public static bool isForeFace(Point3d pt1, Point3d pt2, Point3d pt3) // pts on a plane
{
Vector3d v1 = new Vector3d(pt2, pt1);
Vector3d v2 = new Vector3d(pt2, pt3);
Vector3d v = v1.CrossProduct(v2);
return(v.DotProduct(new Vector3d(0, 0, 1)) < 0);
}
public static bool isBackFace(Point3d pt1, Point3d pt2, Point3d pt3)
{
Vector3d v1 = new Vector3d(pt2, pt1);
Vector3d v2 = new Vector3d(pt2, pt3);
Vector3d v = v1.CrossProduct(v2);
return(v.DotProduct(new Vector3d(0, 0, 1)) > 0);
}