这一篇其实并不是提出什么新的东西，而且是做了点类似综述的技术调用实验。省流：T-normalization最好用

摘要

现状：Existing solutions usually involve class-balancing strategies, e.g. by loss re-weighting, data re-sampling, or transfer learning from head- to tail-classes, but most of them adhere to the scheme of jointly learning representations and classifiers.
做法：we decouple the learning procedure into representation learning and classification, and systematically explore how different balancing strategies affect them for long-tailed recognition.
结论：The findings are surprising: （1）data imbalance might not be an issue in learning high-quality representations; （2）with representations learned with the simplest instance-balanced sampling, it is also possible to achieve strong long-tailed recognition ability by adjusting only the classifier.

Classification For LONG-TAILED RECOGNITION

cRT（Classifier Re-training）
re-train the classifier with class-balanced sampling. That is, keeping the representations fixed, we ramdomly re-initialize and optimize the classifier weights W and b for a small number of epochs using class-balanced sampling.

NCM （Nearest Class Mean classifier)

1. compute the mean feature representation for each class on the training set
2. perform nearest neightbor search either using cosine similarity or the Euclidean distance computed on L2-normalized mean features.
3. the cosine similarity alleviates the weight imbalance problem via its inherent normalization.

t-normalized（t-normalized classifier)
inspired: after joint training with instance-balanced sampling, the norms of the weights || wj || are correlated with the cardinality of the classes nj, while, after fine-tuning the classifiers using class-balanced sampling, the norms of the classifier weights tend to be more similar.
做法：adjusting the classifier weight norms directly through the following t-normalization procedure.

LWS（Learnable weight scaling)
Another way of interpreting t-normalization would be to think of it as a re-scaling of the magnitude for each classifier wi keeping the direction unchanged.(将T-norm 转化为一种对每个分类器权重大小的重新缩放)

Sampling Strategies

Instance-balanced sampling：the most common way of sampling data, where each training example has equal probability of being selected.

Class-balanced sampling：each class has an equal probability of being selected. One can see this as a two-stage sampling strategy, where first a class is selected uniformly from the set of classes, and then an instance from that class is subsequently uniformly sampled.

Square-root sampling
A number of variants of the previous sampling strategies have been explored.

Progressive-balanced sampling
This involves first using instance-balanced sampling for a number of epochs, and then class-balanced sampling for the last epochs.

Experiments

As illustrated in Fig.4, this yields a wider classfication boundary in feature space, allowing the classifier to have much higher accuracy on data-rich classes, but hurting data-scarce classes. t-normalized classifiers alleviate this issue to some extent by providing more balanced classifier weight magnitudes.

官方提供的代码链接： https://github.com/facebookresearch/classifier-balancing.