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########################################################################
##
## Copyright (C) 2014-2024 The Octave Project Developers
##
## See the file COPYRIGHT.md in the top-level directory of this
## distribution or <https://octave.org/copyright/>.
##
## This file is part of Octave.
##
## Octave 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.
##
## Octave 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 Octave; see the file COPYING. If not, see
## <https://www.gnu.org/licenses/>.
##
########################################################################
## -*- texinfo -*-
## @deftypefn {} {@var{map} =} cubehelix ()
## @deftypefnx {} {@var{map} =} cubehelix (@var{n})
## @deftypefnx {} {@var{map} =} cubehelix (@var{n}, @var{start}, @var{rots}, @var{hue}, @var{gamma})
## Create cubehelix colormap.
##
## This colormap varies from black to white going though blue, green, and red
## tones while maintaining a monotonically increasing perception of intensity.
## This is achieved by traversing a color cube from black to white through
## a helix, hence the name cubehelix, while taking into account the perceived
## brightness of each channel according to the NTSC specifications from 1953.
##
## @example
## rgbplot (cubehelix (256))
## @end example
##
## The argument @var{n} must be a scalar.
## If unspecified, the length of the current colormap, or 64, is used.
##
## Reference: Green, D. A., 2011,
## @cite{A @nospell{colour} scheme for the display of astronomical intensity
## images}, Bulletin of the Astronomical Society of India, 39, 289.
##
## @seealso{colormap}
## @end deftypefn
function map = cubehelix (n, start = 0.5, rots = -1.5, hue = 1, gamma = 1)
if (nargin > 0)
if (! isscalar (n))
error ("cubehelix: N must be a scalar");
endif
n = double (n);
else
hf = get (0, "currentfigure");
if (! isempty (hf))
n = rows (get (hf, "colormap"));
else
n = 64;
endif
endif
if (n > 1)
coeff = [ -0.14861 -0.29227 1.97294
1.78277 -0.90649 0.00000];
fract = ((0:n-1) / (n-1))';
angle = 2 * pi * (start/3 + 1 + rots*fract);
fract .^= gamma;
amp = hue * fract .* (1-fract) /2;
map = fract + amp .* ([cos(angle) sin(angle)] * coeff);
## Clip values (only in case users have changed values of hue or gamma)
map(map < 0) = 0;
map(map > 1) = 1;
elseif (n > 0)
map = [0, 0, 0];
else
map = zeros (0, 3);
endif
endfunction
## A better demo of this colormap would be a 3-D plot in NTSC instead of
## RGB values. That would really show what this colormap is about.
%!demo
%! ## Show the 'cubehelix' colormap profile and as an image
%! cmap = cubehelix (256);
%! subplot (2, 1, 1);
%! rgbplot (cmap, "composite");
%! subplot (2, 1, 2);
%! rgbplot (cmap);
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