A
cycloid
is the trace of a point on a disk when this disk rolls along a line.
The shape of the cycloid depends on two parameters, the radius r of
the circle and the distance d of the point generating the cycloid to the
center of rolling disk. The mathematical expression of a
cycloid is
Cycloid[r, d](t) = (r t + d sin(t),
r - d cos(t)).
