This paper deals with a problem in computer vision: how to recover the
motion of a disk, thrown toward an observer, from a sequence of image
s acquired by a pinhole camera. Polynomial equations describing the mo
tion are established, and techniques from algebraic geometry are used
to show that in general a sequence of three images is sufficient for t
he recovery of motion of the disk when it is known to be moving along
a straight Line, and that five images suffice in the more general situ
ation in which the disk travels in a gravitational field. Examples are
worked out in detail to illustrate our results. (C) 1996 Academic Pre
ss, Inc.