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fspecial.m

## Copyright (C) 2005 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 2 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, see <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn {Function File} {@var{filter} = } fspecial(@var{type}, @var{arg1}, @var{arg2})
## Create spatial filters for image processing.
##
## @var{type} determines the shape of the filter and can be
## @table @t
## @item "average"
## Rectangular averaging filter. The optional argument @var{arg1} controls the
## size of the filter. If @var{arg1} is an integer @var{N}, a @var{N} by @var{N}
## filter is created. If it is a two-vector with elements @var{N} and @var{M}, the
## resulting filter will be @var{N} by @var{M}. By default a 3 by 3 filter is
## created.
## @item "disk"
## Circular averaging filter. The optional argument @var{arg1} controls the
## radius of the filter. If @var{arg1} is an integer @var{N}, a 2 @var{N} + 1
## filter is created. By default a radius of 5 is used.
## @item "gaussian"
## Gaussian filter. The optional argument @var{arg1} controls the size of the
## filter. If @var{arg1} is an integer @var{N}, a @var{N} by @var{N}
## filter is created. If it is a two-vector with elements @var{N} and @var{M}, the
## resulting filter will be @var{N} by @var{M}. By default a 3 by 3 filter is
## created. The optional argument @var{arg2} sets spread of the filter. By default
## a spread of @math{0.5} is used.
## @item "log"
## Laplacian of Gaussian. The optional argument @var{arg1} controls the size of the
## filter. If @var{arg1} is an integer @var{N}, a @var{N} by @var{N}
## filter is created. If it is a two-vector with elements @var{N} and @var{M}, the
## resulting filter will be @var{N} by @var{M}. By default a 5 by 5 filter is
## created. The optional argument @var{arg2} sets spread of the filter. By default
## a spread of @math{0.5} is used.
## @item "laplacian"
## 3x3 approximation of the laplacian. The filter is approximated as
## @example
## (4/(@var{alpha}+1))*[@var{alpha}/4,     (1-@var{alpha})/4, @var{alpha}/4; ...
##                (1-@var{alpha})/4, -1,          (1-@var{alpha})/4;  ...
##                @var{alpha}/4,     (1-@var{alpha})/4, @var{alpha}/4];
## @end example
## where @var{alpha} is a number between 0 and 1. This number can be controlled
## via the optional input argument @var{arg1}. By default it is @math{0.2}.
## @item "unsharp"
## Sharpening filter. The following filter is returned
## @example
## (1/(@var{alpha}+1))*[-@var{alpha},   @var{alpha}-1, -@var{alpha}; ...
##                 @var{alpha}-1, @var{alpha}+5,  @var{alpha}-1; ...
##                -@var{alpha},   @var{alpha}-1, -@var{alpha}];
## @end example
## where @var{alpha} is a number between 0 and 1. This number can be controlled
## via the optional input argument @var{arg1}. By default it is @math{0.2}.
## @item "motion"
## Moion blur filter of width 1 pixel. The optional input argument @var{arg1}
## controls the length of the filter, which by default is 9. The argument @var{arg2}
## controls the angle of the filter, which by default is 0 degrees.
## @item "sobel"
## Horizontal Sobel edge filter. The following filter is returned
## @example
## [ 1,  2,  1;
##   0,  0,  0;
##  -1, -2, -1 ]
## @end example
## @item "prewitt"
## Horizontal Prewitt edge filter. The following filter is returned
## @example
## [ 1,  1,  1;
##   0,  0,  0;
##  -1, -1, -1 ]
## @end example
## @item "kirsch"
## Horizontal Kirsch edge filter. The following filter is returned
## @example
## [ 3,  3,  3;
##   3,  0,  3;
##  -5, -5, -5 ]
## @end example
## @end table
## @end deftypefn

## Remarks by Søren Hauberg (jan. 2nd 2007)
## The motion filter and most of the documentation was taken from Peter Kovesi's
## GPL'ed implementation of fspecial from 
## http://www.csse.uwa.edu.au/~pk/research/matlabfns/OctaveCode/fspecial.m

function f = fspecial (type, arg1, arg2)
  if (!ischar (type))
    error ("fspecial: first argument must be a string");
  endif
  
  switch lower(type)
    case "average"
      ## Get filtersize
      if (nargin > 1 && isreal (arg1) && length (arg1 (:)) <= 2)
        fsize = arg1 (:);
      else
        fsize = 3;
      endif
      ## Create the filter
      f = ones (fsize);
      ## Normalize the filter to integral 1
      f = f / sum (f (:));

