About custom cutting and drawing lines

I plan to create a plane parallel to the screen and draw lines on this plane and then do the cutting operation.
I have two questions.
How do I get the normal vector of the current window plane?
How does vtk create a plane parallel to the window screen?
I think if the first problem is solved, the second problem should also be solved.

I have solved this problem, vtkCamera has two functions, GeteyePlaneNomal and GetViewPlaneNomal, I confused them