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REVIEW 5 major objections 6 minor 38 references

VARD: Efficient and Dense Fine-Tuning for Diffusion Models with Value-based RL

T0 review · 5 major / 6 minor · reviewed 2026-08-07 · deepseek-v4-flash

Pith's one-line read A Monte Carlo-trained value function replaces sparse final rewards with dense differentiable supervision across the diffusion trajectory for stable RL fine-tuning with differentiable or non-differentiable rewards.

desk verdict VARD is a genuine new combination of known ideas, with honest limitations, but the non-differentiable protein experiment is weaker than the rest and needs a redo or a reframing before the headline claim is credible. read the letter →

arxiv 2505.15791 v2 pith:3LLDM5CB submitted 2025-05-21 cs.CV cs.LG

classification cs.CVcs.LG
keywords diffusionmodelsreinforcementlearningfine-tuningprocessrewardmodelvaluefunctiondensesupervisionnon-differentiablerewardsKLregularization
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

VARD (Value-based Reinforced Diffusion) claims that the sparse, final-step-only rewards that limit RL fine-tuning of diffusion models can be replaced by a learned value function $V_\phi(x_{T-t}, c)$ that predicts, from any intermediate denoised state, the expected final reward of all trajectories passing through that state. Because the value function is differentiable, the same backpropagation recipe works whether the external reward is differentiable (human-preference scorers) or not (protein secondary-structure composition, JPEG file size), avoiding the high-variance gradients of policy-gradient methods. The paper argues that this dense, trajectory-wide supervision converges faster, uses samples more efficiently, and sits closer to the pretrained distribution than final-step backpropagation (DRaFT, ReFL) or policy gradients (DDPO). If the claim holds, the standard dilemma of diffusion RL fine-tuning dissolves: stability and efficiency no longer require a differentiable reward, and non-differentiable objectives no longer require unstable policy gradients.

What carries the argument

The central object is the value function, also called a process reward model: a network $V_\phi(x_{T-t}, c)$ that maps any intermediate noisy state, together with its prompt, to the expected final reward of all trajectories that pass through that state. It is trained by Monte Carlo regression (Eq. 8) on final rewards of trajectories from the fixed pretrained policy, which lets the network generalize the final reward backward across the denoising chain. During fine-tuning it becomes the differentiation engine: the policy climbs the predicted values in Eq. (9), turning a sparse or non-differentiable reward into a dense, smooth signal. The companion mechanism is Lemma 1, which states that the gradient of the squared distance between the updated and pretrained samples equals $2\sigma^2$ times the gradient of the KL divergence between the two Gaussian denoising steps, so the KL anchor costs nothing extra in the same backprop. The value function receives lightweight online updates (learning rates $10^{-6}$ to $5\times10^{-6}$) to track the moving policy, and for differentiable rewards it is implemented as the reward model itself with LoRA adapters.

What would settle it

Two concrete checks would settle the claim. First, freeze the value function after pretraining (no online updates) and compare reward curves with full VARD: if the frozen variant matches it, the online recalibration is doing no work, and if it collapses, calibration is the load-bearing part. Second, replace the learned value with the true final reward backpropagated through every denoising step on a differentiable reward: if that dense final-reward baseline matches VARD, the learned value adds nothing beyond densification. A diagnostic third check is to measure Eq. (8)'s prediction error on states sampled from the moving policy as training progresses and ask whether rewards keep improving while this error grows large.

Watch

Extended reading notes

Core claim

The paper's central claim is that a Monte Carlo-trained value function can carry the entire reward signal of diffusion-model fine-tuning. The value function $V_\phi$ is trained by Eq. (8), regressing the final rewards of complete trajectories sampled from the pretrained policy onto each intermediate state, and it is then used in the fine-tuning objective of Eq. (9): $J(\theta) = -\mathbb{E}[V_\phi(x_{T-t}, c) + \eta \|x_{T-t} - x^0_{T-t}\|^2]$, where the second term anchors the updated policy to the pretrained one through a per-step squared-distance surrogate for the KL divergence (Lemma 1). Backpropagating through the value function gives the policy a dense, smooth gradient at every denoising step, and the same mechanism serves non-differentiable rewards because the value function, not the reward, is what is differentiated. The authors report steeper reward-versus-sample curves than DRaFT, ReFL, and DDPO on Aesthetic Score, PickScore, and ImageReward; more $\beta$-sheet-rich and structurally diverse proteins from the FoldFlow-2 base model; and resistance to reward hacking, with prompt alignment and low FID to the base model preserved even at high reward.