    case "disk"
      ## Get the radius
      if (nargin > 1 && isreal (arg1) && length (arg1 (:)) == 1)
        radius = arg1;
      else
        radius = 5;
      endif
      ## Create the filter
      [x, y] = meshgrid (-radius:radius, -radius:radius);
      r = sqrt (x.^2 + y.^2);
      f = (r <= radius);
      ## Normalize the filter to integral 1
      f = f / sum (f (:));

    case "gaussian"
      ## Get hsize
      if (nargin > 1 && isreal (arg1))
        if (length (arg1 (:)) == 1)
          hsize = [arg1, arg1];
        elseif (length (arg1 (:)) == 2)
          hsize = arg1;
        else
          error ("fspecial: second argument must be a scalar or a vector of two scalars");
        endif
      else
        hsize = [3, 3];
      endif
      ## Get sigma
      if (nargin > 2 && isreal (arg2) && length (arg2 (:)) == 1)
        sigma = arg2;
      else
        sigma = 0.5;
      endif
      h1 = hsize (1)-1; h2 = hsize (2)-1; 
      [x, y] = meshgrid(0:h2, 0:h1);
      x = x-h2/2; y = y-h1/2;
      gauss = exp( -( x.^2 + y.^2 ) / (2*sigma^2) );
      f = gauss / sum (gauss (:));

    case "laplacian"
      ## Get alpha
      if (nargin > 1 && isscalar (arg1))
        alpha = arg1;
        if (alpha < 0 || alpha > 1)
          error ("fspecial: second argument must be between 0 and 1");
        endif
      else
        alpha = 0.2;
      endif
      ## Compute filter
      f = (4/(alpha+1))*[alpha/4,     (1-alpha)/4, alpha/4; ...
                         (1-alpha)/4, -1,          (1-alpha)/4;  ...
                         alpha/4,     (1-alpha)/4, alpha/4];
    case "log"
      ## Get hsize
      if (nargin > 1 && isreal (arg1))
        if (length (arg1 (:)) == 1)
          hsize = [arg1, arg1];
        elseif (length (arg1 (:)) == 2)
          hsize = arg1;
        else
          error ("fspecial: second argument must be a scalar or a vector of two scalars");
        endif
      else
        hsize = [5, 5];
      endif
      ## Get sigma
      if (nargin > 2 && isreal (arg2) && length (arg2 (:)) == 1)
        sigma = arg2;
      else
        sigma = 0.5;
      endif
      ## Compute the filter
      h1 = hsize (1)-1; h2 = hsize (2)-1; 
      [x, y] = meshgrid(0:h2, 0:h1);
      x = x-h2/2; y = y = y-h1/2;
      gauss = exp( -( x.^2 + y.^2 ) / (2*sigma^2) );
      f = ( (x.^2 + y.^2 - 2*sigma^2).*gauss )/( 2*pi*sigma^6*sum(gauss(:)) );

    case "motion"
      ## Taken (with some changes) from Peter Kovesis implementation 
      ## (http://www.csse.uwa.edu.au/~pk/research/matlabfns/OctaveCode/fspecial.m)
      ## FIXME: The implementation is not quite matlab compatible.
      if (nargin > 1 && isreal (arg1))
        len = arg1;
      else
        len = 9;
      endif
      if (mod (len, 2) == 1)
        sze = [len, len];
      else
        sze = ;
      end
      if (nargin > 2 && isreal (arg2))
        angle = arg2;
      else
        angle = 0;
      endif
          
      ## First generate a horizontal line across the middle
      f = zeros (sze);
      f (floor (len/2)+1, 1:len) = 1;

      # Then rotate to specified angle
      f = imrotate (f, angle, "bilinear", "loose");
      f = f / sum (f (:));

    case "prewitt"
      ## The filter
      f = ;
      
    case "sobel"
      ## The filter
      f = ;
      
    case "kirsch"
      ## The filter
      f = ;
    
    case "unsharp"
      ## Get alpha
      if (nargin > 1 && isscalar (arg1))
        alpha = arg1;
        if (alpha < 0 || alpha > 1)
          error ("fspecial: second argument must be between 0 and 1");
        endif
      else
        alpha = 0.2;
      endif
      ## Compute filter
      f = (1/(alpha+1))*[-alpha,   alpha-1, -alpha; ...
                          alpha-1, alpha+5,  alpha-1; ...
                         -alpha,   alpha-1, -alpha];

    otherwise
      error ("fspecial: filter type '%s' is not supported", type);
  endswitch
endfunction

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