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Title: INSTANCE CROSS ENTROPY FOR DEEP METRIC LEARNING

Paper content:
Loss functions play a crucial role in deep metric learning thus a variety of them have been proposed.
Some supervise the learning process by pairwise or tripletwise similarity constraints while others take the advantage of structured similarity information among multiple data points.
In this work, we approach deep metric learning from a novel perspective.
We propose instance cross entropy (ICE) which measures the difference between an estimated instance-level matching distribution and its ground-truth one.
ICE has three main appealing properties.
Firstly, similar to categorical cross entropy (CCE), ICE has clear probabilistic interpretation and exploits structured semantic similarity information for learning supervision.
Secondly, ICE is scalable to infinite training data as it learns on mini-batches iteratively and is independent of the training set size.
Thirdly, motivated by our relative weight analysis, seamless sample reweighting is incorporated.
It rescales samples’ gradients to control the differentiation degree over training examples instead of truncating them by sample mining.
In addition to its simplicity and intuitiveness, extensive experiments on three real-world benchmarks demonstrate the superiority of ICE.
Deep metric learning (DML) aims to learn a non-linear embedding function (a.k.a. distance metric) such that the semantic similarities over samples are well captured in the feature space (Tadmor et al., 2016; Sohn, 2016) .
Due to its fundamental function of learning discriminative representations, DML has diverse applications, such as image retrieval (Song et al., 2016) , clustering (Song et al., 2017) , verification (Schroff et al., 2015) , few-shot learning (Vinyals et al., 2016) and zero-shot learning (Bucher et al., 2016) .
A key to DML is to design an effective and efficient loss function for supervising the learning process, thus significant efforts have been made (Chopra et al., 2005; Schroff et al., 2015; Sohn, 2016; Song et al., 2016; Law et al., 2017; Wu et al., 2017) .
Some loss functions learn the embedding function from pairwise or triplet-wise relationship constraints (Chopra et al., 2005; Schroff et al., 2015; Tadmor et al., 2016) .
However, they are known to not only suffer from an increasing number of non-informative samples during training, but also incur considering only several instances per loss computation.
Therefore, informative sample mining strategies are proposed (Schroff et al., 2015; Wu et al., 2017; Wang et al., 2019b) .
Recently, several methods consider semantic relations among multiple examples to exploit their similarity structure (Sohn, 2016; Song et al., 2016; Law et al., 2017) .
Consequently, these structured losses achieve better performance than pairwise and triple-wise approaches.
In this paper, we tackle the DML problem from a novel perspective.
Specifically, we propose a novel loss function inspired by CCE.
CCE is well-known in classification problems owing to the fact that it has an intuitive probabilistic interpretation and achieves great performance, e.g., ImageNet classification (Russakovsky et al., 2015) .
However, since CCE learns a decision function which predicts the class label of an input, it learns class-level centres for reference (Zhang et al., 2018; Wang et al., 2017a) .
Therefore, CCE is not scalable to infinite classes and cannot generalise well when it is directly applied to DML (Law et al., 2017) .
With scalability and structured information in mind, we introduce instance cross entropy (ICE) for DML.
It learns an embedding function by minimising the cross entropy between a predicted instance-level matching distribution and its corresponding ground-truth.
In comparison with CCE, given a query, CCE aims to maximise its matching probability with the class-level context vector (weight vector) of its ground-truth class, whereas ICE targets at maximising its matching probability with it similar instances.
As ICE does not learn class-level context vectors, it is scalable to infinite training classes, which is an intrinsic demand of DML.
Similar to (Sohn, 2016; Song et al., 2016; Law et al., 2017; Goldberger et al., 2005; Wu et al., 2018) , ICE is a structured loss as it also considers all other instances in the mini-batch of a given query.
We illustrate ICE with comparison to other structured losses in Figure 1 .
A common challenge of instance-based losses is that many training examples become trivial as model improves.
Therefore, we integrate seamless sample reweighting into ICE, which functions similarly with various sample mining schemes (Sohn, 2016; Schroff et al., 2015; Shi et al., 2016; Wu et al., 2017) .
Existing mining methods require either separate time-consuming process, e.g., class mining (Sohn, 2016) , or distance thresholds for data pruning (Schroff et al., 2015; Shi et al., 2016; Wu et al., 2017) .
Instead, our reweighting scheme works without explicit data truncation and mining.
It is motivated by the relative weight analysis between two examples.
The current common practice of DML is to learn an angular embedding space by projecting all features to a unit hypersphere surface (Song et al., 2017; Law et al., 2017; MovshovitzAttias et al., 2017) .
We identify the challenge that without sample mining, informative training examples cannot be differentiated and emphasised properly because the relative weight between two samples is strictly bounded.
We address it by sample reweighting, which rescales samples' gradient to control the differentiation degree among them.
Finally, for intraclass compactness and interclass separability, most methods (Schroff et al., 2015; Song et al., 2016; Tadmor et al., 2016; Wu et al., 2017) use distance thresholds to decrease intraclass variances and increase interclass distances.
In contrast, we achieve the target from a perspective of instance-level matching probability.
Without any distance margin constraint, ICE makes no assumptions about the boundaries between different classes.
Therefore, ICE is easier to apply in applications where we have no prior knowledge about intraclass variances.
Our contributions are summarised: (1) We approach DML from a novel perspective by taking in the key idea of matching probability in CCE.
We introduce ICE, which is scalable to an infinite number of training classes and exploits structured information for learning supervision.
(2) A seamless sample reweighting scheme is derived for ICE to address the challenge of learning an embedding subspace by projecting all features to a unit hypersphere surface.
(3) We show the superiority of ICE by comparing with state-of-the-art methods on three real-world datasets.
We remark that Prototypical Networks, Matching Networks (Vinyals et al., 2016) and NCA are also scalable and do not require distance thresholds.
Therefore, they are illustrated and differentiated in Figure 1 .
Matching Networks are designed specifically for one-shot learning.
Similarly, (Triantafillou et al., 2017) design mAP-SSVM and mAP-DLM for few-shot learning, which directly optimises the retrieval performance mAP when multiple positives exist.
FastAP (Cakir et al., 2019) is similar to (Triantafillou et al., 2017) and optimises the ranked-based average precision.
Instead, ICE processes one positive at a time.
Beyond, the setting of few-shot learning is different from deep metric learning: Each mini-batch is a complete subtask and contains a support set as training data and a query set as validation data in few-shot learning.
Few-shot learning applies episodic training in practice.
Remarkably, TADAM formulates instances versus class centres and also has a metric scaling parameter for adjusting the impact of different class centres.
Contrastively, ICE adjusts the influence of other instances.
Furthermore, ours is not exactly distance metric scaling since we simply apply naive cosine similarity as the distance metric at the testing stage.
That is why we interpret it as a weighting scheme during training.
In this paper, we propose a novel instance-level softmax regression framework, named instance cross entropy, for deep metric learning.
Firstly, the proposed ICE has clear probability interpretation and exploits structured semantic similarity information among multiple instances.
Secondly, ICE is scalable to infinitely many classes, which is required by DML.
Thirdly, ICE has only one weight scaling hyper-parameter, which works as mining informative examples and can be easily selected via cross-validation.
Finally, distance thresholds are not applied to achieve intraclass compactness and interclass separability.
This indicates that ICE makes no assumptions about intraclass variances and the boundaries between different classes.
Therefore ICE owns general applicability.

assistant: text: We propose instance cross entropy (ICE) which measures the difference between an estimated instance-level matching distribution and its ground-truth one.