REVIEW 5 major objections 7 minor 30 references
EndoMUST: Monocular Depth Estimation for Robotic Endoscopy via End-to-end Multi-step Self-supervised Training
T0 review · 5 major / 7 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read The paper claims that dividing each end-to-end training epoch into three isolated steps — optical flow registration, multiscale intrinsic image decomposition, and rigid transformation alignment — produces state-of-the-art self-supervised…
desk verdict Solid empirical integration paper with a reproducible SOTA claim held back by an unstated scale-alignment procedure and test-set selection of a warm-up step. 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 load-bearing mechanism is the staged epoch: Step I trains optical-flow and appearance-flow networks with a registration loss against a visibility-masked target; Step II decomposes images at scales 1.0, 0.75, and 0.5 into reflectance and shading and supervises reconstruction; Step III aligns the source image, its intrinsic images, and the reflectance image to the target using predicted depth, ego-motion, and intrinsics, adding depth and appearance smoothness terms. The final component is DV-LoRA, a low-rank adaptation that modifies each transformer block's QKV and feed-forward linear layers with trainable diagonal scaling matrices around frozen weights, giving the depth network new capacity while adding only about 0.2 million trainable parameters.
What would settle it
Rerun the SCARED and Hamlyn evaluations with an explicitly specified scale alignment (for example, per-image median scaling versus a least-squares fit to ground truth) for every method; if EndoMUST's advantage over EndoDAC and MonoPCC shrinks or reverses under one consistent convention, the reported ranking depends on an unstated choice rather than on the three-step training.
Extended reading notes
Core claim
The central claim is that a three-step, end-to-end training strategy, built on top of a parameter-efficiently fine-tuned Depth Anything foundation model, reaches state-of-the-art self-supervised depth estimation for robotic endoscopy. In each epoch, optical flow and appearance flow networks are optimized alone; then an intrinsic-image-decomposition network is trained at three image scales; finally depth, ego-motion, camera-intrinsics, and appearance networks are trained together through photometric and illumination-free reflectance alignments. Reported numbers on SCARED are RelAbs 0.046, RelSq 0.313, RMSE 4.276, RMSElog 0.067, delta 0.984, beating the previous best compared method, and zero-shot Hamlyn results are RelAbs 0.145 and delta 0.807. The paper attributes the improvement to isolating constraints so that irrelevant information cannot mislead a given module, together with full-module DV-LoRA fine-tuning that keeps trainable parameters low.
Load-bearing premise
The comparison assumes that all predicted depth maps were aligned to ground truth by the same standard procedure, but the paper never describes how the scale-ambiguous predicted depth was scaled before computing RelAbs, RMSE, or delta.
Editorial extensions
If this is right
- Self-supervised depth for endoscopy does not require ground truth, and with staged training it can reach sub-5% relative error on SCARED.
- The same SCARED-trained weights generalize zero-shot to Hamlyn, over 90,000 frames, while keeping the best reported accuracy among compared methods.
- Parameter-efficient fine-tuning lets a large foundation model adapt with only about 1.8M trainable parameters in the depth network, keeping inference above 100 fps on SCARED and above 200 fps on Hamlyn.
- Ego-motion and camera-intrinsics estimates also improve under the third step, with focal-length relative error dropping below 1% in the reported sequences.
- If the results hold, monocular endoscopic depth becomes a low-cost basis for 3D reconstruction, registration, and augmented-reality navigation during robot-assisted surgery.
Reading between the lines
- The staged schedule may be transferable to other self-supervised geometric tasks where multiple constraints compete, such as monocular odometry in low-texture medical scenes; the paper does not test this.
- The ablation tables suggest warm-up length of DV-LoRA is a sensitive hyperparameter, since 5000 warm-up steps degrade results, so part of the reported gain may be due to the tuned warm-up rather than the three-step ordering.
- Because the paper omits the scale-alignment protocol for metrics, the precise numerical state-of-the-art claim is not independently reproducible until that protocol is specified; a reader should treat the ranking as tentative.
