Class-Incremental Learning: A Survey

TL;DR

This survey reviews recent advances in class-incremental learning, addressing catastrophic forgetting, evaluating 17 methods on benchmarks, and emphasizing fair comparison protocols considering memory constraints.

Contribution

It provides a comprehensive overview of CIL methods, a unified evaluation framework, and highlights the importance of fair comparison considering memory budgets.

Findings

Different algorithms exhibit varied strengths and weaknesses.

Memory budget significantly impacts model performance and comparison fairness.

Memory-agnostic measures provide more reliable evaluation.

Abstract

Deep models, e.g., CNNs and Vision Transformers, have achieved impressive achievements in many vision tasks in the closed world. However, novel classes emerge from time to time in our ever-changing world, requiring a learning system to acquire new knowledge continually. Class-Incremental Learning (CIL) enables the learner to incorporate the knowledge of new classes incrementally and build a universal classifier among all seen classes. Correspondingly, when directly training the model with new class instances, a fatal problem occurs -- the model tends to catastrophically forget the characteristics of former ones, and its performance drastically degrades. There have been numerous efforts to tackle catastrophic forgetting in the machine learning community. In this paper, we survey comprehensively recent advances in class-incremental learning and summarize these methods from several aspects. We also provide a rigorous and unified evaluation of 17 methods in benchmark image classification tasks to find out the characteristics of different algorithms empirically. Furthermore, we notice that the current comparison protocol ignores the influence of memory budget in model storage, which may result in unfair comparison and biased results. Hence, we advocate fair comparison by aligning the memory budget in evaluation, as well as several memory-agnostic performance measures. The source code is available at https://github.com/zhoudw-zdw/CIL_Survey/