inside triangle

inside_triangle is used to check if a point P is inside the triangle P1P2P3 or not.

Description

%inside_triangle is used to check if a point P is inside
%the triangle P1P2P3 or not.
%
%Inputs: P, P1, P2 and P3 are vectors of length 2 or three of the
% form [x y z] or [x y]
%
%Output: True
%True=1 => P is on or inside P1P2P3
%True=0 => P is outside P1P2P3
%
%Example:
%True=inside_triangle([0.5 0.5],[0 0],[0 2],[2 0]);
%
%The following algorithm is implemented
% If P is ON or INSIDE the triangle
%
% Area(PP1P2) + Area(PP2P3) + Area(PP3P1) = Area(P1P2P3)
%
% If P is OUTSIDE then,
%
% Area(PP1P2) + Area(PP2P3) + Area(PP3P1) > Area(P1P2P3)
%
% Area of a triangle can be found using the determinant:
%
% Area = abs(1/2 * |x1 y1 1| )
% |x2 y2 1|
% |x3 y3 1|
%

• MATLAB release: MATLAB 7.0.1 (R14SP1)

[seda]

I tried to use this function, but, it does not work for all test, for example
True=inside_triangle([25 20],[20 10],[25 30],[30 20]), basically it gives correct result that sum:75, area:75, but it gives true=0

or for another example;
True=inside_triangle([28.76 41.00],[28.7060 40.7020],[29.0592 41.0657],[28.7730 41.1810]) these values are lat. and lon. in degrees, and are in the triangle normally, but function gives True=0
What is the reason of that??

Seda Yelkenci

[Oren]

Looks like it will work.
Is it needed to input in specific order?
For example:
P = [2 3]
P1 = [0 0]
P2 = [-5 10]
P3 = [5 10]
so:
True=inside_triangle(P,P1,P2,P3)
or
True=inside_triangle(P,P2,P1,P3)
?