Rethinking preventing class-collapsing in metric learning with margin-based losses

TL;DR

This paper identifies a class collapse problem in margin-based metric learning losses and proposes a simple sampling modification that allows multiple sub-clusters per class, improving retrieval performance.

Contribution

The paper introduces a novel sampling strategy for margin-based losses that prevents class collapse and supports multiple sub-clusters within classes.

Findings

Improved retrieval accuracy on fine-grained datasets.

Qualitative evidence of better class separation in embeddings.

The method is compatible with various margin-based losses.

Abstract

Metric learning seeks perceptual embeddings where visually similar instances are close and dissimilar instances are apart, but learned representations can be sub-optimal when the distribution of intra-class samples is diverse and distinct sub-clusters are present. Although theoretically with optimal assumptions, margin-based losses such as the triplet loss and margin loss have a diverse family of solutions. We theoretically prove and empirically show that under reasonable noise assumptions, margin-based losses tend to project all samples of a class with various modes onto a single point in the embedding space, resulting in a class collapse that usually renders the space ill-sorted for classification or retrieval. To address this problem, we propose a simple modification to the embedding losses such that each sample selects its nearest same-class counterpart in a batch as the positive element in the tuple. This allows for the presence of multiple sub-clusters within each class. The adaptation can be integrated into a wide range of metric learning losses. The proposed sampling method demonstrates clear benefits on various fine-grained image retrieval datasets over a variety of existing losses; qualitative retrieval results show that samples with similar visual patterns are indeed closer in the embedding space.