Flownomaly: Single-Step Conditional-Mean Recovery for Few-Shot Anomaly Detection

Bernardo Perrone Ribeiro; Erkut Akdag; Egor Bondarev

cs.CVOct 23, 2026

Flownomaly: Single-Step Conditional-Mean Recovery for Few-Shot Anomaly Detection

Authors: Bernardo Perrone Ribeiro, Erkut Akdag, Egor Bondarev

Organizations: AIMS Group, Department of Electrical Engineering, Eindhoven University of Technology

Abstract

Few-shot industrial anomaly detection requires modeling category-specific normality from only a handful of defect-free images. Common approaches tie inference to stored exemplars, impose rigid linear structure, or risk reproducing anomalous content. Flownomaly instead casts the problem as normal-counterpart recovery in frozen DINOv2 feature space. Given normal supports, a velocity field is learned through conditional flow matching over frozen patch tokens to transport Gaussian noise toward normal features. Along straight paths, a single recovery step using one evaluation of the optimal field yields the posterior mean of the clean normal token without trajectory integration. The operating time governs how strongly recovery modifies a query token. With no anomalous validation samples available for tuning, a normal-only Jacobian gain-to-noise criterion selects this time from the supports alone. Squared residuals between query tokens and their recovered counterparts yield anomaly signals for both dense localization and image-level detection. Across 1-, 2-, and 4-shot benchmarks on MVTec-AD and VisA, together with sample-shot evaluations on Real-IAD, Flownomaly achieves state-of-the-art performance against recent category-specific methods. In the 1-shot setting, image- and pixel-level AUROC reach 97.8% and 98.0% on MVTec-AD and 95.3% and 98.7% on VisA, with a 2.1-point image-level AUROC improvement over the strongest baseline on VisA. Code is available at https://github.com/b-rbmp/Flownomaly.

Figures & tables

Figure 1: Conceptual illustration of Flownomaly. A query patch token is (1) perturbed with Gaussian noise and (2) recovered using a single evaluation of the velocity field fitted to the normal supports. Tokens consistent with normal appearance undergo little displacement (left), whereas less compatible tokens tend to undergo larger changes toward the normal support features (right). The squared distance between a query token and its recovered counterpart defines the anomaly score.

Figure 2: Overview of Flownomaly. (a) Disjoint augmented views of the $k$ normal supports are encoded into standardized multi-depth DINOv2 tokens (Sec. 3.1 ): fitting views train the velocity field $v_{\theta}$ via conditional flow matching (Sec. 3.2 ), validation views select the checkpoint $v_{\widehat{\theta}}$ , and calibration views determine the operating time $t^{\star}$ via a normal-only criterion (Sec. 3.5 ). (b) At inference, query tokens perturbed with Gaussian noise at $t^{\star}$ are recovered in one batched evaluation of $v_{\widehat{\theta}}$ (Sec. 3.3 ). Squared residuals to recovered counterparts form the anomaly score map, which is smoothed and upsampled for dense localization (Sec. 3.4 ).

Setup	Method	MVTec-AD						VisA
		Image-level			Pixel-level			Image-level			Pixel-level
		AUROC	AUPR	$F_{1,\max}$	AUROC	AUPRO	$F_{1,\max}$	AUROC	AUPR	$F_{1,\max}$	AUROC	AUPRO	$F_{1,\max}$
1-shot	PatchCore [ 27 ]	83.4	92.2	90.5	92.0	79.7	50.4	79.9	82.8	81.7	95.4	80.5	38.0
	WinCLIP [ 16 ]	93.1	96.5	93.7	95.2	87.1	55.9	83.8	85.1	83.1	96.4	85.1	41.3
	PromptAD [ 20 ]	94.6	97.1	95.2	95.9	87.9	53.2	86.9	88.4	82.9	96.7	85.1	35.0
	AnomalyDINO [ 6 ]	96.6	98.2	95.8	96.8	92.7	60.2	87.4	89.0	84.3	97.8	92.5	45.1

Table 1: Few-shot anomaly detection performance on MVTec-AD and VisA for category-specific methods. Flownomaly is reported as mean

\pm

standard deviation over 3 seeds. All values are percentages. Bold and underlined indicate the best and second-best results, respectively.

