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## -*- texinfo -*-
## @deftypefn  {} {@var{map} =} jet ()
## @deftypefnx {} {@var{map} =} jet (@var{n})
## Create color colormap.  This colormap ranges from dark blue through blue,
## cyan, green, yellow, red, to dark red.
##
## The argument @var{n} must be a scalar.
## If unspecified, the length of the current colormap, or 64, is used.
##
## Programming Note: The @code{jet} colormap is not perceptually uniform.
## Try the @code{viridis} colormap if that is important.  For a drop-in
## replacement for @code{jet} with better perceptual characteristics try
## the @code{turbo} colormap.
## @seealso{colormap, turbo}
## @end deftypefn

function map = jet (n)

  if (nargin == 1)
    if (! isscalar (n))
      error ("jet: 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 = [0, 1, 1];
  elseif (n == 2)
    map = [0, 0, 1
           0, 1, 1];
  elseif (n > 2)
    nel = ceil (n/4);           # number of elements
    idx1 = ceil (3/8 * n) + 1;  # ~3/8*n for large n
    if (mod (n, 8) == 2)
      idx1 += 1;
    endif
    idx2 = idx1 + nel - 1;      # ~5/8*n for large n
    idx3 = min (idx2 + nel, n); # ~7/8*n for large n

    r = zeros (n, 1);
    r(idx1:idx2, 1) = [1:nel] / nel;
    r(idx2:idx3, 1) = 1;
    nel2 = n - idx3;
    r(idx3:(idx3+nel2), 1) = [nel:-1:(nel - nel2)] / nel;

    idx1 = idx1 - nel;          # ~1/8*n for large n
    idx2 = idx1 + nel - 1;      # ~3/8*n for large n
    idx3 = min (idx2 + nel, n); # ~5/8*n for large n

    g = zeros (n, 1);
    g(idx1:idx2, 1) = [1:nel] / nel;
    g(idx2:idx3, 1) = 1;
    nel2 = min (nel, n - idx3);
    g(idx3:(idx3+nel2), 1) = [nel:-1:(nel - nel2)] / nel;

    idx1 = max (idx2 - nel, 1); # ~1/8*n for large n
    idx2 = idx2;                # ~3/8*n for large n
    idx3 = idx3;                # ~5/8*n for large n

    b = zeros (n, 1);
    nel2 = min (nel, idx1-1);
    b(1:idx1, 1) = [(nel - nel2):nel] / nel;
    b(idx1:idx2, 1) = 1;
    nel2 = min (nel, n - idx3);
    b(idx2:(idx2+nel2), 1) = [nel:-1:(nel - nel2)] / nel;

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

endfunction


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