Generalized Out-of-Distribution Detection: A Survey

Jingkang YangKaiyang ZhouYixuan LiZiwei Liu

article2021IJCV1,542 citations

Establishes a unified taxonomy that connects out-of-distribution detection with anomaly detection, novelty detection, open set recognition, and outlier detection to clarify problem boundaries and guide future methodology.

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Modern machine learning models are typically trained under the closed-world assumption that operational data will match training data. In open-world deployments, however, models frequently encounter out-of-distribution inputs that can cause dangerous, overconfident misclassifications. This issue poses serious safety and operational risks in critical systems such as autonomous driving and automated diagnostics. Although related sub-fields—including anomaly detection, novelty detection, open set recognition, out-of-distribution detection, and outlier detection—address this core challenge, they have historically evolved in isolation with fragmented terminology and inconsistent benchmarks.

The article establishes a unified framework, termed generalized out-of-distribution detection, to systematically categorize and compare these five distinct sub-fields. It specifically aims to clarify their theoretical differences, survey the rapid methodological developments in out-of-distribution detection, and benchmark representative techniques under standardized experimental conditions.

The authors analyze the problem space using four clear criteria: whether the distribution shift is sensory (covariate) or semantic (label-based), whether the target distribution contains single or multiple classes, whether in-distribution classification must be preserved, and whether learning is inductive (train-then-test) or transductive (evaluating all data together). Using this taxonomy, the article conducts a comprehensive literature review across classification-based, density-based, distance-based, and reconstruction-based methods. To support fair comparison, the authors evaluate these methods on standardized vision benchmarks using the OpenOOD platform with unified backbones and training configurations.

The analysis yields four central findings. First, simple model uncertainty and data augmentation strategies during training, such as PixMix and CutMix, prove exceptionally effective, with PixMix achieving a leading 93.1% AUROC score on challenging near-distribution benchmarks. Second, inference-only, post-hoc methods—such as deep nearest neighbors (KNN) and activation rectifications—consistently match or outperform specialized training-heavy approaches while avoiding retraining overhead. Third, exposing models to auxiliary outlier datasets during training provides marginal practical advantage over modern outlier-free, post-hoc methods, while introducing severe risks of training contamination. Finally, specialized anomaly detection methods designed for pixel-level irregularities transfer effectively to identifying far out-of-distribution samples.

These findings have direct operational and economic implications for deploying reliable artificial intelligence systems. Organizations do not necessarily need to invest heavy compute budgets into retraining models with massive synthetic or auxiliary datasets to achieve safe decision-making. Instead, deploying modular, post-hoc inference filters over well-regularized base classifiers offers a cost-effective path to significantly mitigate the risk of high-confidence failures on novel data.

Decision-makers should prioritize integrating non-parametric, post-hoc detection algorithms alongside standard data-augmentation pipelines when deploying safety-critical vision systems. Future technical efforts should move away from relying on uncurated auxiliary outlier sets and focus on full-spectrum detection, which ensures systems simultaneously generalize to harmless sensory variations while reliably rejecting true semantic novelties. Further validation should be conducted on large-scale benchmarks like ImageNet and real-world multi-modal tasks.

Confidence in these findings is high for standard computer vision classification benchmarks, supported by rigorous, standardized evaluations in OpenOOD. However, stakeholders should remain cautious when translating these conclusions directly to large language models, dense perception tasks like segmentation, or domain-specific settings where benchmark contamination and fine-grained semantic overlaps remain active research challenges.

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Abstract

Out-of-distribution (OOD) detection is critical to ensuring the reliability and safety of machine learning systems. For instance, in autonomous driving, we would like the driving system to issue an alert and hand over the control to humans when it detects unusual scenes or objects that it has never seen during training time and cannot make a safe decision. The term, OOD detection, first emerged in 2017 and since then has received increasing attention from the research community, leading to a plethora of methods developed, ranging from classification-based to density-based to distance-based ones. Meanwhile, several other problems, including anomaly detection (AD), novelty detection (ND), open set recognition (OSR), and outlier detection (OD), are closely related to OOD detection in terms of motivation and methodology. Despite common goals, these topics develop in isolation, and their subtle differences in definition and problem setting often confuse readers and practitioners. In this survey, we first present a unified framework called generalized OOD detection, which encompasses the five aforementioned problems, i.e., AD, ND, OSR, OOD detection, and OD. Under our framework, these five problems can be seen as special cases or sub-tasks, and are easier to distinguish. We then review each of these five areas by summarizing their recent technical developments, with a special focus on OOD detection methodologies. We conclude this survey with open challenges and potential research directions.

