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REVIEW 4 major objections 5 minor 62 references

DocShaDiffusion: Diffusion Model in Latent Space for Document Image Shadow Removal

T0 review · 4 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash

Pith's one-line read The paper claims that latent-space diffusion with shadow-mask guidance removes document shadows, including colored ones, better than existing methods.

desk verdict The forward SDE contradicts the method's own pipeline, so this needs major revision despite a potentially useful dataset. read the letter →

arxiv 2507.01422 v1 pith:QA4GRO6I submitted 2025-07-02 cs.CV cs.AI

classification cs.CVcs.AI
keywords documentimageshadowremovallatentdiffusionmodelsoft-maskcolorshadowssyntheticdatasetmask-guidedshadow-robustperceptualloss
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

This paper aims to make document image shadow removal work for colored shadows, not just the constant-color shadows that earlier methods target. It proposes DocShaDiffusion, a diffusion model that operates in latent space and uses a predicted shadow soft-mask to concentrate noise and denoising on shadowed regions, together with a shadow-robust perceptual loss. The authors also build SDCSRD, a 17,624-image synthetic dataset designed to mimic real color-shadow distributions, and report that their method outperforms prior state-of-the-art methods on the Adobe, FSDSRD, and SDCSRD benchmarks, with particularly large gains on their own dataset. The practical goal is a usable document shadow remover plus a training set that other methods can adopt.

What carries the argument

The load-bearing object is the shadow soft-mask $I^m$ produced by SSGM (Algorithm 1), a normalized map with values in $[0,1]$, set to 0 outside shadows and near 1 in the darkest shadow interior, obtained by sorting filtered grayscale pixels and thresholding with the mean of the darkest fraction. Inserted into the forward SDE $dx = \theta_t(\mu - x)\,dt - \sigma_t(1 - I^m)\,dw$, the mask is intended to concentrate random perturbation and denoising effort on shadow regions while leaving non-shadow areas largely untouched. The same mask guides a latent-space denoising network built from NAFBlocks with time embeddings, and the loss adds a VGG-19-based perceptual term that is designed to be less sensitive to shadow regions.

What would settle it

Take any SDCSRD shadow image for which the ground truth is known and measure the color of a shadowed patch before and after removing the shadow. If the synthetic shadow only scales RGB values by a scalar factor, the hue angle in the shadowed patch equals the hue in the ground-truth patch; a dataset that claims color shadows should show a shift in hue or saturation. A second check compares SSGM masks with manually annotated shadow boundaries on images with colored shadows on colored paper; low IoU on such images would mean the mask guidance fails exactly in the regime the paper targets.

Watch

Extended reading notes

Core claim

On its own terms, the paper establishes that document shadow removal can be cast as a mask-guided latent diffusion problem. The shadow soft-mask generation module (SSGM) derives a soft mask from a single shadow image through grayscale conversion, dilation, median filtering, and luminance sorting, while the shadow mask-aware guided diffusion module (SMGDM) uses that mask to control the noise term in an IR-SDE forward process and to supervise the reverse denoising. Training couples the diffusion objective with a shadow-robust perceptual feature loss computed from VGG-19 features. According to the reported experiments, the resulting system outperforms existing heuristic and neural document shadow removal methods on Adobe, FSDSRD, and SDCSRD, with the largest margins on the synthetic color-shadow dataset it introduces.

Load-bearing premise

The load-bearing premise is that synthetic shadows generated by Eq. 1 with a scalar $C_k$ adequately represent real colored shadows, and that the luminance-sorting heuristic in SSGM can locate those shadows. If $C_k$ is truly scalar, the synthetic images are darker versions of the same hue, so the dataset contains no genuine color shadows and the paper's core motivation is not exercised.

Editorial extensions

If this is right

  • Document shadow removal can be formulated as a mask-guided latent diffusion problem, so shadow regions receive targeted noise and denoising while background detail is preserved.
  • The synthetic color-shadow dataset SDCSRD, generated with random shadow weight and shadow color parameters, gives other methods a large-scale training resource that existing synthetic datasets such as FSDSRD do not provide.
  • Because only shadow regions need to be denoised, the approach is expected to train faster and to avoid unnecessary changes to non-shadowed text and background.
  • The reported numbers on Adobe, FSDSRD, and SDCSRD indicate the model transfers to real document images without retraining on those sets, although on the RDD dataset it does not surpass CBENet, which was trained on RDD.
  • The ablation results suggest that the NAFBlock encoder-decoder, the mask-aware diffusion guidance, and the perceptual loss each contribute to the final performance, with the architecture change giving the largest single jump in PSNR and LPIPS.

