function zi = sliceg(x, y, z, xi, yi, method)

% sliceg -- Interpolation along a non-monotonic track.
%  sliceg('demo') demonstrates itself.
%  sliceg(N) demonstrates itself with an N-by-N surface
%   and N points along a random track.  (default = 10).
%  sliceg(x, y, xi, yi, 'method') uses the syntax of
%   "interp2" to interpolate the (x, y, z) grid along
%   a (possibly) non-monotonic (xi, yi) track. The
%   track points must be unique.
%
% Also see: "help interp2".
 
% Copyright (C) 2000 Dr. Charles R. Denham, ZYDECO.
%  All Rights Reserved.
%   Disclosure without explicit written consent from the
%    copyright owner does not constitute publication.
 
% Version of 20-Jan-2000 12:17:03.
% Updated    20-Jan-2000 21:13:47.

if nargin < 1, x = 'demo'; help(mfilename), end

if isequal(x, 'demo'), x = 10; end

if ischar(x), x = eval(x); end

if length(x) < 2
	n = x;
	i = linspace(0, 1, n+1);
	[x, y] = meshgrid(i, i);
	z = x + y;
	xi = sort(rand(1, n));
	yi = rand(size(xi));
	result = feval(mfilename, x, y, z, xi, yi);
	surf(x, y, z);
	hold on
	plot3(xi, yi, 0*result-1, '-g', 'LineWidth', 3);
	plot3(xi, yi, 0*result+3, '-g', 'LineWidth', 3);
	plot3([xi;xi], [yi; yi], [0*result-1; 0*result+3], '-ko')
	plot3(xi, yi, result+0.01, '-k', 'LineWidth', 3);
	hold off
	box on
	shading faceted
	rotate3d
	figure(gcf)
	s = [mfilename ' ' int2str(n)];
	set(gcf, 'Name', s)
	return
end

% Sort the data and remember original indices.
%  We assume that there are no points identical.

[yi, i] = sort(yi);
xi = xi(i);
[xi, j] = sort(xi);
yi = yi(j);
i = i(j);

if nargin < 6
	zi = interp2(x, y, z, xi, yi);
else
	zi = interp2(x, y, z, xi, yi, method);
end

zi(i) = zi;