  Open-vocabulary object detection (OVOD) aims at localizing and recognizing
visual objects from novel classes unseen at the training time. Whereas,
empirical studies reveal that advanced detectors generally assign lower scores
to those novel instances, which are inadvertently suppressed during inference
by commonly adopted greedy strategies like Non-Maximum Suppression (NMS),
leading to sub-optimal detection performance for novel classes. This paper
systematically investigates this problem with the commonly-adopted two-stage
OVOD paradigm. Specifically, in the region-proposal stage, proposals that
contain novel instances showcase lower objectness scores, since they are
treated as background proposals during the training phase. Meanwhile, in the
object-classification stage, novel objects share lower region-text similarities
(i.e., classification scores) due to the biased visual-language alignment by
seen training samples. To alleviate this problem, this paper introduces two
advanced measures to adjust confidence scores and conserve erroneously
dismissed objects: (1) a class-agnostic localization quality estimate via
overlap degree of region/object proposals, and (2) a text-guided visual
similarity estimate with proxy prototypes for novel classes. Integrated with
adjusting techniques specifically designed for the region-proposal and
object-classification stages, this paper derives the aggregated confidence
estimate for the open-vocabulary object detection paradigm (AggDet). Our AggDet
is a generic and training-free post-processing scheme, which consistently
bolsters open-vocabulary detectors across model scales and architecture
designs. For instance, AggDet receives 3.3% and 1.5% gains on OV-COCO and
OV-LVIS benchmarks respectively, without any training cost.
