  This paper proposes Elastic Tracker, a flexible trajectory planning framework
that can deal with challenging tracking tasks with guaranteed safety and
visibility. Firstly, an object detection and intension-free motion prediction
method is designed. Then an occlusion-aware path finding method is proposed to
provide a proper topology. A smart safe flight corridor generation strategy is
designed with the guiding path. An analytical occlusion cost is evaluated.
Finally, an effective trajectory optimization approach enables to generate a
spatio-temporal optimal trajectory within the resultant flight corridor.
Particular formulations are designed to guarantee both safety and visibility,
with all the above requirements optimized jointly. The experimental results
show that our method works more robustly but with less computation than the
existing methods, even in some challenging tracking tasks.
