public Plane(Vec3 p1, Vec3 p2, Vec3 p3)
{
Vec3 v1 = p2 - p1;
Vec3 v2 = p3 - p1;
Vec3 plane_norm = v1.Cross(v2);
plane_norm.Normalize();
A = plane_norm.X;
B = plane_norm.Y;
C = plane_norm.Z;
D = -A * p1.X - B * p1.Y - C * p1.Z;
}