Table of Contents

  • 1 Introduction
  • 2 Generalized OOD Detection
  • 2.1 Anomaly Detection
  • 2.2 Novelty Detection
  • 2.3 Open Set Recognition
  • 2.4 Out-of-Distribution Detection
  • 2.5 Outlier Detection
  • 2.6 Related Topics
  • 2.7 Organization of Remaining Sections
  • 3 OOD Detection: Methodology
  • 3.1 Classification-based Methods
  • 3.1.1 Output-based Methods
  • 3.1.2 Methods with Outlier Exposure
  • 3.1.3 Gradient-based Methods
  • 3.1.4 Bayesian Models
  • 3.1.5 OOD Detection for Foundation Models
  • 3.2 Density-based Methods
  • 3.3 Distance-based Methods
  • 3.4 Reconstruction-based Methods
  • 3.5 Theoretical Analysis
  • 3.6 Discussion
  • 4 Methodologies from Other Sub-tasks
  • 4.1 Open Set Recognition
  • 4.2 Anomaly Detection & Novelty Detection
  • 4.3 Outlier Detection
  • 5 Benchmarks and Experiments
  • 5.1 Benchmarks and Metrics
  • 5.2 Experimental Setup
  • 5.3 Experimental Results and Findings
  • 5.4 Exclusion of Covariate-Shift Detection
  • 6 Challenges and Future Directions
  • 6.1 Challenges
  • 6.2 Future Directions
  • 7 Conclusion
  • References

Knowls

  1. Knowl 1 — Taxonomy of Generalized Out-of-Distribution Detection

    definition

    Generalized out-of-distribution (OOD) detection is a unified taxonomy that categorizes open-world detection problems into five sub-tasks: Anomaly Detection (AD), Novelty Detection (ND), Open Set Recognition (OSR), Out-of-Distribution (OOD) Detection, and Outlier Detection (OD). The classification of these tasks is determined by four foundational dichotomies:

    1. Type of Distribution Shift:

      • Covariate shift: Changes occur only in the input distribution P(X)P(X) while the label distribution P(Y)P(Y) remains unchanged (e.g., domain shifts, image corruptions).
      • Semantic shift: Changes occur in the label space P(Y)P(Y) (e.g., the emergence of novel categories), which naturally induces a shift in P(X)P(X).
    2. In-Distribution (ID) Data Cardinality:

      • Single-class: ID data represents a single homogeneous class (common in classic AD and one-class ND).
      • Multi-class: ID data contains multiple distinct semantic categories (standard in OSR, multi-class ND, and OOD detection).
    3. ID Classification Requirement:

      • Required: The system must simultaneously maintain high closed-set multi-class classification accuracy on ID samples while detecting OOD instances (as in OSR and OOD detection).
      • Not required: The task is formulated purely as binary identification (ID vs. non-ID), without requiring differentiation among individual ID classes (as in AD and ND).
    4. Learning Paradigm:

      • Inductive: Follows a train-test split where the ID distribution is learned during training, and unseen samples are evaluated during inference (AD, ND, OSR, OOD detection).
      • Transductive: All observations (contaminated with an unknown fraction of anomalies) are provided simultaneously without a separate training set, and the goal is to identify the minority anomalies from the majority ID data (as in OD).
  2. Knowl 2 — Formal Formulations of Generalized OOD Detection Sub-tasks

    definition

    Let X\mathcal{X} denote the input space of observations and Y\mathcal{Y} denote the label space of semantic classes. Training data is drawn from an in-distribution (ID) joint probability distribution P(X,Y)P(X, Y), and test data is drawn from a potentially shifted distribution P′(X,Y)P'(X, Y):

    • Sensory Anomaly Detection (Sensory AD): Focuses exclusively on covariate shift on low-level sensory observations without semantic label changes: P′(X)≠P(X)andP′(Y)=P(Y)P'(X) \neq P(X) \quad \text{and} \quad P'(Y) = P(Y) The ID training set is treated as a single class, and ID classification is not required.

