  We study three orientation-based shape descriptors on a set of continuously
moving points: the first principal component, the smallest oriented bounding
box and the thinnest strip. Each of these shape descriptors essentially defines
a cost capturing the quality of the descriptor and uses the orientation that
minimizes the cost. This optimal orientation may be very unstable as the points
are moving, which is undesirable in many practical scenarios. If we bound the
speed with which the orientation of the descriptor may change, this may lower
the quality of the resulting shape descriptor. In this paper we study the
trade-off between stability and quality of these shape descriptors. We first
show that there is no stateless algorithm, an algorithm that keeps no state
over time, that both approximates the minimum cost of a shape descriptor and
achieves continuous motion for the shape descriptor. On the other hand, if we
can use the previous state of the shape descriptor to compute the new state, we
can define "chasing" algorithms that attempt to follow the optimal orientation
with bounded speed. We show that, under mild conditions, chasing algorithms
with sufficient bounded speed approximate the optimal cost at all times for
oriented bounding boxes and strips. The analysis of such chasing algorithms is
challenging and has received little attention in literature, hence we believe
that our methods used in this analysis are of independent interest.
