is-front-face

#include <stdio.h>
#include <stdlib.h>

struct Point{
    int x;
    int y;
};

// OpenGL front face.
int is_front_face(Point p0, Point p1, Point p2) {
    Point d0 = {p1.x - p0.x, p1.y - p0.y};
    Point d1 = {p2.x - p1.x, p2.y - p1.y};

    int det = d0.x * d1.y - d1.x * d0.y;

    if (det >= 0) {
        return 1;
    }
    else {
        return 0;
    }
}

int main(int argc, char* argv[]) {
    // OpenGL sequence.
    Point p0 = {0, 0};
    Point p1 = {1, 0};
    Point p2 = {1, 1};

    printf("%d\n", is_front_face(p0, p1, p2));

    // DirectX sequence.
    printf("%d\n", is_front_face(p2, p1, p0)); 

    return 0;
}