Load-bearing premise

The load-bearing premise is that the value function stays accurate for the states the updated policy visits: it is pretrained on trajectories of the frozen pretrained model and only lightly updated during fine-tuning, and the paper's own Limitation section names value-function accuracy as the primary risk, so if $V_\phi$ becomes miscalibrated on the moving policy's states, the dense gradient in Eq. (9) optimizes a stale surrogate rather than the true reward.

Editorial extensions

If this is right

  • Any reward that can be scored on final samples, differentiable or not, becomes trainable by stable backpropagation once distilled into the value function, removing the differentiability constraint that limits final-step backpropagation methods.
  • Fine-tuning needs far fewer reward evaluations, since the reported reward-versus-sample curves rise faster than DDPO, ReFL, and DRaFT, so a given reward level is reached with less sampling.
  • The per-step KL anchor keeps the optimized policy near the pretrained distribution, which the experiments tie to reduced reward hacking, lower per-prompt FID to the base model, and higher HPSv2 scores on unseen prompts.
  • The recipe transfers across domains (text-to-image and protein backbones) and across base models (SD 1.4 and SDXL with LoRA), suggesting the value-function mechanism rather than a task-specific trick delivers the gains.
  • Trajectory-level supervision generalizes beyond the training reward: the VARD-trained model scores highest on all four HPDv2 test splits when trained with HPSv2 on the training split.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Reasoning from the paper's own limitation section, the method's ceiling is the value function's calibration: if $V_\phi$'s prediction error grows on the states the updated policy visits during fine-tuning, the dense gradient in Eq. (9) optimizes a stale surrogate. A direct test is to plot that held-out error against training step and ask whether reward gains track value accuracy.
  • The protein experiment is a sharper test than the image experiments because the value function there is trained on clean SCOPe structures rather than on generated trajectories (Appendix A.4); if offline-curated value pretraining matches on-policy pretraining elsewhere, value learning could be decoupled from the generative model entirely.
  • Because Lemma 1's KL equivalence assumes Gaussian conditional steps with shared covariance, the anchor's justification weakens for non-Gaussian samplers such as DDIM or distilled few-step models; an explicit KL estimator is the natural extension for those samplers.
  • The trajectory-wise reward alignment claim presumes that a scalar expected value adequately summarizes the future; for heavy-tailed or multi-modal rewards, a distributional or quantile value head would test whether the mean is the right statistic.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

5 major / 6 minor

Summary. The paper proposes VARD (Value-based Reinforced Diffusion), a method for fine-tuning pretrained diffusion models with reinforcement learning. VARD first trains a value function (also called a process reward model) to predict the expected final reward from intermediate diffusion states, using Monte Carlo regression on trajectories of the frozen pretrained policy (Eq. 8). It then fine-tunes the diffusion policy by backpropagating through this value function with a KL-regularization term that penalizes deviation from the pretrained model (Eq. 9). The method is evaluated on non-differentiable rewards (protein secondary-structure diversity, image compressibility) and differentiable rewards (Aesthetic Score, PickScore, ImageReward, HPSv2), with additional experiments on unseen-prompt generalization, cross-reward validation, and prior preservation. The central claim is that dense value-based supervision yields faster convergence, better sample efficiency, and stronger reward alignment than final-step backpropagation (DRaFT, ReFL) or policy-gradient (DDPO) baselines, while staying close to the pretrained distribution.

