z ,while changing
        if x(k) == eob               %to the next column whenever
            k = k+1; break;          %an eob symbol is found.
        else
            z(i,j) = x(k);
            k = k +1;
        end
    end
end

z = z(rev,:);                        %Restore order
x = col2im(z,[8 8],[xm xn],'distinct'); %Form matrix blocks
x = blkproc(x,[8 8],'x.*P1',m);         %Denormalize DCT
t = dctmtx(8);                          %Get 8 * 8 DCT matrix
x = blkproc(x,[8 8],'P1 * x * P2',t',t); %Compute block DCT-1
x = x +double(2^(y.bits -1));            %Level shift
if y.bits <= 8
    x = uint8(x);
else
    x = uint16(x);
end