A Two-Stage Triplet Network Training Framework for Image Retrieval

In this paper, we propose a novel framework for instance-level image retrieval. Recent methods focus on fine-tuning the Convolutional Neural Network (CNN) via a Siamese architecture to improve off-the-shelf CNN features. They generally use the ranking loss to train such networks, and do not take full use of supervised information for better network training, especially with more complex neural architectures. To solve this, we propose a two-stage triplet network training framework, which mainly consists of two stages. First, we propose a Double-Loss Regularized Triplet Network (DLRTN), which extends basic triplet network by attaching the classification sub-network, and is trained via simultaneously optimizing two different types of loss functions. Double-loss functions of DLRTN aim at specific retrieval task and can jointly boost the discriminative capability of DLRTN from different aspects via supervised learning. Second, considering feature maps of the last convolution layer extracted from DLRTN and regions detected from the region proposal network as the input, we then introduce the Regional Generalized-Mean Pooling (RGMP) layer for the triplet network, and re-train this network to learn pooling parameters. Through RGMP, we pool feature maps for each region and aggregate features of different regions from each image to Regional Generalized Activations of Convolutions (R-GAC) as final image representation. R-GAC is capable of generalizing existing Regional Maximum Activations of Convolutions (R-MAC) and is thus more robust to scale and translation. We conduct the experiment on six image retrieval datasets including standard benchmarks and recently introduced INSTRE dataset. Extensive experimental results demonstrate the effectiveness of the proposed framework.