Significance. If the central claims hold, VARD would be a useful contribution to the growing literature on RL fine-tuning of diffusion models, particularly for non-differentiable rewards where policy-gradient methods are sample-inefficient. The idea of using a learned value function to provide dense, differentiable supervision throughout the diffusion trajectory is natural and potentially impactful. The paper includes comparisons on both protein and image domains, a generalization study, and a discussion of reward hacking. However, the current demonstration is weakened by a likely sign error in the main objective, a post hoc change to the protein reward weights, and the acknowledged but unaddressed issue of value-function calibration on the moving policy's states. If these are corrected, the method could be a solid contribution; as presented, the evidence does not yet firmly establish the advantages claimed.

major comments (5)
  1. [Eq. (9), Section 3.2] The KL regularization term in Eq. (9) has the wrong sign. The objective is written as J(θ) = -E[Vφ(x_{T-t}, c) + η·||x_{T-t} - x^0_{T-t}||²]. Minimizing J with respect to θ is equivalent to maximizing E[V] + η·E[||x - x^0||²], which would push the fine-tuned model away from the pretrained model rather than keeping it close. The text states that the KL term 'serves to prevent model collapse' by 'minimizing the gap' between the outputs, and Lemma 1 is invoked to justify this. The correct objective for KL-penalized reward maximization should be J(θ) = -E[V] + η·E[||x - x^0||²], i.e., a minus sign before the η term. This is load-bearing: all experiments and the KL ablation (Figure 10) depend on the actual objective, and if the sign in the text is a typo while the implementation uses the corrected sign, the paper needs to state this explicitly. Please correct the equation and clarify the implemented objective.
  2. [Section 7, Eq. (8), Appendix A.4, Tables 2-3] The central assumption that the value function Vφ, trained on trajectories of the frozen pretrained policy p_{θ0}, remains an accurate predictor of expected final reward on states visited by the moving policy p_θ is not validated. The paper's own Limitation section (Section 7) names value-function accuracy as the primary limitation, and the online update schedules are very light: value learning rates of 1e-6 to 5e-6 over 30 to 1,000 training steps (Tables 2-3), with only 8 value-pretraining steps for differentiable rewards. For the protein experiment, Appendix A.4 states that the value function was trained on the SCOPe dataset rather than on FoldFlow-2 trajectories, so it does not follow Eq. (8) at all. Without evidence that Vφ is calibrated on the fine-tuned policy's states, the dense supervision in Eq. (9) may simply be optimizing a stale surrogate, and the claimed advantage over final-step backpropagation is not established. Please provide calibration plots (e.g., predicted value vs. Monte Carlo return on held-out states from the fine-tuned policy) or otherwise demonstrate that the value function tracks the moving policy's state distribution.
  3. [Appendix A.4, Table 4, Section 4.1] The protein secondary-structure reward weight w_b was changed from 2 to 5 after observing that the original-weight FoldFlow-2-ReFT baseline underperformed the base model. Table 4 shows that with w_b=2, FoldFlow-2-ReFT has a lower reward than FoldFlow-2-base (0.76 vs. 0.83), and only with w_b=5 does the baseline outperform base (1.27 vs. 0.85). This post hoc adjustment, made 'so that the reward of FoldFlow-2-ReFT can be higher' than the base, is not a principled a priori choice and directly affects the comparison between VARD and the baseline. The claim that the original w_b=2 is a 'typo' is not substantiated. Please report results for both weight settings, or justify the chosen weights independently of the outcomes.
  4. [Table 1, Appendix A.4] Table 1 compares cross-prompt generalization scores but does not disclose that DDPO and PRDP are trained with 512,000 sampled images, while ReFL, DRaFT-1, DRaFT-LV, and VARD are trained with 160,000 (as stated in Appendix A.4). Since the paper emphasizes sample efficiency, the comparison should hold the sample budget fixed or visibly mark the sample counts in the table. As presented, the reader cannot assess whether VARD's advantage stems from the method or from a different training budget.
  5. [Eq. (7), Section 3.2] The definition of the value function is notationally unclear. The expression Vφ(x_{T-t}, c) := E_{t∈[0,T]}[Σ_{t'=t}^T R(...) | x_{T-t'}] appears to condition on x_{T-t'} for a range of t', which is not the standard conditional expectation given the current state x_{T-t}. Presumably the expectation is over future denoising steps given x_{T-t} and c; please rewrite this definition with explicit randomness and conditioning to avoid confusion.
minor comments (6)
  1. [Throughout] The notation (x, x^0)_{T-t} ∼ (p_θ, p_{θ0})(·|x_{T-t+1}, c) in Eq. (9) is compressed; it would be clearer to define the two samples separately, as done in the text following the equation.
  2. [Section 2.1] There are minor typographical errors, e.g., 'predifined' for 'predefined', and 'approximates' for 'approximate'.
  3. [Abstract] The abstract contains the typo 'expection' instead of 'expectation'.
  4. [Section 3.2] The statement 'This can also be supported by Lemma 1' would benefit from a brief explanation of how the lemma connects the MSE term to a KL divergence, since the derivation is not immediate.
  5. [Figure 6] The caption 'Reward v.s. sampled images' is ambiguous; it should say something like 'Reward as a function of the number of training samples.'
  6. [Appendix A.4] The sentence 'we refer to the FoldFlow-2-ReFT as FoldFlow-2-base after the reinforced fine-tuning introduced in the original FoldFlow-2 paper' is confusing; it seems to conflate the baseline and the fine-tuned model. Please clarify the naming.

