Projective billiards are bounded domains whose boundary is endowed with a field of transverse lines called projective lines (see here, here or here for more details).
The following interactive animation shows polygonal examples of projective billiards, in which
You can:
You can observe the following properties (which are proved here):
PROP. 1: The trajectories of the projective billiard called right-spherical are periodic of period 3 in the case of a triangle.
PROP. 2: The trajectories of the projective billiards called centrally polygonal are: