REVIEW 4 major objections 5 minor 39 references
Non-Normal Diffusion Models
T0 review · 4 major / 5 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read This paper claims that diffusion models keep the same continuous-time limit no matter the distribution of their noise steps, provided each step has the matching mean and variance.
desk verdict The invariance idea is right but not new, and the loss derivations rest on an invalid ELBO; the paper needs major revision before it contributes. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is the structured random walk, whose increments decompose as $$\$\Delta$ x_k = f(x_k,t_k)\,\$\Delta$ t_k + g(t_k)\sqrt{\$\Delta$ t_k}\,z_k$$ with $\mathbb{E}[z_k]=0$ and $\operatorname{Var}(z_k)=1$, plus a finite-fourth-moment condition on $z_k$. Theorem 3.2 is the load-bearing result: it supplies the invariance that lets any such zero-mean, unit-variance noise define a valid diffusion model in the small-step limit. Closed-form KL divergences among Gaussian, Laplace, and uniform increment families then translate that invariance into concrete loss functions.
What would settle it
Take a simple Ornstein-Uhlenbeck SDE and simulate structured random walks with $z_k$ drawn from Gaussian, Laplace, uniform, and a two-point distribution with matching mean and variance; if the finite-dimensional distributions at a fixed time $t$ do not all converge to the same OU Gaussian law as $\Delta t_k \to 0$, Theorem 3.2 is false. Separately, one can check Eq. (5) directly: for a Laplace-increment model with a fixed score network, compute the left- and right-hand sides by Monte Carlo on a small dataset and see whether the claimed lower bound actually holds.
Extended reading notes
Core claim
The paper's central claim is Theorem 3.2, the Structured Invariance Principle. It states that, under mild regularity conditions (Lipschitz drift and diffusion, linear growth, and finite fourth moments of the noise variables), the càdlàg continuous-time interpolation of any structured random walk converges in distribution to the solution of the corresponding Itô SDE as the time steps go to zero. Because a structured random walk only pins down the first two conditional moments of each increment, the theorem says any zero-mean, unit-variance noise source works in the limit. The paper uses this to argue that the reversed diffusion process, and therefore the likelihood bound and training losses, is invariant to the distribution of the increment $\Delta x_k$ when the step size is small, so Gaussian increments are a choice, not a requirement.
Load-bearing premise
All the proposed non-Gaussian loss functions inherit their validity from the likelihood bound in Eq. (5), which is asserted without derivation; if that bound is not a genuine evidence lower bound for arbitrary increment distributions, the new training objectives have no theoretical justification.
Editorial extensions
If this is right
- In the small-step limit, the choice of increment family $\{z_k\}$ --- Gaussian, Laplace, uniform, or other --- does not change the continuous-time process being sampled.
- New training objectives become available: a Laplace/Laplace model yields a loss that behaves like a weighted $\ell^1$ error on the score residual, while uniform/Laplace pairs produce a piecewise $\ell^2$/ $\ell^1$ loss.
- Diffusion models can be designed by picking the increment distribution separately from the score-network parameterization, so the noise family is a tunable stylistic and inductive-bias knob.
- The experimental results indicate that non-Gaussian increments retain competitive likelihood and sample quality while producing visually different samples, such as more saturated colors for Laplace increments.
Reading between the lines
- Beyond the paper: the same invariance suggests that noise distributions with finite variance but non-trivial higher moments could inject prior structure into generation, for example sparse noise for sparse data, although the paper only demonstrates Laplace and uniform cases.
- Beyond the paper: because the proof relies on finite fourth moments, heavy-tailed increment distributions with infinite variance would be expected to break convergence; testing a Cauchy-increment random walk would directly delineate the boundary of the theorem.
- Beyond the paper: the framework implies that the standard score-matching loss is only one point in a continuum of objectives, so a practical next step is to benchmark whether the $\ell^1$-type losses improve robustness to outlier pixels or adversarial perturbations in generated images.