    • Semantic Anomaly Detection / Novelty Detection (ND): Focuses on detecting samples from novel semantic categories: P′(Y)≠P(Y)P'(Y) \neq P(Y) ND does not require classifying the ID classes, even if multiple classes are present in the training data.

    • Open Set Recognition (OSR): Given an ID training set with multiple classes Y={1,…,C}\mathcal{Y} = \{1, \dots, C\}, the model must simultaneously perform accurate CC-way closed-set classification for ID inputs (Y∈YY \in \mathcal{Y}) and reject test inputs exhibiting semantic shift (Y∉YY \notin \mathcal{Y}).

    • Out-of-Distribution (OOD) Detection: Formulated canonically identically to OSR for multi-class classification (P′(Y)≠P(Y)P'(Y) \neq P(Y) while preserving ID classification performance), but generalized to broader vision and learning tasks (such as multi-label classification, object detection, and segmentation) and evaluated across multi-dataset distribution shifts.

    • Outlier Detection (OD): Operates transductively on an unpartitioned dataset D={xi}i=1N\mathcal{D} = \{\mathbf{x}_i\}_{i=1}^N to identify anomalous samples that deviate significantly from the majority distribution, driven by either covariate or semantic shift.

  3. Knowl 3 — Distinctions Between Open Set Recognition and OOD Detection

    definition

    While Open Set Recognition (OSR) and Out-of-Distribution (OOD) detection both address semantic shift detection while maintaining in-distribution (ID) classification, they differ in three key aspects:

    1. Evaluation Protocol and Benchmark Design: OSR traditionally constructs benchmarks by partitioning a single multi-class dataset into known (ID) and unknown (OOD) classes based on label splits. In contrast, OOD detection designates an entire dataset (e.g., CIFAR-10) as ID and employs entirely separate datasets (e.g., SVHN, Textures) as OOD, with strict de-duplication to eliminate overlapping classes.

    2. Outlier Data Constraints During Training: By theoretical formulation (such as bounding open-space risk), OSR restricts model training strictly to ID data and discourages the use of external outlier data during training. OOD detection accommodates a wider algorithmic space, including methods utilizing outlier exposure (OE) from real auxiliary datasets or synthesized negative data.

    3. Scope and Solution Space: OSR is predominantly formulated around closed-set classifiers calibrated for reject options, whereas OOD detection extends across multi-label classification, dense prediction tasks (e.g., object detection, segmentation), and multi-modal foundation models.

  4. Knowl 4 — Post-Hoc Output and Activation Manipulation for OOD Detection

    model/method

    Post-hoc OOD detection methods operate on pre-trained neural networks without modifying the training loss or architecture:

    • Maximum Softmax Probability (MSP): Uses the maximum softmax probability as the confidence score, identifying inputs with low maximum probability as OOD.

    • ODIN: Applies temperature scaling T>1T > 1 and input gradient perturbation to widen the softmax score divergence between ID and OOD samples: Si(x;T)=exp⁡(fi(x)/T)∑j=1Cexp⁡(fj(x)/T)S_i(\mathbf{x}; T) = \frac{\exp(f_i(\mathbf{x}) / T)}{\sum_{j=1}^C \exp(f_j(\mathbf{x}) / T)} where fi(x)f_i(\mathbf{x}) is the logit for class ii.

    • Energy-Based OOD Detection: Maps the logits to a scalar Helmholtz free energy score, theoretically aligned with the input's negative log marginal likelihood −log⁡p(x)- \log p(\mathbf{x}): E(x;f)=−T⋅log⁡∑i=1Cexp⁡(fi(x)T)E(\mathbf{x}; f) = -T \cdot \log \sum_{i=1}^C \exp\left(\frac{f_i(\mathbf{x})}{T}\right) Inputs with higher energy scores E(x;f)E(\mathbf{x}; f) are flagged as OOD.

    • Rectified Activations (ReAct): Identifies that mismatched batch normalization statistics from OOD inputs trigger abnormally high activations in penultimate feature layers. ReAct clips unit activations at a threshold cc before the classification layer: hk∗(x)=min⁡(hk(x),c)h_k^*(x) = \min(h_k(x), c) where hk(x)h_k(x) is the activation of the kk-th feature dimension.