Figure 3: Qualitative comparison on MVTec-AD and VisA under the 1-shot setting. Columns show the query image, ground-truth mask, and anomaly maps from the open-source category-specific baselines AnomalyDINO [ 6 ] and SubspaceAD [ 18 ] , followed by Flownomaly. Flownomaly delineates defects more sharply, with fewer spurious background activations and closer agreement with the ground-truth masks on both datasets. Additional qualitative results and representative failure modes are provided in Suppl. D and B.4 , respectively.

1-sample shot (5 views)
	Image-level			Pixel-level
Method	AUROC	AUPR	$F_{1,\max}$	AUROC	AUPRO	$F_{1,\max}$
PatchCore [ 27 ]	$79.5_{\scriptstyle\pm 0.1}$	$77.7_{\scriptstyle\pm 0.1}$	$71.5_{\scriptstyle\pm 0.0}$	$96.9_{\scriptstyle\pm 0.0}$	$84.3_{\scriptstyle\pm 0.1}$	$30.7_{\scriptstyle\pm 0.2}$
PromptAD [ 20 ]	$79.4_{\scriptstyle\pm 0.2}$	$77.2_{\scriptstyle\pm 0.4}$	$71.4_{\scriptstyle\pm 0.1}$	$95.9_{\scriptstyle\pm 0.1}$	$81.7_{\scriptstyle\pm 0.1}$	$30.8_{\scriptstyle\pm 0.2}$
AnomalyDINO [ 6 ]	$82.5_{\scriptstyle\pm 0.1}$	$78.3_{\scriptstyle\pm 0.4}$	$73.9_{\scriptstyle\pm 0.1}$	$97.6_{\scriptstyle\pm 0.0}$	$91.7_{\scriptstyle\pm 0.2}$	$37.0_{\scriptstyle\pm 0.5}$
SubspaceAD [ 18 ]	$85.2_{\scriptstyle\pm 0.3}$	$81.6_{\scriptstyle\pm 0.4}$	$76.1_{\scriptstyle\pm 0.0}$	$98.8_{\scriptstyle\pm 0.0}$	$\bm{97.1}_{\scriptstyle\pm 0.1}$	$42.7_{\scriptstyle\pm 0.2}$

Table 2: Sample-shot anomaly detection and localization on Real-IAD. Results are percentages reported as mean

\pm

standard deviation over 3 seeds. Bold and underlined indicate the best and second-best results, respectively.

	MVTec-AD		VisA		Latency (ms/image)
NFE	I-AUROC	P-AUROC	I-AUROC	P-AUROC	Velocity	Total model
1	$97.8_{\scriptstyle\pm 0.9}$	$98.0_{\scriptstyle\pm 0.1}$	$95.3_{\scriptstyle\pm 0.7}$	$98.7_{\scriptstyle\pm 0.0}$	2.2	60.9
2	$97.8_{\scriptstyle\pm 1.0}$	$98.1_{\scriptstyle\pm 0.1}$	$95.2_{\scriptstyle\pm 0.7}$	$98.7_{\scriptstyle\pm 0.0}$	3.7	62.4
5	$97.8_{\scriptstyle\pm 1.0}$	$98.1_{\scriptstyle\pm 0.1}$	$95.1_{\scriptstyle\pm 0.8}$	$98.7_{\scriptstyle\pm 0.0}$	8.0	66.7
10	$97.8_{\scriptstyle\pm 1.0}$	$98.1_{\scriptstyle\pm 0.1}$	$95.1_{\scriptstyle\pm 0.8}$	$98.7_{\scriptstyle\pm 0.0}$	15.1	73.8
20	$97.8_{\scriptstyle\pm 1.0}$	$98.1_{\scriptstyle\pm 0.1}$	$95.1_{\scriptstyle\pm 0.8}$	$98.7_{\scriptstyle\pm 0.0}$	29.2	87.9

Table 3: Effect of the number of velocity-field evaluations (NFE) in the 1-shot setting.