- The paper leaves the statistical guarantees of these alternative objectives open; verifying consistency and asymptotic normality for the non-Gaussian losses would determine whether they are sound estimators beyond their empirical performance.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes to generalize diffusion models by allowing the step increments Δx_k to be drawn from non-Gaussian distributions. Its main theoretical contribution is Theorem 3.2, a 'structured invariance principle' stating that a random walk with increments Δx_k = f(x_k,t_k)Δt_k + g(t_k)√Δt_k z_k, where z_k has zero mean and unit variance, converges in distribution to the Itô diffusion (1) as the step size goes to zero. On this basis the paper derives several training losses (Eqs. 9, 10, 13, 14) corresponding to Laplace and Uniform choices for q and p_θ, and reports CIFAR-10 experiments in Table 2 claiming competitive likelihood and sample quality.
Significance. If Theorem 3.2 were correctly proved and the likelihood bound (5) were valid, the paper would identify a genuinely useful design degree of freedom: the finite-step increment distribution in diffusion models. The proposed losses are explicit and are stated in a way that is directly testable, and the KL computations in Appendix A.1–A.3 are checkable. However, the central derivation is not sound. The ELBO in Eq. (5) is asserted without proof and is not a valid variational bound for non-Gaussian increments, and the tightness argument for Theorem 3.2 contains a concrete mathematical error. Because the training losses in Section 4 all depend on Eq. (5), the central claim of a generalized training framework is unsupported as it stands. The contribution is therefore conditional on a substantive reworking of the theory rather than on local corrections.
major comments (4)
- [Section 2, Eq. (5); Appendix A.5] Equation (5) is the likelihood bound from which every loss in Section 4 is derived, but no derivation is given and, as written, it is not a valid variational lower bound for the reverse model in Eq. (4). A valid ELBO must involve a variational family over the latent trajectory conditioned on x0; in standard diffusion models this is q(x_{k-1} | x_k, x_0), whose dependence on x0 is essential. The paper instead writes KL(q(Δx_k | x_{k+1}) || p_θ(Δx_k | x_{k+1})) with no conditioning on x0 and no explicit variational family. For non-Gaussian structured increments, q(Δx_k | x_{k+1}) is not determined by the first two moments and is generally intractable. Moreover, the appendix derivations in Lemmas A.1–A.4 do not evaluate the expression in Eq. (5): they compute KL divergences between forward transition densities conditioned on x_k, e.g., p_θ(Δx_k | x_k) = N(Δx_k; f̂_θ(x_k,t_k)Δt_k, g(t_k)^2 Δt_k), rather than the reverse kernels q(Δx_k | x_{k+1}) and p_θ(Δx_k | x_{k+1}) appearing in Eq. (4). Therefore Eqs. (9), (10), (13), and (14) are not justified as ELBO-based losses. This gap is load-bearing: without Eq. (5), the paper's central claim of a generalized training framework is unsupported, and Theorem 3.2, even if repaired, concerns only the forward process and does not repair this issue.
- [Appendix B.3, Lemma B.1, Eq. (68)] The tightness argument in Lemma B.1 does not establish the Kolmogorov criterion it invokes. Equation (68) gives an upper bound E||Δx_k||^4 ≤ C√Δt (the displayed constant is O(√Δt)), but the preceding line actually suggests a bound of order Δt for the drift term and √Δt for the diffusion term, so the claimed order is already not justified. More importantly, in Case 1 the paper derives E||x_n(s) − x_n(t)||^4 ≤ (n|t−s|) E||Δx_k||^4 ≤ C_1 √n |t−s|, and then, using |t−s| ≤ n^{−1}, asserts a |t−s|^2 bound. This is not a passage from a first moment to a fourth power; it is already the fourth moment, and the bound is O(n^{−1/2}), not O(|t−s|^2). Case 2 has the same defect. Case 3 relies on Lemma B.3, which is stated for sums of independent mean-zero terms and is not connected to the original dependent increments in a way that yields the required |t−s|^2 bound. Thus the tightness half of Theorem 3.2 is not proved.