Circularity Check

0 steps flagged · score 2.0 of 10

No material circularity: VARD's value function is a fitted surrogate, but the paper's central claims are validated by external reward models, cross-reward checks, and FID distance; the single self-citation is background and not load-bearing.

full rationale

The derivation chain is a standard actor-critic construction. Equation (8) fits V_phi by Monte Carlo regression to final rewards on pretrained-policy trajectories; Equation (9) then optimizes that value estimate (plus an MSE surrogate for KL, whose gradient identity is proved in Lemma 1 and Appendix A.2). This is not a circular reduction: the paper's reported successes are measured on the actual reward models (Aesthetic, PickScore, ImageReward), on HPSv2 cross-reward validation, and on FID distance to the pretrained model, none of which are V_phi's own predictions. The admitted limitation in Section 7 that value-function accuracy 'critically influences the entire fine-tuning process' is an approximation-risk statement, not an admission that the objective equals its input. The non-differentiable protein demonstration is weakened by training the value function on SCOPe rather than FoldFlow-2 trajectories (Appendix A.4) and by adjusting wb from 2 to 5 after the ReFT baseline underperformed (Table 4); these are experimental-validity concerns, not construction-level circularity. The only self-citation (Zhuang et al. 2023) appears in the offline-RL background sentence and is not load-bearing. Hence score 2 reflects a minor self-citation and approximate-surrogate caveats, with no identified circular step.

Assumptions & free parameters 4 free parameters · 4 assumptions · 0 invented entities

The central claim rests on two hand-tuned weights (eta and the protein w_b), one hand-chosen training horizon, one value-pretraining budget, and four domain assumptions about the MDP framing and value-function calibration. The value function itself is a learned regression model, not a free scalar parameter, but its accuracy on the moving policy's states is the main unverified premise. No new physical entities are introduced.

free parameters (4)
  • KL regularization weight (eta) = 0.1 (protein), 1 (compressibility), 100 (Aesthetic), 0.5 (PickScore), 20 (ImageReward)
    Hand-chosen per task; ablations (Appendix A.5.1, Figure 10) show higher eta preserves priors at the cost of slower reward growth, so the value directly controls the reported prior-preservation result.
  • Protein secondary-structure reward weights (w_a, w_b, w_c) = w_a=1, w_b=5, w_c=0.5 (w_b raised from 2)
    w_b was increased from the FoldFlow-2 paper's value of 2 after the authors observed that with w_b=2 the ReFT baseline underperformed the base model (Table 4). This post hoc choice is part of the non-differentiable reward showcase.
  • Image fine-tuning horizon = final 10 of 50 denoising steps for images; all 50 for protein
    Chosen by hand based on the observation that human-preference and compressibility rewards are set by late-stage details (Appendix A.4). The abstract's 'throughout the entire diffusion process' claim is implemented over only the last 20 percent of steps for images.
  • Value-pretraining budget for differentiable rewards = 8 training steps (batch 32)
    Selected in preliminary experiments because the reward-model-initialized value function 'learns rapidly' (Appendix A.3). In effect the method skips the pretraining stage it describes and relies on the pretrained reward model as the value function.
assumptions (4)
  • domain assumption Conditional distributions p_theta and p_theta0 are Gaussians with identical covariance sigma^2 I, making the gradient of the sample MSE equal to 2 sigma^2 times the gradient of the KL (Lemma 1).
    Appendix A.2 (Eq. 15-18) derives the identity under equal covariance. DDPM schedules fix the covariance per step, so the gradient identity holds, but the main text's 'equivalent to minimizing the KL' overstates an objective-level equivalence; the MSE contains an extra constant 2 sigma^2 d term.
  • domain assumption The denoising process is an MDP with deterministic transitions and a sparse terminal reward (Eq. 6).
    Section 3.1 defines states as noisy latents plus prompt and rewards only the final step. This is the standard DDPO-style framing and is not questioned in the paper.
  • domain assumption A value function regressed on the pretrained model's trajectories remains an accurate predictor of expected final reward on the intermediate states visited by the updated policy during fine-tuning.
    Eq. (8) fits V_phi on p_theta0 trajectories; after p_theta moves, the estimates become off-distribution. The Limitation section (Section 7) admits value-function accuracy is the primary limitation and that pretraining is sometimes needed. For protein, the value function is trained on clean SCOPe structures, making this gap larger.
  • domain assumption Diffusion trajectories are consistent enough that intermediate states carry reliable information about the final reward.
    Section 1 states 'diffusion trajectories generally maintain greater consistency throughout the process' to justify importing the PRM idea from LLMs. This is a qualitative empirical premise that determines whether the dense signals are informative.

