Placing Arrows in Directed Graph Drawings
  We consider the problem of placing arrow heads in directed graph drawings without them overlapping other drawn objects. This gives drawings where edge directions can be deduced unambiguously. We show hardness of the problem, present exact and heuristic algorithms, and report on a practical study. 
Carla Binucci, Markus Chimani, Walter Didimo, Giuseppe Liotta,   Fabrizio Montecchiani
cs.DS