OpenCIL: Benchmarking Out-of-Distribution Detection in Class-Incremental Learning

TL;DR

This paper introduces OpenCIL, a comprehensive benchmark for evaluating out-of-distribution detection in class-incremental learning models, highlighting biases and proposing a new regularization method to improve detection performance.

Contribution

It establishes the first large-scale benchmark for OOD detection in CIL, evaluates multiple models and methods, and proposes the Bi-directional Energy Regularization to mitigate biases.

Findings

CIL models are biased towards OOD samples and new classes in open environments.

The proposed BER method improves OOD detection by mitigating biases.

Open-source code and datasets facilitate future research.

Abstract

Class incremental learning (CIL) aims to learn a model that can not only incrementally accommodate new classes, but also maintain the learned knowledge of old classes. Out-of-distribution (OOD) detection in CIL is to retain this incremental learning ability, while being able to reject unknown samples that are drawn from different distributions of the learned classes. This capability is crucial to the safety of deploying CIL models in open worlds. However, despite remarkable advancements in the respective CIL and OOD detection, there lacks a systematic and large-scale benchmark to assess the capability of advanced CIL models in detecting OOD samples. To fill this gap, in this study we design a comprehensive empirical study to establish such a benchmark, named $\textbf{OpenCIL}$. To this end, we propose two principled frameworks for enabling four representative CIL models with 15 diverse OOD detection methods, resulting in 60 baseline models for OOD detection in CIL. The empirical evaluation is performed on two popular CIL datasets with six commonly-used OOD datasets. One key observation we find through our comprehensive evaluation is that the CIL models can be severely biased towards the OOD samples and newly added classes when they are exposed to open environments. Motivated by this, we further propose a new baseline for OOD detection in CIL, namely Bi-directional Energy Regularization ($\textbf{BER}$), which is specially designed to mitigate these two biases in different CIL models by having energy regularization on both old and new classes. Its superior performance is justified in our experiments. All codes and datasets are open-source at https://github.com/mala-lab/OpenCIL.