- [Theorem 3.2 and Section 2] Even if Theorem 3.2 were proved, it would not establish the abstract's claim that the reversed process is invariant to the distribution of the increments. The theorem is a forward-time weak convergence statement for the càdlàg interpolation of the structured random walk. The reverse-time generative model in Eqs. (2)–(4) requires convergence of the one-step reverse conditionals q(x_k | x_{k+1}) (or q(Δx_k | x_{k+1})) to the reverse transitions of the limiting SDE. Weak convergence of forward paths does not imply convergence of these reverse conditionals for non-Gaussian increments; the reverse conditionals depend on the full marginal density of x_{k+1}, not just on f, g, and the first two moments of z_k. No lemma in Appendix B addresses reverse-time conditioning. Consequently the paper's main conceptual claim—that the reversed process is invariant to the increment distribution—is formally unsupported.
- [Section 4.2, Eq. (11)] The asymptotic claim in Eq. (11) is mathematically incorrect. Let a = v_k ||ε − ε_θ(x_k,t_k)||_1 and d(a) = exp(−a) − 1 + a. As t_k → 0, for fixed ε−ε_θ, v_k → 0 and hence a → 0. In this limit d(a) ~ a^2/2, so a/d(a) ~ 2/a → ∞, not 1. Equation (11) therefore does not show that the Laplace loss behaves like a weighted L1 norm when t is small. Equation (12) is correct as ||r_k||_1 → ∞, but it does not rescue Eq. (11). This is not merely cosmetic: the interpretation of the non-Gaussian losses as L1-type objectives is part of the paper's motivation and is used to explain the Laplace models' behavior.
minor comments (5)
- [Abstract, Eq. (4), Definition 1] The sign convention for Δx_k is inconsistent: the abstract and Eq. (4) define Δx_k := x_k − x_{k+1}, while Definition 1 and the appendices use Δx_k := x_{k+1} − x_k. This creates confusion in the derivation of the reverse losses and should be fixed throughout.
- [Section 4.3, Eq. (13), Table 1] The constant C is reported inconsistently. Equation (13) states C = 1/2(1 + log(π/6)) ≈ 0.34, but 1/2(1 + log(π/6)) ≈ 0.18, while Table 1 writes 1/2(1 + log√(π/6)) ≈ 0.34. The numerical value and the expression should be reconciled.
- [Appendix A.4] The 'Linear Sub-Gaussian' KL derivation is incomplete and is not connected to any loss in Section 4 or Table 1; the section ends mid-derivation, and the notation s_1, s_2 is never defined in the main text. Either complete the derivation or remove the section.
- [Section 5, Table 2] The experimental support is difficult to assess. Table 2 reports BPD and FID for four configurations, but the footnote says these metrics are computed with different architectures, no standard DDPM/VDM baselines are given, no error bars or training details are provided, and the sentence in Section 5 mentions down-sampled ImageNet but no ImageNet results are shown.
- [Section 3.2] The paper states that 'we verify that we can recover Donsker's theorem when we let f = 0 and g = 1,' but no derivation is shown. Since Donsker's theorem requires independent increments, while the increments in Definition 1 are not independent when f ≠ 0, a brief verification or reference would be helpful.
Circularity Check
No significant circularity: Section 4 losses are closed-form KL divergences with no fitted constants, and Theorem 3.2 is an external convergence result; the unproven ELBO (5) is a soundness gap, not a circular step.