Figure 4: Operating-time robustness on MVTec-AD and VisA in the 1-shot setting. Curves show mean category-wise image-level regret; lower is better. The dashed lines mark the median category-wise JGN selection at

t=0.65

	MVTec-AD		VisA
Scoring rule	I-AUROC	P-AUROC	I-AUROC	P-AUROC
Recovery residual	$\bm{97.8}_{\scriptstyle\pm 0.9}$	$\bm{98.0}_{\scriptstyle\pm 0.1}$	$\bm{95.3}_{\scriptstyle\pm 0.7}$	$\bm{98.7}_{\scriptstyle\pm 0.0}$
Velocity norm	$96.9_{\scriptstyle\pm 0.9}$	$97.3_{\scriptstyle\pm 0.1}$	$94.3_{\scriptstyle\pm 0.2}$	$98.4_{\scriptstyle\pm 0.0}$
Cosine residual	$96.7_{\scriptstyle\pm 0.6}$	$97.9_{\scriptstyle\pm 0.1}$	$90.2_{\scriptstyle\pm 0.4}$	$97.8_{\scriptstyle\pm 0.2}$

Table 4: Effect of the scoring rule (1-shot setting), with all other factors fixed. Best results are in bold .

		MVTec-AD		VisA
Backbone	Latency (ms)	I-AUROC	P-AUROC	I-AUROC	P-AUROC
DINOv2-S	18.1	$94.7_{\scriptstyle\pm 1.3}$	$97.7_{\scriptstyle\pm 0.1}$	$91.8_{\scriptstyle\pm 1.0}$	$98.0_{\scriptstyle\pm 0.1}$
DINOv2-B	25.5	$96.4_{\scriptstyle\pm 1.0}$	$97.8_{\scriptstyle\pm 0.1}$	$93.4_{\scriptstyle\pm 0.8}$	$98.3_{\scriptstyle\pm 0.0}$
DINOv2-L	51.9	$97.3_{\scriptstyle\pm 0.9}$	$97.9_{\scriptstyle\pm 0.1}$	$94.3_{\scriptstyle\pm 0.8}$	$98.5_{\scriptstyle\pm 0.0}$
DINOv2-g	60.9	$\bm{97.8}_{\scriptstyle\pm 0.9}$	$\bm{98.0}_{\scriptstyle\pm 0.1}$	$\bm{95.3}_{\scriptstyle\pm 0.7}$	$\bm{98.7}_{\scriptstyle\pm 0.0}$

Table 5: Backbone-scale ablation in the 1-shot setting. Best results are in bold .

Appendix figures & tables12 assets

Supplementary material from the paper’s appendix.

Appendix

Figure S1 : Recovery relative to the support-derived normal token set. Left: recovery residual versus pre-recovery support distance, with both quantities normalized by their category-wise normal-token medians and aggregated equally across categories. Middle:

d_{\mathrm{after}}/d_{\mathrm{before}}

, where values below one indicate contraction toward the normal support features. Right: cosine alignment between the recovery update and the direction toward the nearest support token, where positive values indicate movement toward the normal support features. Bars and error bars report the median and interquartile range across categories. Ground-truth masks serve only for post-hoc stratification.

		Regret (pp)
Dataset	$t^{\star}$ range	Mean	Median	Maximum
MVTec-AD	$0.60$ – $0.75$	0.15	0.06	0.82
VisA	$0.65$ – $0.70$	0.54	0.14	4.82

Appendix

Table S1 : Retrospective image-AUROC regret of the normal-only JGN selection, measured in percentage points relative to the category-wise test-label oracle. Regret statistics are computed over categories for seed 42; the

t^{\star}

column reports the range of per-category selections across all three seeds.