Reading between the lines

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

  • If Eq. 1 is read literally with scalar $C_k$, the synthetic shadows are only darker versions of the same hue, so the dataset would not actually contain chromatic color shadows; the color-shadow story requires $C_k$ to be a color vector or channel-wise blending, which the text does not state.
  • The very large performance gap on SDCSRD compared with the more modest gains on RDD suggests that much of the reported advantage comes from matching the synthetic degradation model; a decisive test would be evaluation on real captured colored shadows on colored paper.
  • The same SSGM-plus-SMGDM recipe could plausibly transfer to other localized document degradations such as stains, watermarks, or demoiréing, since the mask mechanism is agnostic to the specific degradation type.
  • A direct reading of Eq. 5 gives noise coefficient $1-I^m$, which suppresses noise where the mask is 1, the opposite of the text's claim that noise is amplified in shadow regions; reconciling this sign would clarify what the mask is actually doing.
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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

4 major / 5 minor

Summary. The paper proposes DocShaDiffusion, a latent-space diffusion model for document image shadow removal. The method consists of a shadow soft-mask generation module (SSGM) that produces a soft mask from the luminance ordering of the input, and a shadow mask-aware guided diffusion module (SMGDM) built on the IR-SDE framework. The authors also introduce a synthetic dataset, SDCSRD, generated by a linear blending formula, and a shadow-robust perceptual feature loss based on VGG-19 features. Experiments are reported on Adobe, FSDSRD, SDCSRD, and RDD, with the central claim that DocShaDiffusion outperforms the state of the art, especially on color shadows. The paper states that code and dataset will be released.

Significance. If correct, the paper would make two contributions: a diffusion-based architecture tailored to document shadow removal, and a synthetic color-shadow dataset that could serve as training data for other methods. The ablation study shows a large positive effect of the proposed components (PSNR increases from 23.20 to 32.54 when all components are included), and the reported numbers on Adobe and FSDSRD are competitive with existing methods. However, the significance assessment is weakened by the absence of code/data, the lack of statistical validation, and, more importantly, by internal inconsistencies in the mathematical formulation of the core diffusion module and in the color-shadow dataset construction. The claimed SOTA result on the authors' own synthetic dataset is also not established under a fair comparison protocol.

