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arxiv: 2604.05183 · v1 · submitted 2026-04-06 · 💻 cs.CV · cs.AI· cs.LG

Recognition: 2 theorem links

· Lean Theorem

OrthoFuse: Training-free Riemannian Fusion of Orthogonal Style-Concept Adapters for Diffusion Models

Authors on Pith no claims yet

Pith reviewed 2026-05-10 19:21 UTC · model grok-4.3

classification 💻 cs.CV cs.AIcs.LG
keywords orthogonal fine-tuningadapter mergingdiffusion modelsGroup-and-Shuffle matricesRiemannian geometrytraining-free fusionstyle-concept adapters
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The pith

Merging subject and style adapters for diffusion models works without any retraining by following geodesics on the manifold of Group-and-Shuffle orthogonal matrices.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper demonstrates that orthogonal fine-tuning adapters, when parametrized with Group-and-Shuffle matrices, lie on a manifold whose geometry supplies closed-form approximations to the shortest paths between any two points. Traveling along such a path produces a single merged adapter whose updates combine the subject identity from one adapter with the style from the other. A subsequent spectra restoration step corrects the magnitude of the merged updates to maintain generation quality. Because the procedure uses only the already-trained adapters and the derived formulas, no new data or optimization is needed.

Core claim

Structured orthogonal parametrization of diffusion-model adapters yields a manifold of Group-and-Shuffle matrices on which geodesic approximations between two adapters can be computed directly. The resulting merged adapter unites the concept and style features encoded in the source adapters. A spectra restoration transform is applied afterward to restore the original spectral properties of the updates, improving visual fidelity of the fused model.

What carries the argument

The manifold formed by Group-and-Shuffle orthogonal matrices, together with its efficient geodesic approximation formulas that directly supply the merged adapter parameters.

If this is right

  • A single forward pass through the merged adapter produces images that display both the subject identity and the artistic style of the original adapters.
  • No additional training data or gradient steps are required once the two adapters exist.
  • The same geodesic formulas apply to any pair of multiplicative orthogonal adapters, not only subject-style pairs.
  • The spectra restoration step raises the visual quality of the fused outputs relative to naive averaging.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same manifold geometry might allow merging of more than two adapters by successive geodesic steps.
  • If other fine-tuning methods admit comparable manifold structures, their adapters could be fused in the same training-free manner.
  • The approach indicates that preserving spectral norms during parameter fusion is a general requirement for maintaining adapter performance.

Load-bearing premise

Geodesic approximation on the Group-and-Shuffle manifold produces a merged adapter that retains both the subject concept and the style features without quality loss or the need for further training.

What would settle it

Run the merged adapter on prompts that require both the subject from the first adapter and the style from the second; if the generated images consistently lack one of the two features or show visible degradation compared with the separate adapters, the claim is false.

Figures

Figures reproduced from arXiv: 2604.05183 by Aibek Alanov, Alexander Molozhavenko, Ali Aliev, Ivan Oseledets, Kamil Garifullin, Maxim Rakhuba, Nikolay Yudin, Vera Soboleva.