MVTec-AD			VisA
Category	$t^{\star}$	Regret (pp)	Category	$t^{\star}$	Regret (pp)
bottle	0.75	0.08	candle	0.70	0.09
cable	0.65	0.82	capsules	0.65	0.36
capsule	0.65	0.08	cashew	0.65	0.02
carpet	0.70	0.00	chewinggum	0.65	0.04
grid	0.75	0.00	fryum	0.65	0.23

Appendix

Table S2 : Category-level operating-time diagnostics for seed 42. Regret is the image-AUROC difference, in percentage points, from the retrospective test-label oracle.

Figure S2 : Distribution of category-specific operating times selected by JGN, pooled across three seeds. Each observation represents one category–seed pair.

	MVTec-AD		VisA
Configuration	I-AUROC	P-AUROC	I-AUROC	P-AUROC	Median $t^{*}$ (MVTec-AD/VisA)
Default: 40–80%, standardized	$97.8_{\scriptstyle\pm 0.9}$	$98.0_{\scriptstyle\pm 0.1}$	$95.3_{\scriptstyle\pm 0.7}$	$98.7_{\scriptstyle\pm 0.0}$	0.65/0.65
Single block at 80% depth	$97.6_{\scriptstyle\pm 1.2}$	$98.0_{\scriptstyle\pm 0.0}$	$92.0_{\scriptstyle\pm 0.7}$	$98.9_{\scriptstyle\pm 0.0}$	0.70/0.70
Shallow blocks at 20–40% depth	$85.7_{\scriptstyle\pm 0.9}$	$95.9_{\scriptstyle\pm 0.1}$	$86.0_{\scriptstyle\pm 1.7}$	$93.5_{\scriptstyle\pm 0.3}$	0.60/0.60
Without layer standardization	$97.7_{\scriptstyle\pm 1.3}$	$98.2_{\scriptstyle\pm 0.1}$	$94.0_{\scriptstyle\pm 0.9}$	$98.8_{\scriptstyle\pm 0.0}$	0.85/0.85
Without rotation-derived views †	$93.8_{\scriptstyle\pm 1.4}$	$97.4_{\scriptstyle\pm 0.1}$	$93.3_{\scriptstyle\pm 0.5}$	$98.5_{\scriptstyle\pm 0.0}$	0.60/0.60

Appendix

Table S3 : Feature and support-view ablations in the 1-shot setting. AUROC results are mean

\pm

standard deviation over three seeds. The final column reports the median selected

t^{\star}

over categories for MVTec-AD/VisA under seed 42. † Disabling rotations also removes diversity from the support-derived validation and calibration pools.

Figure S3 : Failure modes where the predicted anomaly map remains essentially empty. Columns show the query image, ground-truth mask, and Flownomaly anomaly map. Top: Logical anomaly with swapped wires (MVTec-AD cable ). Bottom: Missing component revealing previously seen background (VisA pcb3 ).

		MVTec-AD		VisA
Backbone	Res.	I-AUROC	P-AUROC	I-AUROC	P-AUROC
DINOv2-g	$672^{2}$	$97.8_{\scriptstyle\pm 0.9}$	$98.0_{\scriptstyle\pm 0.1}$	$95.3_{\scriptstyle\pm 0.7}$	$98.7_{\scriptstyle\pm 0.0}$
DINOv3-7B	$672^{2}$	$96.1_{\scriptstyle\pm 1.0}$	$98.0_{\scriptstyle\pm 0.1}$	$94.8_{\scriptstyle\pm 0.1}$	$98.5_{\scriptstyle\pm 0.0}$
DINOv3-7B	$768^{2}$	$96.0_{\scriptstyle\pm 1.6}$	$97.9_{\scriptstyle\pm 0.1}$	$94.9_{\scriptstyle\pm 0.2}$	$98.5_{\scriptstyle\pm 0.0}$

Appendix

Table S4 : DINOv3 vs. DINOv2 backbone ablation in the 1-shot setting across MVTec-AD and VisA. DINOv3 (

16\times 16

patch size) is evaluated at

672\times 672

(

42\times 42

token grid) and at

768\times 768

to match DINOv2’s

48\times 48

token grid.