major comments (4)
  1. [Section IV.B, Eqs. (5)-(6)] The forward SDE in Eq. (5) is internally inconsistent with the stated restoration goal. The text defines x as the shadowed image and mu as the shadow-free target. Under that assignment, the drift theta_t(mu - x) pushes the state toward the clean image, so X_T at the end of the forward process is approximately clean-plus-noise. The reverse SDE in Eq. (6) is the time reversal of that map and would send clean-plus-noise back toward shadowed images, not the other way around. The pipeline described in Section IV.A and Fig. 4 instead constructs X_T by adding mask-modulated noise to the shadowed input and then denoises to obtain a shadow-free image, which requires the opposite convention, namely mu equal to the shadowed observation, as in the cited Refusion work. Independently, the noise coefficient in Eq. (5), -sigma_t(1 - I_m)dw, vanishes where I_m = 1, i.e., in the darkest shadow regions, whereas the text in Section IV.B and the contribution bullet in the Introduction state that noise is added 'specially to shadow regions.' The two statements cannot both be true. This issue is load-bearing: as written, the core SMGDM module does not implement the described restoration process, and without released code or data the experimental tables cannot resolve which convention was actually used.
  2. [Section III.A, Eq. (1)] The dataset construction formula in Eq. (1) cannot generate color shadows as claimed. The text states that both a and C_k are scalar values, and I_sf, I_m are images in RGB color space. A scalar C_k multiplied by a scalar-valued mask (after the stated histogram equalization) scales all three color channels identically, producing grayscale shadow intensity shifts but no chromatic shift. Thus SDCSRD, as specified, is a gray-shadow dataset, not a color-shadow dataset, directly contradicting the paper's central motivation that existing methods 'ignore color shadows' and that SDCSRD 'simulates the distribution of realistic color shadows.' To produce colored shadows, C_k would need to be a three-dimensional vector (or otherwise channel-dependent). The authors should either correct Eq. (1) to include per-channel color factors or revise the claim that the dataset addresses color shadows.
  3. [Section V.B, Tables II and IV] The claim of outperforming the state of the art is not supported by the evaluation on SDCSRD. In Table II, DocShaDiffusion is trained on SDCSRD, while all baseline methods are evaluated with their original weights or provided results, as stated in Section V.B ('The results of other models are provided by MS-GAN, DCShadow-Net, CBENet'). The very large margins on SDCSRD (e.g., PSNR 41.66 vs. CBENet's 24.58) therefore largely reflect a training-set advantage, not model superiority. The averaging in Table IV mixes heterogeneous conditions: CBENet is trained on RDD while the proposed method is trained on SDCSRD. A valid SOTA claim requires a fair protocol, e.g., retraining all baselines on SDCSRD or testing all methods under the same cross-dataset conditions, or at minimum clearly labeling the tables as 'trained on SDCSRD' vs. 'not trained on SDCSRD.' Without this, the central 'superiority over state-of-the-art' assertion is not established.
  4. [Section V.A and V.C] The paper reports no error bars, repeated runs, or significance tests for any of the quantitative claims, including the statements in Section V.C that NAFBlock, SMGDM, and L_fea 'play important role' and 'prove beneficial.' Additionally, the feature-loss weights w_i in Eq. (8) are never specified, so the loss in Eq. (9) is incompletely defined. Given the single-run nature of the tables and the unspecified weights, the 'significantly outperforms' language in Section V.B and the conclusion is not backed by statistical evidence.
minor comments (5)
  1. [Throughout] There are numerous typographical errors and inconsistencies: 'datset' for dataset, 'tansformer' for transformer, 'P SN R' spacing artifacts in Tables I-III, and 'the k-th slice (k = 0, 1, ... , 4)' in Section IV.C, where k should range over 0..4 consistently.
  2. [Section IV.C, Eq. (7)] The notation is unclear: the loss uses ∥e_t − ϵ∥, where e_t is described as the predicted noise map, but e_t is not defined in the text; presumably it should be ϵθ, the neural network's noise prediction. Please clarify.
  3. [Fig. 3 and Section III.B] The dataset 'HS' is referenced in the caption of Fig. 3 and in Section III.B, but it is never introduced or defined in the paper. Please identify which real dataset is meant.
  4. [Tables I and II] The venue/year for 'Our Method' is listed as 'CVPR/2024,' which is not appropriate for a submitted manuscript and does not match the paper's arXiv date of July 2025. This should be corrected to avoid confusing the reader.
  5. [Section IV.B] The sentence 'The experimental results indicate that executing the denoising process in pixel space would necessitate excessive computational resources' reports a result that is never shown; either provide the supporting experiment or remove the unsupported claim.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity found: the paper's central claim is empirical and externally benchmarked; the noted SDE and mask issues are correctness concerns, not input-output equivalence.

full rationale

DocShaDiffusion's derivation chain does not reduce to its inputs. The SSGM shadow soft-mask is a heuristic preprocessing step (Algorithm 1 and Eqs. 2-4), not a fitted quantity renamed as a prediction. The SMGDM inherits the IR-SDE forward and reverse equations from external prior work (Refusion, reference [54]), and the training objective is a standard diffusion noise-prediction loss (Eq. 7) combined with a VGG-based perceptual loss (Eq. 9); no parameter is fitted to the target and then reported as a prediction. The main empirical claim is tested against external benchmarks (Adobe, FSDSRD, and RDD) as well as the authors' own SDCSRD, so the state-of-the-art comparison is not statistically forced by construction. The paper's self-citations ([5], [17], [18], [33]) refer to prior heuristic baselines and are not load-bearing for the diffusion design. The internal inconsistencies noted by the reviewer (Eq. 1 using a scalar Ck to model color shadows, and the apparent forward/reverse semantics of Eqs. 5-6 together with the (1-I_m) mask weighting) are correctness or clarity risks rather than circularity, because the text never identifies a predicted quantity with a fitted input. The absence of released code and data is a reproducibility limitation, not a circular step. Therefore no circular step can be quoted or exhibited, and the appropriate finding is no significant circularity.