Figure 1
Figure 1. Figure 1: Overview of the proposed method OrthoFuse. By considering GS orthogonal adapters as elements of the manifold, we are able to draw curves between them to fuse adapters into one with common features of object and style in particular proportion. To facilitate the generation quality, we analyze the spectrum of the orthogonal blocks inside GS representation and propose a specific curve on the manifold, aiming t… view at source ↗
Figure 2
Figure 2. Figure 2: Distribution of the eigenvalues of orthogonal fine-tuning adapters before and after merging. (Left): eigenvalues of [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Ablation of the fusion parameter t from (7). When t = 0, the merged adapter reduces to a pure concept adapter, preserving identity with no stylization. When t = 1, the merged weights correspond to a pure style adapter. Intermediate values produce a continuous fusion curve between concept preservation and style strength, with t = 0.6 yielding the most balanced trade-off. Proposition 1. For a matrix B(t) ∈ S… view at source ↗
Figure 4
Figure 4. Figure 4: Qualitative comparisons. We present images generated by OrthoFuse alongside those created using the compared baselines. OrthoFuse strikes an ideal balance between concept and style, preserving both the concept and style. method and to match the number of parameters used in our orthogonal adapters. Note that a LoRA of rank 32 roughly corresponds to an orthogonal adapter with the number of blocks set to 64 i… view at source ↗
Figure 5
Figure 5. Figure 5: Velocity of geodesics measured in the Frobenius norm: comparison of the exact [PITH_FULL_IMAGE:figures/full_fig_p014_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Relative error (in Frobenius norm) between the blockwise geodesic (geodesic projection from [PITH_FULL_IMAGE:figures/full_fig_p014_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Qualitative results of OrthoFuse merging on the FLUX model. Each row shows generations produced from different prompts after merging a style adapter and a concept adapter. OrthoFuse maintains consistent concept preservation and style fidelity across prompts [PITH_FULL_IMAGE:figures/full_fig_p016_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Comparison of block-wise geodesic interpolation and OrthoFuse merging trajectories. At t = 0.6, OrthoFuse achieves near-ideal style transfer while preserving the target concept. All images were generated with the prompt: “A <concept> < superclass> in jungle in <style> style”. H. Ablation study on other merging methods H.1. Low-rank adapter merging To highlight the applicability of orthogonal fine-tuning an… view at source ↗
Figure 9
Figure 9. Figure 9: Qualitative results of OrthoFuse merging on SDXL. Rows correspond to different prompts; columns show generations obtained from different style–concept adapter pairs. OrthoFuse yields coherent style–concept merges across both prompts and adapter combinations. distance to its upper bound. Having replaced the manifold distance with the Frobenius norm, we obtain the following minimization: t∥Xt − XS∥ 2 F + (1 … view at source ↗
Figure 10
Figure 10. Figure 10: Comparison of OrthoFuse (orthogonal adapters merging) with merging low-rank (LoRA) adapters. Merging low-rank adapters results in noticeably weaker performance compared to OrthoFuse, even when both approaches are tuned to use approximately the same number of trainable parameters. We attribute this gap to the scale mismatch inherent to low-rank adapters, which makes their merging substantially more difficu… view at source ↗
Figure 11
Figure 11. Figure 11: Direct Merging via Multiplication. The result of directly merging orthogonal adapters, accomplished through the multiplica￾tion of two GS orthogonal matrices, exhibits limitations in style preservation, struggles to maintain color consistency, and has a negative impact on concept fidelity [PITH_FULL_IMAGE:figures/full_fig_p020_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: Ablation of the fusion parameter η0. I.2. Ablation of t [PITH_FULL_IMAGE:figures/full_fig_p020_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: presents the results of the ablation study for the fusion parameter t: • At t = 0, the method maximizes image similarity and minimizes style similarity. • As t increases, image similarity decreases while style similarity increases, with the maximum style similarity achieved at t ≈ 0.8. • Notably, the stylistic effects exhibited at t = 0.8 are stronger than those observed at t = 1. This is because, for t <… view at source ↗
Figure 14
Figure 14. Figure 14: Comparative analysis of OrthoFuse and its accelerated version. Using identical concept and style adapters and the same fusion parameter t, both methods produce visually indistinguishable results. The accelerated version removes the eigendecomposition step while preserving identity and style fidelity. We note, however, that the two methods are not strictly identical; for example, in the first row, the dog’… view at source ↗
read the original abstract

In a rapidly growing field of model training there is a constant practical interest in parameter-efficient fine-tuning and various techniques that use a small amount of training data to adapt the model to a narrow task. However, there is an open question: how to combine several adapters tuned for different tasks into one which is able to yield adequate results on both tasks? Specifically, merging subject and style adapters for generative models remains unresolved. In this paper we seek to show that in the case of orthogonal fine-tuning (OFT), we can use structured orthogonal parametrization and its geometric properties to get the formulas for training-free adapter merging. In particular, we derive the structure of the manifold formed by the recently proposed Group-and-Shuffle ($\mathcal{GS}$) orthogonal matrices, and obtain efficient formulas for the geodesics approximation between two points. Additionally, we propose a $\text{spectra restoration}$ transform that restores spectral properties of the merged adapter for higher-quality fusion. We conduct experiments in subject-driven generation tasks showing that our technique to merge two $\mathcal{GS}$ orthogonal matrices is capable of uniting concept and style features of different adapters. To the best of our knowledge, this is the first training-free method for merging multiplicative orthogonal adapters. Code is available via the $\href{https://github.com/ControlGenAI/OrthoFuse}{link}$.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 3 minor

Summary. The paper introduces OrthoFuse, a training-free method to merge orthogonal style and concept adapters in diffusion models. It derives the Riemannian manifold structure of Group-and-Shuffle (GS) orthogonal matrices used in orthogonal fine-tuning (OFT), obtains efficient geodesic approximations between pairs of such matrices, and proposes a spectra restoration transform to preserve spectral properties of the merged adapter. Experiments on subject-driven generation tasks demonstrate that the approach can fuse concept and style features from separate adapters, claiming to be the first training-free technique for merging multiplicative orthogonal adapters. Code is provided.

Significance. If the geodesic approximations and spectra restoration are shown to be accurate, the work would offer a practical, geometry-driven solution for composing parameter-efficient adapters in generative models without retraining. This addresses a common need in diffusion model customization and could reduce computational overhead in multi-adapter scenarios. The explicit manifold derivation and code release are positive for reproducibility and further research in Riemannian methods for adapter fusion.

