  Safety is a central requirement for autonomous system operation across
domains. Hamilton-Jacobi (HJ) reachability analysis can be used to construct
"least-restrictive" safety filters that result in infrequent, but often
extreme, control overrides. In contrast, control barrier function (CBF) methods
apply smooth control corrections to guard the system against an often
conservative safety boundary. This paper provides an online scheme to construct
an implicit CBF through HJ reach-avoid differential dynamic programming in a
receding-horizon framework, enabling smooth safety filtering with infinite-time
safety guarantees. Simulations with the Dubins car and 5D bicycle dynamics
demonstrate the scheme's ability to preserve safety smoothly without the
conservativeness of handcrafted CBFs.
