dequant.m

jpeg的matlab代码

% NAME 
%      	dequant - unapply quantizing matrix
%
% SYNOPSIS
%	dequant
%	dequant(X)
%	dequant(X, Q)
%
% DESCRIPTION
%	dequant cuts an mxn matrix X into NxN matrices and dequantizes
%	using the matrix Q. If X is not given, then X=ans. If Q is not 
%	given, then Q=QMAT.

% Copyright (C) 1995-1997 Darrel Hankerson and Greg A. Harris
%
% This file is part of the JPEGtool collection of scripts for Octave
% and Matlab; see http://www.dms.auburn.edu/compression
%
% The JPEGtool collection is free software; you can redistribute it
% and/or modify it under the terms of the GNU General Public License
% as published by the Free Software Foundation; either version 2, or
% (at your option) any later version.
%
% The collection is distributed in the hope that it will be useful,
% but WITHOUT ANY WARRANTY; without even the implied warranty of
% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
% GNU General Public License for more details.
%
% You should have received a copy of the GNU General Public License
% along with this package; see the file COPYING.  If not, write to the
% Free Software Foundation, 59 Temple Place - Suite 330, Boston, MA
% 02111-1307, USA.

function X = dequant(X, Q)

global QMAT

if (~exist('X'))
  X = ans;
end

if (~exist('Q'))
  Q = QMAT;
end

N = size(Q, 1);

dim = fix(size(X)/N); m=dim(1); n=dim(2);
X = X(1:N*m,1:N*n);  	%  snip trailing rows and columns from X

%  Now break up X and un-quantize
for ii=1:m	
  rows = N*(ii-1)+1:N*ii;
  for jj=1:n
    cols = N*(jj-1)+1:N*jj;
    X(rows, cols) = X(rows, cols) .* Q ;
  end
end