function cmap = chMap(M)
%CHMAP Create a monochrome-compatible colour map
% 
%   CMAP = IOSR.FIGURES.CHMAP returns a colour map CMAP (varying black -
%   blue - green - yellow - white) that is monochrome-compatible, i.e. it
%   produces a linear greyscale colour map. CMAP is size Mx3, where M is
%   the length of the current figure's colormap. If no figure exists,
%   MATLAB creates one.
% 
%   CMAP = IOSR.FIGURES.CHMAP(M) returns a colormap of length M.
% 
%   EXAMPLE
% 
%       figure;
%       imagesc(sin(linspace(0,2*pi,1000))'*...
%           sin(linspace(0,2*pi,1000)));
%       colormap(iosr.figures.chMap(256));
%       axis image;
%       colorbar
% 
%   See also IOSR.FIGURES.CMRMAP, GRAY.

%   Copyright 2016 University of Surrey.

    % default colormap size
    if nargin < 1, M = size(get(gcf,'colormap'),1); end

    N = linspace(0,1,M)';

    % Define red and blue components
    R = (sin(((2.*N.^1.5)-1).*(pi/2))+1)./2;
    B = ((N.^1.25)+((sin((N.^1.0).*(2*pi)))./1.75));
    % Calculate green to ensure monotonic luminance
    G = (N - 0.2989.*R - 0.1140.*B)./0.5870;

    % Ensure map is in range [0,1]
    cmap = [R G B];
    cmap = cmap-min(cmap(:));
    cmap = cmap./max(cmap(:));

end