C# - is the point in triangle?

Question

I need some help with my homework. I tried hard to do this and I failed.

We have a triangle with coordinates: A=(-4,4), B=(4,-2), C=(6,6). I need to load from user coordinates of point P=(x,y) and then write on the screen information telling if point P is laying in, out, or on the side of the triangle.

That's what I made this far:

class Program
{
    static void Main(string[] args)
    {
        const int x1 = -4, y1 = 4;
        const int x2 = 4, y2 = -2;
        const int x3 = 6, y3 = 6;

        int x0, y0;
        Console.Write("Podaj współrzędną x: ");
        x0 = Convert.ToInt32(Console.ReadLine());
        Console.Write("Podaj współrzędną y: ");
        y0 = Convert.ToInt32(Console.ReadLine());

        double A_AB = (y2 - y1 / x2 - x1);
        double B_AB = (y1 - (y2 - y1 / x2 - x1) * x1);

        double A_AC = (y3 - y1 / x3 - x1);
        double B_AC = (y1 - (y3 - y1 / x3 - x1) * x1);

        double A_BC = (y3 - y2 / x3 - x2);
        double B_BC = (y2 - (y3 - y2 / x3 - x2) * x2);

        if ((y0 > (A_AB * x0 + B_AB) && y0 > (A_AC * x0 + B_AC) && y0 > (A_BC * x0 + B_BC))) Console.WriteLine("Point is inside of the triangle.");
        else if (y0 == (A_AB * x0 + B_AB) || y0 == (A_AC * x0 + B_AC) || y0 == (A_BC * x0 + B_BC)) Console.WriteLine("Point is on the side of the triangle");
        else Console.WriteLine("Point is outside of the triangle.");

        Console.ReadKey(true);
    }
}

Something is wrong, cause it's hard to hit space in the triangle. Would somebody analyse this? My brain is burning.

Thanks.

Does it make it any difference when x and y arent integers?.. — Sayse, Dec 04 '16 at 14:42
[How to determine if a point is in a triangle?](http://stackoverflow.com/q/2049582/1070452) — Ňɏssa Pøngjǣrdenlarp, Dec 04 '16 at 15:04
Using a debugger to analyze your code can save your brain from a lot of burn. — Cody Gray - on strike, Dec 04 '16 at 17:03

score 0 · Answer 1 · answered Dec 04 '16 at 15:06

The problem is solved using the barycentric coordinate system like so:

    public static void Main(string[] args)
    {
        const double x1 = -4, y1 = 4;
        const double x2 = 4, y2 = -2;
        const double x3 = 6, y3 = 6;

        double x, y;
        x = 0;
        y = 1;

        double a = ((y2 - y3)*(x - x3) + (x3 - x2)*(y - y3)) / ((y2 - y3)*(x1 - x3) + (x3 - x2)*(y1 - y3));
        double b = ((y3 - y1)*(x - x3) + (x1 - x3)*(y - y3)) / ((y2 - y3)*(x1 - x3) + (x3 - x2)*(y1 - y3));
        double c = 1 - a - b;

        if (a == 0 || b == 0 || c == 0) Console.WriteLine("Point is on the side of the triangle");
        else if (a >= 0 && a <= 1 && b >= 0 && b<= 1 && c >= 0 && c <= 1) Console.WriteLine("Point is inside of the triangle.");
        else Console.WriteLine("Point is outside of the triangle.");
    }

C# - is the point in triangle?

1 Answers1