  Instance-level Image Retrieval (IIR), or simply Instance Retrieval, deals
with the problem of finding all the images within an dataset that contain a
query instance (e.g. an object). This paper makes the first attempt that
tackles this problem using instance-discrimination based contrastive learning
(CL). While CL has shown impressive performance for many computer vision tasks,
the similar success has never been found in the field of IIR. In this work, we
approach this problem by exploring the capability of deriving discriminative
representations from pre-trained and fine-tuned CL models. To begin with, we
investigate the efficacy of transfer learning in IIR, by comparing
off-the-shelf features learned by a pre-trained deep neural network (DNN)
classifier with features learned by a CL model. The findings inspired us to
propose a new training strategy that optimizes CL towards learning IIR-oriented
features, by using an Average Precision (AP) loss together with a fine-tuning
method to learn contrastive feature representations that are tailored to IIR.
Our empirical evaluation demonstrates significant performance enhancement over
the off-the-shelf features learned from a pre-trained DNN classifier on the
challenging Oxford and Paris datasets.
