  In this paper, we propose an algorithm, named hashing-based non-maximum
suppression (HNMS) to efficiently suppress the non-maximum boxes for object
detection. Non-maximum suppression (NMS) is an essential component to suppress
the boxes at closely located locations with similar shapes. The time cost tends
to be huge when the number of boxes becomes large, especially for crowded
scenes. The basic idea of HNMS is to firstly map each box to a discrete code
(hash cell) and then remove the boxes with lower confidences if they are in the
same cell. Considering the intersection-over-union (IoU) as the metric, we
propose a simple yet effective hashing algorithm, named IoUHash, which
guarantees that the boxes within the same cell are close enough by a lower IoU
bound. For two-stage detectors, we replace NMS in region proposal network with
HNMS, and observe significant speed-up with comparable accuracy. For one-stage
detectors, HNMS is used as a pre-filter to speed up the suppression with a
large margin. Extensive experiments are conducted on CARPK, SKU-110K,
CrowdHuman datasets to demonstrate the efficiency and effectiveness of HNMS.
Code is released at \url{https://github.com/microsoft/hnms.git}.