	Image-level			Pixel-level
Method	AUROC	AUPR	$F_{1,\max}$	AUROC	AUPRO	$F_{1,\max}$
PatchCore [ 27 ]	99.2	99.8	98.8	98.1	92.6	59.3
AnomalyDINO [ 6 ]	99.5	99.8	99.0	98.2	95.0	64.3
SubspaceAD [ 18 ]	99.2	99.6	98.3	98.2	95.6	64.7
ANoCo [ 29 ]	99.4	99.7	98.5	98.2	96.4	67.8
Flownomaly	99.7	99.9	99.0	98.7	96.4	67.2

Appendix

Table S5 : Full-shot anomaly detection and localization on MVTec-AD. Rotation augmentations are disabled in the full-shot setting. All values are percentages. Bold denotes the best result, and underlined denotes the second-best result.

	Image-level			Pixel-level
Method	AUROC	AUPR	$F_{1,\max}$	AUROC	AUPRO	$F_{1,\max}$
PatchCore [ 27 ]	92.8	91.4	88.0	98.5	91.1	43.7
AnomalyDINO [ 6 ]	97.6	98.0	94.5	98.8	96.1	53.8
SubspaceAD [ 18 ]	98.2	98.3	95.0	99.1	96.9	53.0
ANoCo [ 29 ]	98.7	98.9	96.1	99.2	95.3	54.7
Flownomaly	98.9	98.9	96.5	99.3	97.0	54.8

Appendix

Table S6 : Full-shot anomaly detection and localization on VisA. Rotation augmentations are disabled in the full-shot setting. All values are percentages. Bold denotes the best result, and underlined denotes the second-best result.

Figure S4 : Additional qualitative localization results on representative MVTec-AD samples under the 1-shot setting. Columns show the input image, ground-truth mask, baseline anomaly maps (AnomalyDINO [ 6 ] , SubspaceAD [ 18 ] ), and Flownomaly’s predicted anomaly map.

Figure S5 : Additional qualitative localization results on representative VisA samples under the 1-shot setting. Columns show the input image, ground-truth mask, baseline anomaly predictions (AnomalyDINO [ 6 ] , SubspaceAD [ 18 ] ), and Flownomaly.

Figure S6 : Qualitative localization results on representative Real-IAD samples under the 1-sample setting (spanning five multi-view captures of a single normal physical sample). Columns show the input query image, ground-truth mask, baseline anomaly maps (AnomalyDINO [ 6 ] , SubspaceAD [ 18 ] ), and Flownomaly.

Few-Shot Anomaly Detection

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SubspaceAD: Training-Free Few-Shot Anomaly Detection via Subspace Modeling

Camile Lendering, Erkut Akdag, Egor Bondarev

AIMS Group, Department of Electrical Engineering, Eindhoven University of Technology

Few-Shot Anomaly Detection Anomaly Detection Industrial Anomaly Detection Reconstruction-Based Anomaly Detection

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Detecting visual anomalies in industrial inspection often requires training with only a few normal images per category. Recent few-shot methods achieve strong results employing foundation-model features, but typically rely on memory banks, auxiliary datasets, or multi-modal tuning of vision-language models. We therefore question whether such complexity is necessary given the feature representations of vision foundation models. To answer this question, we introduce SubspaceAD, a training-free method, that operates in two simple stages. First, patch-level features are extracted from a small set of normal images by a frozen DINOv2 backbone. Second, a Principal Component Analysis (PCA) model is fit to these features to estimate the low-dimensional subspace of normal variations. At inference, anomalies are detected via the reconstruction residual with respect to this subspace, producing interpretable and statistically grounded anomaly scores. Despite its simplicity, SubspaceAD achieves state-of-the-art performance across one-shot and few-shot settings without training, prompt tuning, or memory banks. In the one-shot anomaly detection setting, SubspaceAD achieves image-level and pixel-level AUROC of 97.1% and 97.5% on the MVTec-AD dataset, and 93.2% and 98.2% on the VisA dataset, respectively, surpassing prior state-of-the-art results. Code and demo are available at https://github.com/CLendering/SubspaceAD.

Abstract

Few-Shot Anomaly Detection

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