jpeg2im.m

简易的JPEG压缩程序(Matlab实现)

codefunction 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.

% Copyright 2002-2004 R. C. Gonzalez, R. E. Woods, & S. L. Eddins
% Digital Image Processing Using MATLAB, Prentice-Hall, 2004
% $Revision: 1.4 $ $Date: 2003/10/26 18:39:08 $

error(nargchk(1, 1, nargin)); % Check input arguments

m = [16 11 10 16 24 40 51 61 % JPEG normalizing array
12 12 14 19 26 58 60 55 % and zig-zag reordering
14 13 16 24 40 57 69 56 % pattern.
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 x 8 DCT matrix
x = blkproc(x, [8 8], 'P1 * x * P2', t', t); % Compute block DCT-1
x = uint8(x + 128); % Level shift