  A massive particle under the influence of a constant gravitational force that
is bouncing inside an ideal reflecting mirror described by some function $f(x)$
is considered. For the associated flight trajectories we derive the parametric
curves, named foci curves. All foci points of the parabolas for a given initial
position and energy lie on these curves. From these foci curves the associated
flight parabola envelopes are derived resulting, together with the mirror
surface, in a confined domain for all possible particle trajectories in the
non-periodic orbit case. The general results are briefly discussed and
visualized for three concrete mirror surfaces.
