Add a useful package (from Source forge) for octave

[CreaPhase.git] / octave_packages / image-1.0.15 / entropy.m

diff --git a/octave_packages/image-1.0.15/entropy.m b/octave_packages/image-1.0.15/entropy.m
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+## Copyright (C) 2008 Søren Hauberg
+## 
+## This program 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 3 of the License, or
+## (at your option) any later version.
+## 
+## This program 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 program; if not, write to the Free Software
+## Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {@var{E} =} entropy (@var{im})
+## @deftypefnx{Function File} {@var{E} =} entropy (@var{im}, @var{nbins})
+## Computes the entropy of an image.
+##
+## The entropy of the elements of the image @var{im} is computed as
+##
+## @example
+## @var{E} = -sum (@var{P} .* log2 (@var{P})
+## @end example
+##
+## where @var{P} is the distribution of the elements of @var{im}. The distribution
+## is approximated using a histogram with @var{nbins} cells. If @var{im} is
+## @code{logical} then two cells are used by default. For other classes 256 cells
+## are used by default.
+##
+## When the entropy is computed, zero-valued cells of the histogram are ignored.
+##
+## @seealso{entropyfilt}
+## @end deftypefn
+
+function retval = entropy (I, nbins = 0)
+  ## Check input
+  if (nargin == 0)
+    error ("entropy: not enough input arguments");
+  endif
+  
+  if (!ismatrix (I))
+    error ("entropy: first input must be a matrix");
+  endif
+  
+  if (!isscalar (nbins))
+    error ("entropy: second input argument must be a scalar");
+  endif
+  
+  ## Get number of histogram bins
+  if (nbins <= 0)
+    if (islogical (I))
+      nbins = 2;
+    else
+      nbins = 256;
+    endif
+  endif
+  
+  ## Compute histogram
+  P = hist (I (:), nbins, true);
+  
+  ## Compute entropy (ignoring zero-entries of the histogram)
+  P += (P == 0);
+  retval = -sum (P .* log2 (P));
+endfunction