Projection Regret: Reducing Background Bias for Novelty Detection via Diffusion Models

Sungik Choi, Hankook Lee, Honglak Lee, Moontae Lee

Keywords: Novelty detection, out-of-distribution detection, consistency models, diffusion models, score-based generative models

TL;DR: We propose an efficient out-of-distribution detection model based on pre-trained diffusion model and the LPIPS metric

Abstract: Novelty detection is a fundamental task of machine learning which aims to detect abnormal (*i.e.* out-of-distribution (OOD)) samples. Since diffusion models have recently emerged as the de facto standard generative framework with surprising generation results, novelty detection via diffusion models has also gained much attention. Recent methods have mainly utilized the reconstruction property of in-distribution samples. However, they often suffer from detecting OOD samples that share similar background information to the in-distribution data. Based on our observation that diffusion models can *project* any sample to an in-distribution sample with similar background information, we propose *Projection Regret (PR)*, an efficient novelty detection method that mitigates the bias of non-semantic information. To be specific, PR computes the perceptual distance between the test image and its diffusion-based projection to detect abnormality. Since the perceptual distance often fails to capture semantic changes when the background information is dominant, we cancel out the background bias by comparing it against recursive projections. Extensive experiments demonstrate that PR outperforms the prior art of generative-model-based novelty detection methods by a significant margin.

Submission Number: 9354