    • Directed Sparsification (DICE): Prunes noisy, non-salient network weights based on contribution rankings, reducing output variance on OOD data and sharpening the ID/OOD separation.

    • Activation Shaping (ASH): Removes a large fraction (e.g., 90%90\%) of late-layer feature representations based on a top-KK criterion and adjusts the remaining activations by scaling or constant assignments.

  5. Knowl 5 — Distance-Based OOD Scoring: Mahalanobis and Deep Nearest Neighbors

    model/method

    Distance-based OOD detection measures the proximity of test features to in-distribution representations in the intermediate or penultimate feature space of a neural network:

    • Parametric Mahalanobis Distance: Assumes that pre-trained deep feature representations z=f(x)\mathbf{z} = f(\mathbf{x}) follow a class-conditional Gaussian distribution N(μc,Σ)\mathcal{N}(\boldsymbol{\mu}_c, \boldsymbol{\Sigma}) with shared covariance Σ\boldsymbol{\Sigma} across classes c∈{1,…,C}c \in \{1, \dots, C\}. The confidence score is defined by the minimum Mahalanobis distance to any class centroid: M(x)=max⁡c−(z−μc)⊤Σ−1(z−μc)M(\mathbf{x}) = \max_{c} -(\mathbf{z} - \boldsymbol{\mu}_c)^\top \boldsymbol{\Sigma}^{-1} (\mathbf{z} - \boldsymbol{\mu}_c)

    • Non-Parametric Nearest-Neighbor Distance (KNN): Computes the Euclidean distance between the normalized penultimate feature representation z∗=z/∥z∥2\mathbf{z}^* = \mathbf{z} / \|\mathbf{z}\|_2 of a test sample x\mathbf{x} and the kk-th nearest neighbor in the stored set of normalized training embeddings {zi∗}i=1N\{\mathbf{z}_i^*\}_{i=1}^N: Dk(x)=∥z∗−z(k)∗∥2D_k(\mathbf{x}) = \|\mathbf{z}^* - \mathbf{z}_{(k)}^*\|_2 Non-parametric distance scoring avoids parametric Gaussian assumptions, making it robust across high-dimensional feature spaces and complex multi-modal class distributions.

  6. Knowl 6 — Feature-Space Virtual Outlier Synthesis for Outlier Exposure

    model/method

    While outlier exposure (OE) using auxiliary real datasets significantly improves ID/OOD separability, collecting clean, non-overlapping real outliers is often impractical, and pixel-space GAN synthesis is computationally expensive in high dimensions.

    Feature-space synthesis models the feature representations of in-distribution data directly in the latent space Z\mathcal{Z}:

    • Parametric Virtual Outlier Synthesis (VOS): Estimates class-conditional Gaussian distributions N(μc,Σc)\mathcal{N}(\boldsymbol{\mu}_c, \boldsymbol{\Sigma}_c) over penultimate layer features and samples virtual outliers from the low-likelihood boundary region of the feature space. An auxiliary energy-regularization loss forces the classifier to produce high energy on these synthesized virtual representations during training.

    • Non-Parametric Outlier Synthesis (NPOS): Generates synthetic outliers in feature space along the boundary of non-parametric ID nearest-neighbor envelopes without assuming Gaussianity.

  7. Knowl 7 — Near-OOD vs. Far-OOD Benchmarking Protocol with Semantic De-duplication

    experimental setup

    In the standard OpenOOD benchmarking evaluation, OOD detection datasets are partitioned into Near-OOD and Far-OOD sets relative to an In-Distribution (ID) dataset:

    • Near-OOD: Test sets that share the same general visual domain as the ID dataset but differ purely by semantic categories (e.g., CIFAR-100 and TinyImageNet when CIFAR-10 is used as ID).
    • Far-OOD: Test sets that exhibit both semantic shift and severe covariate/domain shifts (e.g., MNIST, SVHN, Textures, and Places365 when CIFAR-10 is used as ID).

