  Aerial imagery has been increasingly adopted in mission-critical tasks, such
as traffic surveillance, smart cities, and disaster assistance. However,
identifying objects from aerial images faces the following challenges: 1)
objects of interests are often too small and too dense relative to the images;
2) objects of interests are often in different relative sizes; and 3) the
number of objects in each category is imbalanced. A novel network structure,
Points Estimated Network (PENet), is proposed in this work to answer these
challenges. PENet uses a Mask Resampling Module (MRM) to augment the imbalanced
datasets, a coarse anchor-free detector (CPEN) to effectively predict the
center points of the small object clusters, and a fine anchor-free detector
FPEN to locate the precise positions of the small objects. An adaptive merge
algorithm Non-maximum Merge (NMM) is implemented in CPEN to address the issue
of detecting dense small objects, and a hierarchical loss is defined in FPEN to
further improve the classification accuracy. Our extensive experiments on
aerial datasets visDrone and UAVDT showed that PENet achieved higher precision
results than existing state-of-the-art approaches. Our best model achieved 8.7%
improvement on visDrone and 20.3% on UAVDT.
