/usr/share/octave/packages/image-2.2.2/imgradientxy.m is in octave-image 2.2.2-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 | ## Copyright (C) 2013 Brandon Miles <brandon.miles7@gmail.com>
##
## 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, see <http://www.gnu.org/licenses/>.
## -*- texinfo -*-
## @deftypefn {Function File} {[@var{gradX}, @var{gradY}] =} imgradientxy (@var{img})
## @deftypefnx {Function File} {[@var{gradX}, @var{gradY}] =} imgradientxy (@var{img}, @var{method})
## Compute the x and y gradients of an image using various methods.
##
## The first input @var{img} is the gray scale image to compute the edges
## on. The second input @var{method} controls the method used to calculate
## the gradients.
##
## The first output @var{gradX} returns the gradient in the x direction.
## The second output @var{gradY} returns the gradient in the y direction.
##
## The @var{method} input argument can be any of the following strings:
##
## @table @asis
## @item @qcode{"sobel"} (default)
## Calculates the gradient using the Sobel approximation to the
## derivatives.
##
## @item @qcode{"prewitt"}
## Calculates the gradient using the Prewitt approximation to the
## derivatives. This method works just like Sobel except a different
## approximation of the gradient is used.
##
## @item @qcode{"central difference"}
## Calculates the gradient using the central difference approximation to the
## derivatives: @code{(x(i-1) - x(i+1))/2}.
##
## @item @qcode{"intermediate difference"}
## Calculates the gradient in using the intermediate difference approximation
## to the derivatives: @code{x(i) - x(i+1)}.
##
## @end table
##
## @seealso{edge, imgradient}
## @end deftypefn
function [gradX, gradY] = imgradientxy (img, method = "sobel")
if (nargin < 1 || nargin > 2)
print_usage ();
elseif (! isgray (img))
error ("imgradientxy: IMG must be a gray-scale image");
elseif (! ischar (method))
error ("imgradientxy: METHOD must be a string");
endif
switch (tolower (method))
case {"sobel", "prewitt"}
ker = fspecial (method); # horizontal
case {"centraldifference"}
ker = [0.5; 0; -0.5];
case {"intermediatedifference"}
ker = [1; -1];
otherwise
error("imgradientxy: unrecognized METHOD %s", method);
endswitch
gradX = conv2 (img, ker', "same");
if (nargout > 1)
gradY = conv2 (img, ker, "same");
endif
endfunction
%!test
%! A = [0 1 0
%! 1 1 1
%! 0 1 0];
%!
%! [gxSobel, gySobel] = imgradientxy (A);
%! [gxSobel2, gySobel2] = imgradientxy (A, "Sobel");
%! assert (gxSobel,
%! [ 3 0 -3
%! 4 0 -4
%! 3 0 -3]);
%! assert (gySobel,
%! [ 3 4 3
%! 0 0 0
%! -3 -4 -3]);
%!
%! ## test default method
%! assert(gxSobel, gxSobel2);
%! assert(gySobel, gySobel2);
%!
%! [gxPrewitt, gyPrewitt] = imgradientxy (A, "Prewitt");
%! assert (gxPrewitt,
%! [ 2 0 -2
%! 3 0 -3
%! 2 0 -2]);
%! assert (gyPrewitt,
%! [ 2 3 2
%! 0 0 0
%! -2 -3 -2]);
%!
%! [gxCd, gyCd] = imgradientxy (A, "CentralDifference");
%! assert (gxCd,
%! [ 0.5 0.0 -0.5
%! 0.5 0.0 -0.5
%! 0.5 0.0 -0.5]);
%! assert (gyCd,
%! [ 0.5 0.5 0.5
%! 0 0 0
%! -0.5 -0.5 -0.5]);
%!
%! [gxId, gyId] = imgradientxy(A, "IntermediateDifference");
%! assert (gxId,
%! [ 1 -1 0
%! 0 0 -1
%! 1 -1 0]);
%! assert (gyId,
%! [ 1 0 1
%! -1 0 -1
%! 0 -1 0]);
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