trailnum.m

jpeg的matlab代码

% trailnum - count trailing zeros in zigzag sequence
%
% SYNOPSIS
%	trailnum(X)
%
% DESCRIPTION
% 	JPEG-like compression leads to matrices which usually contain
%	zeros in the high-frequency entries. Counting the number of
%	trailing zeros (in the zigzag pattern) gives an indication of
%	the compression achieved at the quantizing stage; trailnum
%	returns this count.
%
% SEE ALSO
%	jpeg, quant

% 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 c = trailnum(a)

n = size(a,1);

[i, j] = find(a);

if ~size(i,1)  % zero matrix?
  c = n^2;
  return
end

w = sort(i+j); d = w(size(w, 1));

% Count the number of zero entries in trailing zero diagonals
if (d <= n+1)
  c = n^2 - (d-1)*d/2;
else
  c = (2*n-d)*(2*n-d+1)/2;
end

% Now count the number of trailing zeros in the last nonzero diagonal
w = diag(rot90(a), d - n - 1);
if (floor(d/2)*2 ~= d)
  w = w(size(w,1):-1:1);
end
w = find(w); c = c + w(1) - 1;