determine whether point lies inside triangle

Question

The program needs to read the values of three coordinates

• P1(x1,y1)
• P2(x2,y2)
• P3(x3,y3)

as well as another coordinate P(x,

Answer 1

Take the average of the three given points. This new point P4 will always lie inside the triangle.

Now check if P and P4 lie on the same side of each of the three lines P1P2 P2P3 and P3P1. You can do this by checking the signs of the cross products (P -> P1) x (P -> P2) and (P4 -> P1) x (P4 -> P2) (where P->P1 is the vector from P to P1), and then the other two pairs.