AANet: Aggregation and Alignment Network with Semi-hard Positive Sample
  Mining for Hierarchical Place Recognition
Feng Lu Lijun Zhang Shuting Dong Baifan Chen Chun Yuan
Visual place recognition (VPR) is one of the research hotspots in robotics,
which uses visual information to locate robots. Recently, the hierarchical
two-stage VPR methods have become popular in this field due to the trade-off
between accuracy and efficiency. These methods retrieve the top-k candidate
images using the global features in the first stage, then re-rank the
candidates by matching the local features in the second stage. However, they
usually require additional algorithms (e.g. RANSAC) for geometric consistency
verification in re-ranking, which is time-consuming. Here we propose a
Dynamically Aligning Local Features (DALF) algorithm to align the local
features under spatial constraints. It is significantly more efficient than the
methods that need geometric consistency verification. We present a unified
network capable of extracting global features for retrieving candidates via an
aggregation module and aligning local features for re-ranking via the DALF
alignment module. We call this network AANet. Meanwhile, many works use the
simplest positive samples in triplet for weakly supervised training, which
limits the ability of the network to recognize harder positive pairs. To
address this issue, we propose a Semi-hard Positive Sample Mining (ShPSM)
strategy to select appropriate hard positive images for training more robust
VPR networks. Extensive experiments on four benchmark VPR datasets show that
the proposed AANet can outperform several state-of-the-art methods with less
time consumption. The code is released at https://github.com/Lu-Feng/AANet.