function y = im2jpeg(x, quality) 
error(nargchk(1, 2, nargin));             % 檢查輸入參數
if ndims(x) ~= 2 | ~isreal(x) | ~isnumeric(x) | ~isa(x, 'uint8')
   error('The input must be a UINT8 image.');  
end
if nargin ==2 && quality <= 0
   error('Input parameter QUALITY must be greater than zero.');
end
if nargin < 2    
   quality = 1;   
end       

m = [16 11  10  16  24  40  51  61        % JPEG 歸一化矩陣
     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] * quality;

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];

[xm, xn] = size(x);                % 輸入大小
x = double(x) - 128;              
t = dctmtx(8);                     % 計算8 x 8 DCT 矩陣

% 計算 DCT的 8x8 塊和相關系數
y = blkproc(x, [8 8], 'P1 * x * P2', t, t');
y = blkproc(y, [8 8], 'round(x ./ P1)', m);

y = im2col(y, [8 8], 'distinct');  % 把8x8變成列 
xb = size(y, 2);                   % 計算塊的數目
y = y(order, :);                   % 排序

eob = max(y(:)) + 1;               
r = zeros(numel(y) + size(y, 2), 1);
count = 0;
for j = 1:xb                       %  一次處理一塊
   i = max(find(y(:, j)));         % 找到上次的非零元素
   if isempty(i)                   
      i = 0;
   end
   p = count + 1;
   q = p + i;
   r(p:q) = [y(1:i, j); eob];      
   count = count + i + 1;         
end

r((count + 1):end) = [];          
   
y           = struct;
y.size      = uint16([xm xn]);
y.numblocks = uint16(xb);
y.quality   = uint16(quality * 100);
y.huffman   = mat2huff(r);