full rationale
The paper's derivation chain is self-contained with respect to its own inputs. The proposed losses in Section 4 (Eqs. 9, 10, 13, 14) are computed in closed form from the KL divergences between chosen increment distributions, with the derivations in Appendix A.5 and the supporting KL formulas in Appendices A.1-A.3; no parameter is fitted to the experimental results and then renamed as a prediction. Theorem 3.2 is an external weak-convergence claim whose proof rests on Billingsley (2013), Kloeden and Platen (1992), and standard central-limit and Slutsky arguments; it does not assume that the reverse conditionals of the non-Gaussian chain are Gaussian or that the specific Laplace/Uniform families of Section 4 are the correct variational posteriors, so those choices are not forced by the theorem. The paper contains no self-citations by the author, and the experiments compare against external benchmarks (CIFAR-10, down-sampled ImageNet) using standard architectures, rather than evaluating an in-sample fit. The most serious concern is not circularity but an unproven premise: Eq. (5) is simply asserted as a likelihood bound ('Letting q be the density of the Gaussian process Eq. 2, Eqs. (3) and (4) result in the same likelihood bound'), and Lemmas A.1-A.4 condition on x_k rather than the x_{k+1} appearing in Eq. (5). If Eq. (5) is not a valid ELBO for non-Gaussian increments, then the Section 4 losses are unjustified, but that is a soundness or correctness gap, not an instance of the derivation reducing to its own inputs by construction. No step defines X in terms of Y, fits a parameter and calls it a prediction, or imports a conclusion through self-citation. Accordingly, the circularity score is 0.
Assumptions & free parameters
assumptions (4)
- domain assumption Assumption 1: f and g are Lipschitz in x and t
- domain assumption Assumption 2: f and g satisfy linear growth bounds
- domain assumption Assumption 3: E||z_k||^4 < K for the increment noise
- ad hoc to paper ELBO Eq. (5) is valid for arbitrary increment distributions
Cite this review
Pith. "Pith review of Non-Normal Diffusion Models." pith.science (2026). https://pith.science/paper/LOUOBERG
@misc{pith2026241207935,
author = {Pith},
title = {Pith review of: Non-Normal Diffusion Models},
year = {2026},
howpublished = {\url{https://pith.science/paper/LOUOBERG}},
note = {Machine review of arXiv:2412.07935}
}
abstract
Diffusion models generate samples by incrementally reversing a process that turns data into noise. We show that when the step size goes to zero, the reversed process is invariant to the distribution of these increments. This reveals a previously unconsidered parameter in the design of diffusion models: the distribution of the diffusion step $\Delta x_k := x_{k} - x_{k + 1}$. This parameter is implicitly set by default to be normally distributed in most diffusion models. By lifting this assumption, we generalize the framework for designing diffusion models and establish an expanded class of diffusion processes with greater flexibility in the choice of loss function used during training. We demonstrate the effectiveness of these models on density estimation and generative modeling tasks on standard image datasets, and show that different choices of the distribution of $\Delta x_k$ result in qualitatively different generated samples.
Figures
Reference graph
Works this paper leans on
-
[1]
write newline
" write newline "" before.all 'output.state := FUNCTION n.dashify 't := "" t empty not t #1 #1 substring "-" = t #1 #2 substring "--" = not "--" * t #2 global.max substring 't := t #1 #1 substring "-" = "-" * t #2 global.max substring 't := while if t #1 #1 substring * t #2 global.max substring 't := if while FUNCTION format.date year duplicate empty "emp...
