stoplight(n)

function cmap = stoplight(n)
% STOPLIGHT - returns a colormap array with stop light colors (r, y, g)
%
% generates an Nx3 array of the RGB values of a colormap that ranges from
% red at the low end, yellow in the middle, and green at the high end.
%
% USAGE
% cmap = stoplight(n)
%
% INPUTS
%   n - (optional) the number of bins in the colormap.  Default is 64.
%
% OUTPUTS
%   cmap - the Nx3 colormap that can be used with the 'colormap' command
%
% James Roberts
% 02/03/2011

% check the inputs
if nargin == 0
    n = 64;
elseif nargin > 1
    error('too many inputs')
elseif ~isinteger(n) || n <=3
    error('input must be an integer greater than 3')
end


% set the x, y, and z valules and interpolate
x           = [1,ceil(n/2),n];
xnew        = 1:n;
y           = 1:3;
ynew        = 1:3;
[x,y]       = meshgrid(x,y);
[xnew,ynew] = meshgrid(xnew,ynew);
z(:,1)      = [1 0 0]';
z(:,2)      = [1 1 0]';
z(:,3)      = [0 1 0]';
cmap        = interp2(x,y,z,xnew,ynew)';