% ISCOLINEAR - are 3 points colinear%% Usage:  r = iscolinear(p1, p2, p3, flag)%% Arguments:%        p1, p2, p3 - Points in 2D or 3D.%        flag       - An optional parameter set to 'h' or 'homog'%                     indicating that p1, p2, p3 are homogneeous%                     coordinates with arbitrary scale.  If this is%                     omitted it is assumed that the points are%                     inhomogeneous, or that they are homogeneous with%                     equal scale.%% Returns:%        r = 1 if points are co-linear, 0 otherwise% Copyright (c) 2004-2005 Peter Kovesi% School of Computer Science & Software Engineering% The University of Western Australia% http://www.csse.uwa.edu.au/% % Permission is hereby granted, free of charge, to any person obtaining a copy% of this software and associated documentation files (the "Software"), to deal% in the Software without restriction, subject to the following conditions:% % The above copyright notice and this permission notice shall be included in % all copies or substantial portions of the Software.%% The Software is provided "as is", without warranty of any kind.% February 2004% January  2005 - modified to allow for homogeneous points of arbitrary%                 scale (thanks to Michael Kirchhof)function r = iscolinear(p1, p2, p3, flag)    if nargin == 3   % Assume inhomogeneous coords	flag = 'inhomog';    end        if ~all(size(p1)==size(p2)) | ~all(size(p1)==size(p3)) | ...        ~(length(p1)==2 | length(p1)==3)                                      error('points must have the same dimension of 2 or 3');    end        % If data is 2D, assume they are 2D inhomogeneous coords. Make them    % homogeneous with scale 1.    if length(p1) == 2            p1(3) = 1; p2(3) = 1; p3(3) = 1;    end    if flag(1) == 'h'	% Apply test that allows for homogeneous coords with arbitrary        % scale.  p1 X p2 generates a normal vector to plane defined by        % origin, p1 and p2.  If the dot product of this normal with p3        % is zero then p3 also lies in the plane, hence co-linear.	r =  abs(dot(cross(p1, p2),p3)) < eps;    else	% Assume inhomogeneous coords, or homogeneous coords with equal        % scale.	r =  norm(cross(p2-p1, p3-p1)) < eps;    end