% -- External Function: cmap = spectral( nval );
%
% Returns a RGB colormap/palette with `nval' values (default: nval=64) based on
% the diverging palette `spectral' (10 levels) from http://colorbrewer2.org/
% Although there is a more-detailed (11 levels) version available on the
%   website, the odd-numbered versions have an excessively distinct central
%   level that breaks the `perceptual continuity' of the palette. You are
%   primarily using `spectral' as a sequential/qualitative palette (and not as
%   its original `diverging' purpose) and thus the continuity is paramount.
% Copyright Cynthia Brewer, Mark Harrower and The Pennsylvania State University
% Adapted for GNU Octave/Matlab by pst-laurent@vims.edu (2024-10-29).
function cmap = spectral( nval )
  if ( nargin < 1 )
    nval = 64;
  end

% Lookup table: The original palette had anywhere between 3-11 entries.

% orig = [ 213 244 253 254 230 171 102  50; ...
%           62 109 174 224 245 221 194 136; ...
%           79  67  97 139 152 164 165 189 ]';             %  8 entries.
  orig = [ 158 213 244 253 254 230 171 102  50  94; ...
             1  62 109 174 224 245 221 194 136  79; ...
            66  79  67  97 139 152 164 165 189 162 ]';     % 10 entries.
% orig = [ 158 213 244 253 254 255 230 171 102  50  94; ...
%            1  62 109 174 224 255 245 221 194 136  79; ...
%           66  79  67  97 139 191 152 164 165 189 162 ]'; % 11 entries.
    
  xori = (0 : size( orig, 1 ) - 1)' / (size( orig, 1 ) - 1);
  xitp = (0 : nval            - 1)' / (nval            - 1);
 
  cmap = interp1( xori, orig, xitp, 'pchip', 'extrap' );

  clear orig xori xitp;
  
  cmap = cmap / 255.; % 0 <= RGB <= 255.
  cmap( find( cmap > 1. ) ) = 1.;
  cmap( find( cmap < 0. ) ) = 0.;

% Reorder the palette to be cold-to-warm rather than original warm-to-cold.
  cmap = flip( cmap, 1 ); % Now in the same order as `jet'.
end