Assumptions & free parameters 5 free parameters · 5 assumptions · 0 invented entities

The central claim rests on borrowed diffusion math, a luminance-based shadow detector, a synthetic-data generation model whose scalar C_k conflicts with color-shadow claims, and an under-specified feature loss. No new theoretical entities such as particles or forces are introduced; SDCSRD is an empirical dataset, not a postulated entity. Independent evidence for the synthetic shadow realism and for the shadow-invariance of deep VGG features is not supplied.

free parameters (5)
  • lambda (loss weight) = 0.5
    Weight balancing the diffusion objective and the feature loss, chosen by hand in Section V.A.
  • diffusion noise level = 50
    Fixed noise level for the latent diffusion process, chosen by hand in Section V.A.
  • diffusion denoising steps = 100
    Number of reverse denoising steps, chosen by hand in Section V.A.
  • SSGM low-intensity fraction a = not specified
    Eq. 2 selects a fraction of the darkest pixels for the shadow mask, but the value of a is never reported.
  • VGG feature weights w_i = not specified
    Eq. 8 defines a weighted sum of VGG-19 slice features, but the weights are never stated, derived, or cited, making the feature loss under-specified.
assumptions (5)
  • standard math IR-SDE forward and reverse models with mask-dependent noise are a valid generative model for shadow removal.
    Borrowed from Luo et al. [54] and treated as given in Eqs. 5 and 6 without derivation or validation in this document setting.
  • domain assumption Shadows can be localized by luminance sorting after grayscale conversion, dilation, and median filtering.
    Algorithm 1 in Section IV.A assumes this heuristic works for colored shadows and textured document backgrounds, but no validation is provided.
  • domain assumption Linear alpha-blending in Eq. 1 with a scalar shadow color simulates realistic color-shadow distributions.
    Section III.A states C_k is scalar, which contradicts the color-shadow claim; the resemblance to real data is asserted only through histograms in Fig. 3.
  • domain assumption Deep VGG-19 features are progressively less affected by shadows and can be combined into a shadow-robust perceptual loss.
    Section IV.C and Fig. 5 make this observation, but the weights w_i and the invariance claim are not independently tested.
  • domain assumption Latent-space diffusion in a NAFBlock-based autoencoder preserves the document details needed for reconstruction.
    Section IV.B justifies the latent-space choice by computational cost, not by a completeness or reconstruction guarantee.

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Pith. "Pith review of DocShaDiffusion: Diffusion Model in Latent Space for Document Image Shadow Removal." pith.science (2026). https://pith.science/paper/QA4GRO6I

@misc{pith2026250701422,
  author       = {Pith},
  title        = {Pith review of: DocShaDiffusion: Diffusion Model in Latent Space for Document Image Shadow Removal},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/QA4GRO6I}},
  note         = {Machine review of arXiv:2507.01422}
}
read the original abstract

Document shadow removal is a crucial task in the field of document image enhancement. However, existing methods tend to remove shadows with constant color background and ignore color shadows. In this paper, we first design a diffusion model in latent space for document image shadow removal, called DocShaDiffusion. It translates shadow images from pixel space to latent space, enabling the model to more easily capture essential features. To address the issue of color shadows, we design a shadow soft-mask generation module (SSGM). It is able to produce accurate shadow mask and add noise into shadow regions specially. Guided by the shadow mask, a shadow mask-aware guided diffusion module (SMGDM) is proposed to remove shadows from document images by supervising the diffusion and denoising process. We also propose a shadow-robust perceptual feature loss to preserve details and structures in document images. Moreover, we develop a large-scale synthetic document color shadow removal dataset (SDCSRD). It simulates the distribution of realistic color shadows and provides powerful supports for the training of models. Experiments on three public datasets validate the proposed method's superiority over state-of-the-art. Our code and dataset will be publicly available.

Figures

Figures reproduced from arXiv: 2507.01422 by the authors.

Figure 1
Figure 1. An example of our proposed DocShaDiffusion for [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Examples of our dataset (SDCSRD). From up to down: [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Comparison of color distribution histograms between [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗
Figures from the paper (3 more)
Figure 4
Figure 4. Figure 4: Illustration of the proposed DocShaDiffusion, in which the training process is dashed line and sampling process is solid [PITH_FULL_IMAGE:figures/full_fig_p004_4.png]
Figure 5
Figure 5. Figure 5: Illustration of the proposed Lf ea. Is is shadow image. Isf is shadow-free image. Vi is the feature map in VGG-19. [60]. We observe that with an increase in the number of convolutional layers in the VGG-19 network, the extracted feature map, as shown in Fig.5, becomes …
Figure 6
Figure 6. Figure 6: Visual comparison between our method and the SOTA methods. (a)input images, (b)ISR-based [32], (c)3D-PC-based [PITH_FULL_IMAGE:figures/full_fig_p006_6.png]

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Pith tools

Reviewed August 6, 2026 · model on record in the stance chip above.