- A direct way to strengthen the claim would be a per-sequence or per-video analysis showing that staging helps most in exactly the illumination-challenging frames the method targets; the paper reports aggregate numbers only.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes EndoMUST, a self-supervised monocular depth and ego-motion estimation framework for robotic endoscopy. The main contribution is a training strategy that splits each epoch into three steps: optical flow registration, multiscale intrinsic image decomposition, and multiple transformation alignments, with only the task-relevant networks trainable at each step. The depth network is finetuned from a foundation model (Depth Anything) using DV-LoRA. The authors report state-of-the-art results on SCARED for self-supervised depth estimation and on Hamlyn for zero-shot depth estimation, along with ego-motion and camera-intrinsics results. The paper includes ablations over step order, step fusion, the multiscale decomposition, the QKV finetuning, and warm-up steps.
Significance. If the reported results hold, the paper makes a useful practical contribution: it shows that a staged, interference-avoiding training schedule combined with parameter-efficient finetuning of a foundation model improves self-supervised depth estimation in endoscopy. The strength of the paper is its clear and systematic experimental design: the ablations in Table V directly test the paper's central claim that the three-step strategy, rather than any single module, is responsible for the gains. The release of evaluation code is also a positive. However, the headline SOTA claim rests on an incompletely specified evaluation protocol (scale alignment), on hyperparameters that appear to be selected using the test set, and on small performance margins without uncertainty quantification. These issues currently prevent the claim from being verified from the manuscript alone.
major comments (5)
- [Section IV-C, Eqs. (13)-(16)] The paper never specifies how the scale-ambiguous predicted depth is aligned to the ground truth before computing RelAbs, RelSq, RMSE, RMSElog, and delta. Since self-supervised monocular depth is defined only up to an unknown scale, the reported numbers in Tables I and II require an alignment convention, e.g., median scaling per image, least-squares fitting, or per-sequence alignment. The margins over the best baselines are small (RelAbs 0.046 vs 0.051 on SCARED, 0.145 vs 0.153 on Hamlyn), so a different alignment convention could change or invert the comparison. Please state the exact alignment procedure and confirm that it was applied identically to every compared method.
- [Section IV-C, Eqs. (13)-(16)] The metric definitions appear to be mislabeled: Eq. (14) is the standard RMSE-log formula, Eq. (15) is the standard RMSE formula, and Eq. (13) appears to be a squared relative error averaged over pixels rather than the usual RelSq definition. The labels 'RMSE' and 'RMSElog' are also swapped relative to the variables used in the text. This ambiguity makes it difficult to reproduce the reported values and should be corrected.
- [Section IV-F, Table VI] The warm-up steps for DV-LoRA are evaluated on values 25000, 20000, 15000, and 5000, and the best value is selected from these results. No validation split is mentioned for this hyperparameter choice, and the tables do not report performance on a held-out validation set. Selecting the warm-up steps using the test set can inflate the reported test performance. Please describe how warm-up was chosen, or re-run the comparison with a validation-based selection.
- [Tables I, II, and V] All reported numbers appear to be from single runs, with no error bars, repeated trials, or significance tests. Several key comparisons are close: in Table I the gap to EndoDAC is 0.005 in RelAbs and 0.004 in delta, and several ablations in Table V differ by similar amounts. Without uncertainty quantification, the claim that the three-step training strategy is the source of the improvement is not statistically supported.
- [Section IV-D] The baseline reproduction protocol is described only as "some works with public codes or public weights are trained and tested based on the same setting." The paper does not report the exact training iterations, learning-rate schedules, loss weights, or finetuning details used for baselines such as AF-SfMLearner, IID-SfMLearner, MonoPCC, DVSMono, and the Depth Anything variants. Because the SOTA claim is a comparison against these methods, the reproducibility of the baseline setup is load-bearing and should be detailed, for example in an appendix or a supplementary table.
minor comments (7)
- [Section III-E] The heading contains a typo: "parametwr-efficient finetuning" should be "parameter-efficient finetuning."
- [Section IV-D] The sentence "Qualitative results in Fig further display the outstanding performance of the proposed method" has an incomplete figure reference and is then immediately repeated; it should refer to Fig. 4 and be stated once.
- [Tables I and II] The GC column is defined in the table caption as "Given Camera intrinsics," but Section IV-E also reports camera-intrinsics estimation. Please clarify whether the methods marked "Yes" use the ground-truth intrinsics during training, during evaluation, or both, and how this relates to the intrinsics prediction results.
- [Section III-D, Eq. (8)] The coordinate transformation in Eq. (8) is hard to parse: the notation C(·), the homogeneous coordinate representation, and the dimensions of the bracketed terms are not fully defined. A precise definition of each symbol would improve reproducibility.
