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Title: A Closer Look at Few-shot Classification

Paper content:
Few-shot classiﬁcation aims to learn a classiﬁer to recognize unseen classes during training with limited labeled examples.
While signiﬁcant progress has been made, the growing complexity of network designs, meta-learning algorithms, and differences in implementation details make a fair comparison difﬁcult.
In this paper, we present
1) a consistent comparative analysis of several representative few-shot classiﬁcation algorithms, with results showing that deeper backbones signiﬁcantly reduce the gap across methods including the baseline,
2) a slightly modiﬁed baseline method that surprisingly achieves competitive performance when compared with the state-of-the-art on both the mini-ImageNet and the CUB datasets, and
3) a new experimental setting for evaluating the cross-domain generalization ability for few-shot classiﬁcation algorithms.
Our results reveal that reducing intra-class variation is an important factor when the feature backbone is shallow, but not as critical when using deeper backbones.
In a realistic, cross-domain evaluation setting, we show that a baseline method with a standard ﬁne-tuning practice compares favorably against other state-of-the-art few-shot learning algorithms.
Deep learning models have achieved state-of-the-art performance on visual recognition tasks such as image classification.
The strong performance, however, heavily relies on training a network with abundant labeled instances with diverse visual variations (e.g., thousands of examples for each new class even with pre-training on large-scale dataset with base classes).
The human annotation cost as well as the scarcity of data in some classes (e.g., rare species) significantly limit the applicability of current vision systems to learn new visual concepts efficiently.
In contrast, the human visual systems can recognize new classes with extremely few labeled examples.
It is thus of great interest to learn to generalize to new classes with a limited amount of labeled examples for each novel class.The problem of learning to generalize to unseen classes during training, known as few-shot classification, has attracted considerable attention BID29 ; BID27 ; BID6 ; BID25 ; BID28 ; BID9 ; BID24 .
One promising direction to few-shot classification is the meta-learning paradigm where transferable knowledge is extracted and propagated from a collection of tasks to prevent overfitting and improve generalization.
Examples include model initialization based methods BID25 ; BID6 , metric learning methods BID29 ; BID27 ; BID28 , and hallucination based methods BID0 ; BID11 ; BID31 .
Another line of work BID10 ; BID24 also demonstrates promising results by directly predicting the weights of the classifiers for novel classes.Limitations.
While many few-shot classification algorithms have reported improved performance over the state-of-the-art, there are two main challenges that prevent us from making a fair comparison and measuring the actual progress.
First, the discrepancy of the implementation details among multiple few-shot learning algorithms obscures the relative performance gain.
The performance of baseline approaches can also be significantly under-estimated (e.g., training without data augmentation).
Second, while the current evaluation focuses on recognizing novel class with limited training examples, these novel classes are sampled from the same dataset.
The lack of domain shift between the base and novel classes makes the evaluation scenarios unrealistic.Our work.
In this paper, we present a detailed empirical study to shed new light on the few-shot classification problem.
First, we conduct consistent comparative experiments to compare several representative few-shot classification methods on common ground.
Our results show that using a deep backbone shrinks the performance gap between different methods in the setting of limited domain differences between base and novel classes.
Second, by replacing the linear classifier with a distance-based classifier as used in BID10 ; BID24 , the baseline method is surprisingly competitive to current state-of-art meta-learning algorithms.
Third, we introduce a practical evaluation setting where there exists domain shift between base and novel classes (e.g., sampling base classes from generic object categories and novel classes from fine-grained categories).
Our results show that sophisticated few-shot learning algorithms do not provide performance improvement over the baseline under this setting.
Through making the source code and model implementations with a consistent evaluation setting publicly available, we hope to foster future progress in the field.
1 Our contributions.1.
We provide a unified testbed for several different few-shot classification algorithms for a fair comparison.
Our empirical evaluation results reveal that the use of a shallow backbone commonly used in existing work leads to favorable results for methods that explicitly reduce intra-class variation.
Increasing the model capacity of the feature backbone reduces the performance gap between different methods when domain differences are limited.2.
We show that a baseline method with a distance-based classifier surprisingly achieves competitive performance with the state-of-the-art meta-learning methods on both mini-ImageNet and CUB datasets.3.
We investigate a practical evaluation setting where base and novel classes are sampled from different domains.
We show that current few-shot classification algorithms fail to address such domain shifts and are inferior even to the baseline method, highlighting the importance of learning to adapt to domain differences in few-shot learning.
In this paper, we have investigated the limits of the standard evaluation setting for few-shot classification.
Through comparing methods on a common ground, our results show that the Baseline++ model is competitive to state of art under standard conditions, and the Baseline model achieves competitive performance with recent state-of-the-art meta-learning algorithms on both CUB and mini-ImageNet benchmark datasets when using a deeper feature backbone.
Surprisingly, the Baseline compares favorably against all the evaluated meta-learning algorithms under a realistic scenario where there exists domain shift between the base and novel classes.
By making our source code publicly available, we believe that community can benefit from the consistent comparative experiments and move forward to tackle the challenge of potential domain shifts in the context of few-shot learning.

assistant: text: A detailed empirical study in few-shot classification that revealing challenges in standard evaluation setting and showing a new direction.