/usr/share/octave/packages/image-2.2.2/histeq.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 | ## Copyright (C) 2000 Kai Habel <kai.habel@gmx.de>
## Copyright (C) 2008 Jonas Wagner <j.b.w@gmx.ch>
##
## 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{J} =} histeq (@var{I}, @var{n})
## Equalize histogram of grayscale image.
##
## The histogram contains
## @var{n} bins, which defaults to 64.
##
## @var{I}: Image in double format, with values from 0.0 to 1.0.
##
## @var{J}: Returned image, in double format as well.
##
## Note that the algorithm used for histogram equalization gives results
## qualitatively comparable but numerically different from @sc{matlab}
## implementation.
##
## @seealso{imhist, mat2gray, brighten}
## @end deftypefn
function J = histeq (I, n = 64)
if (nargin < 1 || nargin > 3)
print_usage ();
endif
if (isempty (I))
J = [];
return
endif
[r, c] = size (I);
I = mat2gray (I);
[X, map] = gray2ind (I, n);
[nn, xx] = imhist (I, n);
Icdf = 1 / prod (size (I)) * cumsum (nn);
J = reshape (Icdf(X + 1), r, c);
endfunction
## FIXME: the method we are using is different from Matlab so our results
## are slightly different. The following xtest show the Matlab
## results that we should be aiming at.
%!assert (histeq ([]), []);
## One value
%!assert (histeq (0), 1);
%!assert (histeq (1), 1);
%!assert (histeq (1.5), 1);
%!assert (histeq (zeros (100, 200)), ones (100, 200)); # matrix
## Two values
%!xtest assert (histeq ([0 1]), [0.4920634921 1], 10^-8);
%!xtest assert (histeq ([0 1]'), [0.4920634921 1]', 10^-8); # column array
%!xtest assert (histeq ([0 255]), [0.4920634921 1], 10^-8);
%!xtest assert (histeq (uint8 ([0 1])), [ 125 190]); # uint8
%!xtest assert (histeq (uint8 ([0 255])), [ 125 255]);
%!xtest assert (histeq (uint16 ([0 1])), [65535 65535]); # uint16
%!xtest assert (histeq (uint16 ([0 255])), [32247 48891]);
%!xtest assert (histeq (uint16 ([0 256])), [32247 48891]);
%!xtest assert (histeq (uint16 ([0 65535])), [32247 65535]);
## Three values
%!test assert (histeq ([0 1 1] ), [ 1/3 1 1] , 10^-8);
%!test assert (histeq ([0 0 1]'), [ 2/3 2/3 1]', 10^-8);
%!xtest assert (histeq ([0 1 2] ), [ 1/3 1 1] , 10^-8);
%!xtest assert (histeq (uint8 ([0 1 2])), [ 85 125 215]);
%!xtest assert (histeq (uint16 ([0 1 2])), [65535 65535 65535]);
%!xtest assert (histeq (uint16 ([0 100 200])), [43690 43690 55133]);
## Many values
%!xtest
%! J = [20 32 57 81 105 125 150 174 198 223 247];
%! assert (histeq (uint8 (0:10:100)), J);
%!xtest
%! J = [0.0793650794
%! 0.1269841270
%! 0.2222222222
%! 0.3174603175
%! 0.4126984127
%! 0.4920634921
%! 0.5873015873
%! 0.6825396825
%! 0.7777777778
%! 0.8730158730
%! 1.0000000000];
%! assert (histeq (0:0.1:1), J', 10^-8);
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