-
[2]
Anderson, B. D. Reverse-time diffusion equation models. Stochastic Processes and their Applications, 12 0 (3): 0 313--326, 1982
1982
-
[3]
Convergence of probability measures
Billingsley, P. Convergence of probability measures. John Wiley & Sons, 2013
work page 2013
-
[4]
Bouchaud, J.-P. and Georges, A. Anomalous diffusion in disordered media: statistical mechanisms, models and physical applications. Physics reports, 195 0 (4-5): 0 127--293, 1990
work page 1990
-
[5]
Large scale gan training for high fidelity natural image synthesis
Brock, A., Donahue, J., and Simonyan, K. Large scale gan training for high fidelity natural image synthesis. arXiv preprint arXiv:1809.11096, 2018
arXiv 2018
-
[6]
Very deep vaes generalize autoregressive models and can outperform them on images
Child, R. Very deep vaes generalize autoregressive models and can outperform them on images. arXiv preprint arXiv:2011.10650, 2020
arXiv 2011
-
[7]
Generating long sequences with sparse transformers
Child, R., Gray, S., Radford, A., and Sutskever, I. Generating long sequences with sparse transformers. arXiv preprint arXiv:1904.10509, 2019
arXiv 1904
-
[8]
Cugliandolo, L. F. Dynamics of glassy systems. arXiv preprint cond-mat/0210312, 2002
arXiv 2002
Show all 39 references
-
[9]
and Nichol, A
Dhariwal, P. and Nichol, A. Diffusion models beat gans on image synthesis. Advances in Neural Information Processing Systems, 34: 0 8780--8794, 2021
2021
-
[10]
Nice: Non-linear independent components estimation
Dinh, L., Krueger, D., and Bengio, Y. Nice: Non-linear independent components estimation. arXiv preprint arXiv:1410.8516, 2014
2014 arXiv
-
[11]
Density estimation using real nvp
Dinh, L., Sohl-Dickstein, J., and Bengio, S. Density estimation using real nvp. arXiv preprint arXiv:1605.08803, 2016
2016 arXiv
-
[12]
Ethier, S. N. and Kurtz, T. G. Markov processes: characterization and convergence. John Wiley & Sons, 2009
2009
-
[13]
Anomalous diffusion on percolating clusters
Gefen, Y., Aharony, A., and Alexander, S. Anomalous diffusion on percolating clusters. Physical Review Letters, 50 0 (1): 0 77, 1983
1983
-
[14]
Generative adversarial networks
Goodfellow, I., Pouget-Abadie, J., Mirza, M., Xu, B., Warde-Farley, D., Ozair, S., Courville, A., and Bengio, Y. Generative adversarial networks. Communications of the ACM, 63 0 (11): 0 139--144, 2020
2020
-
[15]
Gans trained by a two time-scale update rule converge to a local nash equilibrium
Heusel, M., Ramsauer, H., Unterthiner, T., Nessler, B., and Hochreiter, S. Gans trained by a two time-scale update rule converge to a local nash equilibrium. In Guyon, I., Luxburg, U. V., Bengio, S., Wallach, H., Fergus, R., Vishwanathan, S., and Garnett, R. (eds.), Advances i...
2017
-
[16]
and Barkai, E
Hidalgo-Soria, M. and Barkai, E. Hitchhiker model for laplace diffusion processes. Physical Review E, 102 0 (1): 0 012109, 2020
2020
-
[17]
Denoising diffusion probabilistic models
Ho, J., Jain, A., and Abbeel, P. Denoising diffusion probabilistic models. Advances in Neural Information Processing Systems, 33: 0 6840--6851, 2020
2020
-
[18]
J., Norouzi, M., and Salimans, T
Ho, J., Saharia, C., Chan, W., Fleet, D. J., Norouzi, M., and Salimans, T. Cascaded diffusion models for high fidelity image generation. J. Mach. Learn. Res., 23 0 (47): 0 1--33, 2022
2022
-
[19]
Consistency of pseudolikelihood estimation of fully visible boltzmann machines
Hyv \"a rinen, A. Consistency of pseudolikelihood estimation of fully visible boltzmann machines. Neural Computation, 18 0 (10): 0 2283--2292, 2006
2006
-
[20]
and Dayan, P
Hyv \"a rinen, A. and Dayan, P. Estimation of non-normalized statistical models by score matching. Journal of Machine Learning Research, 6 0 (4), 2005
2005
-
[21]
