triangleArea(pt1, pt2, pt3)

function area = triangleArea(pt1, pt2, pt3)
%TRIANGLEAREA Signed area of a triangle
%
%   AREA = triangleArea(P1, P2, P3)
%   Computes area of the triangle whose vertices are given by P1, P2 and
%   P3. Each vertex is a 1-by-2 row vector. 
%
%   AREA = triangleArea(PTS)
%   Concatenates vertex coordinates in a 3-by-2 array. Each row of the
%   array contains coordinates of one vertex.
%
%
%   Example
%   % Compute area of a Counter-Clockwise (CCW) oriented triangle
%     triangleArea([10 10], [30 10], [10 40])
%     ans = 
%         300
%
%   % Compute area of a Clockwise (CW) oriented triangle
%     triangleArea([10 40], [30 10], [10 10])
%     ans = 
%         -300
%
%   See also
%   polygonArea, triangleArea3d
%
% ------
% Author: David Legland
% e-mail: [e-mail]
% Created: 2011-08-23,    using Matlab 7.9.0.529 (R2009b)
% Copyright 2011 INRA - Cepia Software Platform.

% if data is given as one array, split vertices
if nargin == 1
    pt2 = pt1(2,:);
    pt3 = pt1(3,:);
    pt1 = pt1(1,:);
end

% compute individual vectors
v12 = bsxfun(@minus, pt2, pt1);
v13 = bsxfun(@minus, pt3, pt1);

% compute area from cross product
area = (v13(:,2) .* v12(:,1) - v13(:,1) .* v12(:,2)) / 2;