function?chull=ConvHull2D（xy）
%??chull=ConvHull2D（xy）?returns?indices?into?the?x?and?y?vectors?of?the?points?on
%?????the?convex?hull.?Colinear?points?wont?be?part?of?the?convex?hull.
%?
%????????Input:
%????????????????x[nx1]?:?vectors?of?x?points
%????????????????y[nx1]?:?vectors?of?y?points
%????????Output:
%????????????????chull[mx1]:?index?of?convex?hull?points
%
%?????Example:
%???????N=20;
%???????x=rand（N1）;
%???????y=rand（N1）;
%???????chull=ConvHull2D（xy）;
%???????figure（1）
%???????hold?on
%???????plot（xy‘k.‘）
%???????plot（x（chull）y（chull））;
%???????title（‘Convex?Hull‘）
%
%
%?????See?also?convhulln?delaunay?voronoi
%??????????????polyarea?qhull.
%
%
%Author:?Luigi?Giaccari
%Last?Update:?4/12/2008
%Creation:?27/11/2008

%How?does?it?work?