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Cite this review

Pith. "Pith review of VARD: Efficient and Dense Fine-Tuning for Diffusion Models with Value-based RL." pith.science (2026). https://pith.science/paper/3LLDM5CB

@misc{pith2026250515791,
  author       = {Pith},
  title        = {Pith review of: VARD: Efficient and Dense Fine-Tuning for Diffusion Models with Value-based RL},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/3LLDM5CB}},
  note         = {Machine review of arXiv:2505.15791}
}
read the original abstract

Diffusion models have emerged as powerful generative tools across various domains, yet tailoring pre-trained models to exhibit specific desirable properties remains challenging. While reinforcement learning (RL) offers a promising solution,current methods struggle to simultaneously achieve stable, efficient fine-tuning and support non-differentiable rewards. Furthermore, their reliance on sparse rewards provides inadequate supervision during intermediate steps, often resulting in suboptimal generation quality. To address these limitations, dense and differentiable signals are required throughout the diffusion process. Hence, we propose VAlue-based Reinforced Diffusion (VARD): a novel approach that first learns a value function predicting expection of rewards from intermediate states, and subsequently uses this value function with KL regularization to provide dense supervision throughout the generation process. Our method maintains proximity to the pretrained model while enabling effective and stable training via backpropagation. Experimental results demonstrate that our approach facilitates better trajectory guidance, improves training efficiency and extends the applicability of RL to diffusion models optimized for complex, non-differentiable reward functions.

Figures

Figures reproduced from arXiv: 2505.15791 by the authors.

