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stdfilt.m

## 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., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

## -*- texinfo -*-
## @deftypefn {Function File} {@var{S} =} stdfilt (@var{im})
## @deftypefnx{Function File} {@var{S} =} stdfilt (@var{im}, @var{domain})
## @deftypefnx{Function File} {@var{S} =} stdfilt (@var{im}, @var{domain}, @var{padding}, ...)
## Computes the local standard deviation in a neighbourhood around each pixel in
## an image.
##
## The standard deviation of the pixels of a neighbourhood is computed as
##
## @example
## @var{S} = sqrt ((sum (@var{x} - @var{mu}).^2)/(@var{N}-1))
## @end example
##
## where @var{mu} is the mean value of the pixels in the neighbourhood,
## @var{N} is the number of pixels in the neighbourhood. So, an unbiased estimator
## is used.
##
## The neighbourhood is defined by the @var{domain} binary mask. Elements of the
## mask with a non-zero value are considered part of the neighbourhood. By default
## a 3 by 3 matrix containing only non-zero values is used.
##
## At the border of the image, extrapolation is used. By default symmetric
## extrapolation is used, but any method supported by the @code{padarray} function
## can be used. Since extrapolation is used, one can expect a lower deviation near
## the image border.
##
## @seealso{std2, paddarray, entropyfilt}
## @end deftypefn

function retval = stdfilt (I, domain = true (3), padding = "symmetric", varargin)
  ## Check input
  if (nargin == 0)
    error ("stdfilt: not enough input arguments");
  endif
  
  if (!ismatrix (I))
    error ("stdfilt: first input must be a matrix");
  endif
  
  if (!ismatrix (domain))
    error ("stdfilt: second input argument must be a logical matrix");
  endif
  domain = (domain > 0);
  
  ## Pad image
  pad = floor (size (domain)/2);
  I = padarray (I, pad, padding, varargin {:});
  even = (round (size (domain)/2) == size (domain)/2);
  idx = cell (1, ndims (I));
  for k = 1:ndims (I)
    idx {k} = (even (k)+1):size (I, k);
  endfor
  I = I (idx {:});

  ## Perform filtering
  retval = __spatial_filtering__ (I, domain, "std", I, 0);

endfunction

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