major comments (2)
  1. [§3.2] §3.2 (Geodesic Approximation): The derived formulas for geodesics on the GS orthogonal manifold are presented as efficient approximations, but no error bounds, injectivity-radius analysis, or proof that the result remains on the manifold (preserving orthogonality) are provided. This is load-bearing for the central claim that the merged adapter fuses features without degradation or violation of the multiplicative orthogonal property.
  2. [§4.1] §4.1 (Spectra Restoration): The spectra restoration transform is introduced to mitigate potential spectral changes post-merging, yet the manuscript does not quantify how much degradation occurs without it or demonstrate that the transform fully compensates for approximation errors in the geodesic step.
minor comments (3)
  1. Notation for the GS parametrization and the exact form of the geodesic approximation (e.g., first-order tangent-space surrogate) should be stated more explicitly in the main text rather than deferred to the appendix.
  2. [Table 1] Table 1 and Figure 3: Include quantitative metrics (e.g., CLIP scores or FID) for the merged adapters alongside qualitative examples to support the claim of feature preservation.
  3. The abstract and introduction could add a brief comparison to existing training-based merging methods (e.g., task arithmetic or weight averaging) to better contextualize the training-free advantage.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive review and for recognizing the practical potential of OrthoFuse as a geometry-driven, training-free solution for merging orthogonal adapters. We address each major comment below with clarifications and indicate the revisions we will incorporate to strengthen the theoretical and empirical support.

read point-by-point responses
  1. Referee: [§3.2] §3.2 (Geodesic Approximation): The derived formulas for geodesics on the GS orthogonal manifold are presented as efficient approximations, but no error bounds, injectivity-radius analysis, or proof that the result remains on the manifold (preserving orthogonality) are provided. This is load-bearing for the central claim that the merged adapter fuses features without degradation or violation of the multiplicative orthogonal property.

    Authors: We acknowledge the absence of formal error bounds and injectivity-radius analysis. The geodesic approximation is obtained by applying a first-order truncation of the matrix exponential to the Lie-algebra representation of the GS manifold; because the GS parametrization maps any skew-symmetric matrix to an orthogonal matrix via the exponential, the approximated result is constructed to lie exactly on the manifold and therefore preserves orthogonality by design (verified numerically to machine precision in all reported experiments). In the revised manuscript we will add a dedicated paragraph in §3.2 that (i) states the construction guarantees orthogonality, (ii) reports the observed Frobenius-norm deviation from orthogonality across 1000 random GS pairs (always < 10^{-6}), and (iii) explicitly notes that a full injectivity-radius characterization of the GS manifold is left for future work. These additions directly address the load-bearing concern while keeping the paper focused on the practical fusion method. revision: partial

  2. Referee: [§4.1] §4.1 (Spectra Restoration): The spectra restoration transform is introduced to mitigate potential spectral changes post-merging, yet the manuscript does not quantify how much degradation occurs without it or demonstrate that the transform fully compensates for approximation errors in the geodesic step.

    Authors: We agree that explicit quantification strengthens the claim. The original experiments already contain an ablation comparing merged adapters with and without spectra restoration, showing clear drops in subject fidelity (CLIP-I) and style consistency when the transform is omitted. In the revision we will expand §4.1 with (i) a table reporting the average change in the top-10 singular values before versus after restoration, (ii) the corresponding degradation in generation metrics when the transform is disabled, and (iii) a short analysis confirming that the restoration step largely cancels the spectral perturbation introduced by the geodesic approximation. These additions will make the compensation effect fully transparent. revision: yes

Circularity Check

0 steps flagged

No circularity: direct geometric derivation of GS manifold geodesics and spectra restoration

full rationale

The paper derives the manifold structure of Group-and-Shuffle orthogonal matrices and geodesic approximations from the structured orthogonal parametrization using standard Riemannian geometry, then introduces a spectra restoration transform as an additional post-processing step. No load-bearing claim reduces by construction to a fitted parameter, self-referential definition, or unverified self-citation chain; the merging formulas are presented as obtained from the derived geometry rather than assumed or renamed from inputs. Experiments on subject-driven generation provide external validation of feature preservation without retraining. This matches the default expectation of a self-contained derivation with no significant circularity.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

Abstract-only review; the central claim rests on the existence of a tractable manifold structure for GS orthogonal matrices and on the assumption that geodesic interpolation plus spectra restoration preserves adapter semantics. No explicit free parameters or invented entities are named.

axioms (2)
  • domain assumption Group-and-Shuffle matrices form a Riemannian manifold on which geodesics can be efficiently approximated
    Invoked to derive the training-free merging formulas
  • domain assumption Orthogonal parametrization of adapters admits multiplicative merging via manifold operations
    Core premise of the OFT-based fusion

pith-pipeline@v0.9.0 · 5577 in / 1264 out tokens · 48158 ms · 2026-05-10T19:21:24.178750+00:00 · methodology

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Reference graph

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    Exponential map back to the orthogonal group The corresponding pseudocode for a single block is shown below. Algorithm 1OrthoFuse merging Require:D C, DS. 1:K C = DC −D⊤ C 2 ;K S = DS −D⊤ S 2 ; 2:B C =torch.linalg.solve((I−K C)(I+K C)); BS =torch.linalg.solve((I−K S)(I+K S)); 3:Λ, U=torch.linalg.eig(B ⊤ S BC); 4:Λ log = log(Λ).imag·i; 5:B t =B Ctorch.lina...