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## -*- texinfo -*-
## @deftypefn  {} {@var{map} =} rainbow ()
## @deftypefnx {} {@var{map} =} rainbow (@var{n})
## Create color colormap.  This colormap ranges from red through orange,
## yellow, green, blue, to violet.
##
## The argument @var{n} must be a scalar.
## If unspecified, the length of the current colormap, or 64, is used.
## @seealso{colormap}
## @end deftypefn

## this colormap is not part of matlab, it is like the prism
## colormap map but with a continuous map

function map = rainbow (n)

  if (nargin == 1)
    if (! isscalar (n))
      error ("rainbow: 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)
    map = [1, 0, 0];
  elseif (n > 1)
    x = [0:(n-1)]' / (n - 1);

    r = (  (x < 2/5)
         + (x >= 2/5 & x < 3/5) .* (-5 * x + 3)
         + (x >= 4/5) .* (10/3 * x - 8/3));

    g = (  (x < 2/5) .* (5/2 * x)
         + (x >= 2/5 & x < 3/5)
         + (x >= 3/5 & x < 4/5) .* (-5 * x + 4));

    b = (  (x >= 3/5 & x < 4/5) .* (5 * x - 3)
         + (x >= 4/5));

    map = [r, g, b];
  else
    map = zeros (0, 3);
  endif

endfunction


%!demo
%! ## Show the 'rainbow' colormap profile and as an image
%! cmap = rainbow (256);
%! subplot (2, 1, 1);
%!  rgbplot (cmap, "composite");
%! subplot (2, 1, 2);
%!  rgbplot (cmap);