Trivial Acceptance or Rejection of a Line Segment

Both endpoints of segment AB lie within the window, so the whole segment AB must lie within the window. Therefore, AB can be trivially accepted.

Both endpoints of segment CD lie entirely to one side of the window, so segment CD must lie entirely outside of the window. Therefore, CD can be trivially rejected.

From P. Asokarathinam -- see details 27.11.1996