A style-based generator architecture for generative adversarial networks
Karras, T., Laine, S., and Aila, T. A style-based generator architecture for generative adversarial networks. In Proceedings of the IEEE/CVF conference on computer vision and pattern recognition, pp.\ 4401--4410, 2019
2019
-
[22]
Elucidating the design space of diffusion-based generative models
Karras, T., Aittala, M., Aila, T., and Laine, S. Elucidating the design space of diffusion-based generative models. arXiv preprint arXiv:2206.00364, 2022
2022 arXiv
-
[23]
Variational diffusion models
Kingma, D., Salimans, T., Poole, B., and Ho, J. Variational diffusion models. Advances in neural information processing systems, 34: 0 21696--21707, 2021
2021
-
[24]
Kingma, D. P. and Dhariwal, P. Glow: Generative flow with invertible 1x1 convolutions. Advances in neural information processing systems, 31, 2018
2018
-
[25]
Kingma, D. P. and Welling, M. Auto-encoding variational bayes. arXiv preprint arXiv:1312.6114, 2013
2013 arXiv
-
[26]
Kloeden, P. E. and Platen, E. Stochastic differential equations. Springer, 1992
1992
-
[27]
Learning multiple layers of features from tiny images
Krizhevsky, A., Hinton, G., et al. Learning multiple layers of features from tiny images. Technical Report, 2009
2009
-
[28]
and ksendal, B
ksendal, B. and ksendal, B. Stochastic differential equations. Springer, 2003
2003
-
[29]
Oord, A. v. d., Dieleman, S., Zen, H., Simonyan, K., Vinyals, O., Graves, A., Kalchbrenner, N., Senior, A., and Kavukcuoglu, K. Wavenet: A generative model for raw audio. arXiv preprint arXiv:1609.03499, 2016
2016 arXiv
-
[30]
Salimans, T., Karpathy, A., Chen, X., and Kingma, D. P. Pixelcnn++: Improving the pixelcnn with discretized logistic mixture likelihood and other modifications. arXiv preprint arXiv:1701.05517, 2017
2017 arXiv
-
[31]
and Solin, A
S \"a rkk \"a , S. and Solin, A. Applied stochastic differential equations, volume 10. Cambridge University Press, 2019
2019
-
[32]
Deep unsupervised learning using nonequilibrium thermodynamics
Sohl-Dickstein, J., Weiss, E., Maheswaranathan, N., and Ganguli, S. Deep unsupervised learning using nonequilibrium thermodynamics. In International Conference on Machine Learning, pp.\ 2256--2265. PMLR, 2015
2015
-
[33]
and Ermon, S
Song, Y. and Ermon, S. Generative modeling by estimating gradients of the data distribution. Advances in Neural Information Processing Systems, 32, 2019
2019
-
[34]
Sliced score matching: A scalable approach to density and score estimation
Song, Y., Garg, S., Shi, J., and Ermon, S. Sliced score matching: A scalable approach to density and score estimation. In Uncertainty in Artificial Intelligence, pp.\ 574--584. PMLR, 2020 a
2020
-
[35]
P., Kumar, A., Ermon, S., and Poole, B
Song, Y., Sohl-Dickstein, J., Kingma, D. P., Kumar, A., Ermon, S., and Poole, B. Score-based generative modeling through stochastic differential equations. arXiv preprint arXiv:2011.13456, 2020 b
2011 arXiv
-
[36]
Stroock, D. W. An introduction to Markov processes, volume 230. Springer Science & Business Media, 2013
2013
-
[37]
and Kautz, J
Vahdat, A. and Kautz, J. Nvae: A deep hierarchical variational autoencoder. Advances in neural information processing systems, 33: 0 19667--19679, 2020
2020
-
[38]
Pixel recurrent neural networks
Van Den Oord, A., Kalchbrenner, N., and Kavukcuoglu, K. Pixel recurrent neural networks. In International conference on machine learning, pp.\ 1747--1756. PMLR, 2016
2016
-
[39]
and Romanovsky, M
Vidov, P. and Romanovsky, M. Y. Analytical representations of non-gaussian laws of random walks. Physics of wave phenomena, 17: 0 218--227, 2009
2009
Reviewed August 11, 2026 · model on record in the stance chip above.
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