public static Point3DCollection ExtendLineBothDirections(Point3DCollection inputline, double offsetamount)
{
Vector3D v1 = inputline[0] - inputline[inputline.Count - 1];
Vector3D v2 = inputline[inputline.Count - 1] - inputline[0];
v1.Normalize();
v2.Normalize();
Point3D startPoint = inputline[0];
Point3D endPoint = inputline[inputline.Count - 1];
startPoint.Offset(offsetamount * v1.X, offsetamount * v1.Y, offsetamount * v1.Z);
inputline.Insert(0, startPoint);
endPoint.Offset(offsetamount * v2.X, offsetamount * v2.Y, offsetamount * v2.Z);
inputline.Add(endPoint);
return(inputline);
}