    Semantic De-duplication Requirement: To prevent corrupted evaluation metrics (where true ID samples in the OOD set are falsely penalized when classified with high confidence), overlapping classes must be explicitly purged from OOD datasets:

    • When evaluating on CIFAR-10, 1,207 semantically overlapping images are removed from TinyImageNet, and 1,305 overlapping images are removed from Places365.
    • When evaluating on CIFAR-100, 2,502 semantically overlapping images are removed from TinyImageNet, and 1,305 overlapping images are removed from Places365.
  8. Knowl 8 — Empirical Performance Comparison Across Generalized OOD Detection Families

    empirical result

    Standardized evaluation on ResNet-18 backbones trained on CIFAR-10 and CIFAR-100 across various method categories reveals several empirical findings on the OpenOOD benchmark:

    1. Data Augmentation and Uncertainty Techniques: Methods utilizing rich data augmentation—specifically PixMix, CutMix, and Mixup—demonstrate superior performance. PixMix attains the top AUROC score of 93.1%93.1\% on CIFAR-10 Near-OOD.

    2. Post-Hoc Methods vs. Training-Required Methods: Modern inference-only post-hoc methods (such as KNN, ReAct, and DICE) consistently match or exceed the detection performance of specialized training-required objectives while requiring no retraining overhead.

    3. Utility of Auxiliary Outlier Data: Non-parametric methods without outlier data (e.g., KNN) achieve performance competitive with or superior to outlier exposure methods (e.g., UDG) on Far-OOD and CIFAR-100 Near-OOD, indicating that large auxiliary datasets are not strictly necessary for strong general OOD detection.

    4. Sensory Anomaly Detection Methods on Far-OOD: Anomaly detection techniques engineered for pixel-level defect identification (such as DRAEM and CutPaste) exhibit strong transferability to Far-OOD detection tasks on CIFAR-100 benchmarks.

  9. Knowl 9 — Covariate Shift Sensitivity Dilemma and Full-Spectrum OOD Detection

    limitation

    A fundamental limitation of existing OOD detectors is their unintended sensitivity to benign covariate shifts (e.g., image corruptions, style changes, domain shifts) rather than purely semantic shifts. Because deep models often rely on non-semantic environmental features, standard scoring functions frequently trigger false OOD alarms for covariate-shifted in-distribution samples that the model could otherwise correctly classify.

    To address this dilemma, the full-spectrum OOD detection framework requires the classifier to be robust and generalize correctly across covariate-shifted ID inputs (P′(X)≠P(X),P′(Y)=P(Y)P'(X) \neq P(X), P'(Y) = P(Y)) while reliably rejecting samples that contain true semantic shifts (P′(Y)≠P(Y)P'(Y) \neq P(Y)).

Coverage note — Omitted detailed descriptions of general background surveys on non-deep tabular outlier detection heuristics (e.g., LOF, RANSAC, Isolation Forest) and specific hyperparameter tables from cited external papers, as they represent well-established prior literature rather than core contributions of the generalized framework.

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Citation

MLA
Yang, J., et al. “Generalized Out-of-Distribution Detection: A Survey”. arXiv, 2021, http://arxiv.org/abs/2110.11334v3.
APA
Yang, J., Zhou, K., Li, Y., & Liu, Z. (2021). Generalized Out-of-Distribution Detection: A Survey. arXiv. http://arxiv.org/abs/2110.11334v3
Chicago
Yang, J., K. Zhou, Y. Li, and Z. Liu. 2021. “Generalized Out-of-Distribution Detection: A Survey”. arXiv. http://arxiv.org/abs/2110.11334v3.
Harvard
Yang, J. et al. (2021) “Generalized Out-of-Distribution Detection: A Survey”, arXiv [Preprint]. Available at: http://arxiv.org/abs/2110.11334v3.
Vancouver
1. Yang J, Zhou K, Li Y, Liu Z (2021) Generalized Out-of-Distribution Detection: A Survey. arXiv

BibTeX

@article{yang2021generalized,
  title = {Generalized Out-of-Distribution Detection: A Survey},
  author = {Yang, Jingkang and Zhou, Kaiyang and Li, Yixuan and Liu, Ziwei},
  year = {2021},
  journal = {arXiv},
  url = {http://arxiv.org/abs/2110.11334v3},
  eprint = {2110.11334}
}
Metadata:arXiv

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