Figure 1
Figure 1. Illustration of the proposed VARD. Different colors represent distinct diffusion trajecto￾ries, indexed by the superscripts on states (x i t ) and final rewards (Ri ). At intermediate steps, VARD’s learned PRM calculates the value as the expected final reward, estimated by averaging Ri of relevant trajectories considered to pass through the current state representation. For instance, as depicted for the intermediate… view at source ↗
Figure 2
Figure 2. Qualitative analysis of generated protein structure from FoldFlow-2 base and VARD version. Alpha-helices, beta-sheets and coils are colored yellow, green and gray, respectively. The VARD version produces structures with significantly more beta-sheet regions, whereas the FoldFlow￾2 base model generates almost exclusively alpha-helix regions. • During this stage, the diffusion model generates a new sample xT −t at eac… view at source ↗
Figure 3
Figure 3. Secondary structure distribution com￾parison between FoldFlow-2-base and FoldFlow-2- VARD generated proteins. Proteins are sampled at 50-residue increments between lengths 100-300, with 64 samples per length. Motivated by this issue, we aim to fine￾tune the pretrained SE(3) diffusion frame￾work [Yim et al., 2023] to recalibrate con￾formational sampling. Following FoldFlow￾2 [Huguet et al., 2024], we build a reward m… view at source ↗
Figures from the paper (17 more)
Figure 4
Figure 4. Figure 4: Qualitative analysis of compressibility vs. incompressibility rewards. Generated images correspond to checkpoints saved at 20-step intervals during training. AestheticScore PickScore ImageReward ReFL SD-1.4 DRaFT-1 VARD a painting of a dog a painting of a fox female po…
Figure 5
Figure 5. Figure 5: Comparative visualization. Compared to ReFL and DRaFT-1 aligned with specified reward preferences, VARD achieves demonstrably superior image quality and enhanced realism. experiments are conducted on the 45 animal prompts. As shown in [PITH_FULL_IMAGE:figures/full_fig…
Figure 6
Figure 6. Figure 6: Reward v.s. sampled images. Both VARD w/o KL and VARD attain higher reward values than baselines with substantially fewer training samples. set of 45 animal prompts. For PickScore and ImageReward, we directly adopt their native training prompts (Pick-a-Pic and ImageRew…
Figure 7
Figure 7. Figure 7: A example of reward hacking. Directly backpropagation without KL catas￾trophically loses ability to generate prompt￾aligned images, degenerating into texture-like noise patterns. All of the images is generated from checkpoints optimized with 500 steps. Results [PITH_F…
Figure 8
Figure 8. Figure 8: Evaluated by HPSv2. VARD consistently attains the highest HPSv2 scores while simultaneously preserving a modest reward. Exp3: Generalization FID-reward validation AestheticScore PickScore ImageReward FID AestheticScore PickScore ImageReward FID [PITH_FULL_IMAGE:figure…
Figure 9
Figure 9. Figure 9: Prior maintenance. The image generated by VARD remains the closest to the base model as the reward increases. standard reward backpropagation approaches. VARD w/o KL demonstrates superior unseen-prompt generalization by outperforming ReFL and DRaFT across all four test…
Figure 10
Figure 10. Figure 10: Training trend of FID (left) and reward (right) under KL regularization. Color intensity correlates with η magnitude (darker corresponds to higher values). Higher η configurations maintain lower FID scores (better prior preservation) while exhibiting slower reward gro…
Figure 11
Figure 11. Figure 11: Uncurated protein structures generated by FoldFlow-2-VARD. The showcased samples include proteins with various secondary structures, such as those containing beta-sheets. This illustrates that FoldFlow-2-VARD maintains a broad generative capacity, retaining its abilit…
Figure 12
Figure 12. Figure 12: Uncurated samples of fine-tuning on compressibility and incompressibility as re￾wards. 21 [PITH_FULL_IMAGE:figures/full_fig_p021_12.png]
Figure 13
Figure 13. Figure 13: Uncurated samples of fine-tuning on Aesthetic Score as the reward. Prompt: Rocket Raccoon, furry art, fanart, digital painting Prompt: A 1956 Ford Mustang Cobra painted black with gold stripes, fisheye lens, abandonded industrial complex, high contrast SD 1.4 VARD SD …
Figure 14
Figure 14. Figure 14: Further comparison of the base model and VARD fine-tuned version with PickScore as reward. 22 [PITH_FULL_IMAGE:figures/full_fig_p022_14.png]
Figure 15
Figure 15. Figure 15: Further comparison of the base model and VARD fine-tuned version with ImageRe￾ward as reward. Prompt: Oil on canvas painting by John Singer Sargent of one billion parrots in flight. Prompt: A Goku Lego in 3D render. SD 1.4 VARD SD 1.4 VARD [PITH_FULL_IMAGE:figures/fu…
Figure 16
Figure 16. Figure 16: Further comparison of the base model and VARD fine-tuned version with HPSv2 as reward. 23 [PITH_FULL_IMAGE:figures/full_fig_p023_16.png]
Figure 17
Figure 17. Figure 17: Uncurated samples from fine-tuning SDXL on Aesthetic Score as the reward. 24 [PITH_FULL_IMAGE:figures/full_fig_p024_17.png]
Figure 18
Figure 18. Figure 18: Further comparison of the SDXL and VARD fine-tuned version with PickScore as reward. 25 [PITH_FULL_IMAGE:figures/full_fig_p025_18.png]
Figure 19
Figure 19. Figure 19: Further comparison of the SDXL and VARD fine-tuned version with ImageReward as reward. 26 [PITH_FULL_IMAGE:figures/full_fig_p026_19.png]
Figure 20
Figure 20. Figure 20: Further comparison of the SDXL and VARD fine-tuned version with HPSv2 as reward. 27 [PITH_FULL_IMAGE:figures/full_fig_p027_20.png]

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Pith tools

Reviewed August 7, 2026 · model on record in the stance chip above.