- [Section III-E, Eq. (12)] In Eq. (12), the dimensions and roles of A, B, U, and V are not made explicit. The text says "ΛV and ΛU diagonal matrix of trainable weights U and V" while A and B are also trainable; please define all matrix shapes and state how the diagonal matrices are constructed.
- [Section IV-A] The description "all 92,672 endoscopic frames from the 21 rectified videos[28]" is ambiguous: is the count of frames from the subset used in [28]? Clarify the source of the rectified videos and the frame count.
- [Table V] The last row of Table V is garbled: "II→{I,III} Out of GPU Memory{I,II,III}" is not a clear description of the experiment. Please rephrase to state whether the run failed or was not performed.
Circularity Check
No significant circularity; the SOTA claim rests on empirical comparisons and internal ablations, not on a derivation that reduces to its inputs.
full rationale
The paper's central claim is empirical: a three-step training strategy for self-supervised monocular depth achieves state-of-the-art results on SCARED and zero-shot results on Hamlyn. None of the reported metrics (Eqs. 13-16) are defined in terms of the model's training losses or parameters; they are external error measures against ground-truth depth. The training losses (Eqs. 5, 6, 11) supervise optical flow registration, intrinsic image decomposition, and rigid transformation alignment, respectively; they do not encode the final depth accuracy metric. The improvement of the multi-step strategy is supported by within-paper ablations (Table V) showing that the proposed step ordering and separation outperform fused variants, and by comparisons against methods from other groups (Yang et al., DVSMono, MonoPCC). The paper does cite prior work by overlapping authors: EndoDAC [16] shares co-author L. Bai and supplies the DV-LoRA adaptation module, and AF-SfMLearner [14] may be by the same first-author lineage and supplies appearance-flow components. However, these citations provide building blocks, not the load-bearing claim; the claimed benefit of the multi-step training is validated by ablation and by external baselines, not inferred from the citations themselves. The missing description of how scale-ambiguous monocular depth predictions are aligned to ground truth before computing RelAbs/RMSE/δ is a reproducibility gap, but it is not a circularity: the metric is not defined as a function of the model output by construction. No equation or training objective sets a predicted quantity equal to an input by definition. Therefore no circular step is identified.
Assumptions & free parameters
free parameters (3)
- Warm-up steps for DV-LoRA =
20000
- Loss weighting coefficients =
0.001, 0.01, 0.02 (and ablation fusions 0.02, 0.02)
- DV-LoRA rank =
4
assumptions (4)
- domain assumption Photometric consistency between target and warped source frames is a sufficient supervisory signal after adding appearance flow and intrinsic decomposition.
- domain assumption Visibility mask M from backward optical flow [26] correctly identifies occluded pixels.
- domain assumption The SCARED and Hamlyn preprocessing (320x256, split per [14]) and the evaluation protocol are fair and consistent across all compared methods.
- domain assumption A foundation model pretrained on natural images provides a useful depth prior for endoscopy.
Cite this review
Pith. "Pith review of EndoMUST: Monocular Depth Estimation for Robotic Endoscopy via End-to-end Multi-step Self-supervised Training." pith.science (2026). https://pith.science/paper/BOV4Y3CW
@misc{pith2026250616017,
author = {Pith},
title = {Pith review of: EndoMUST: Monocular Depth Estimation for Robotic Endoscopy via End-to-end Multi-step Self-supervised Training},
year = {2026},
howpublished = {\url{https://pith.science/paper/BOV4Y3CW}},
note = {Machine review of arXiv:2506.16017}
}
abstract
Monocular depth estimation and ego-motion estimation are significant tasks for scene perception and navigation in stable, accurate and efficient robot-assisted endoscopy. To tackle lighting variations and sparse textures in endoscopic scenes, multiple techniques including optical flow, appearance flow and intrinsic image decomposition have been introduced into the existing methods. However, the effective training strategy for multiple modules are still critical to deal with both illumination issues and information interference for self-supervised depth estimation in endoscopy. Therefore, a novel framework with multistep efficient finetuning is proposed in this work. In each epoch of end-to-end training, the process is divided into three steps, including optical flow registration, multiscale image decomposition and multiple transformation alignments. At each step, only the related networks are trained without interference of irrelevant information. Based on parameter-efficient finetuning on the foundation model, the proposed method achieves state-of-the-art performance on self-supervised depth estimation on SCARED dataset and zero-shot depth estimation on Hamlyn dataset, with 4\%$\sim$10\% lower error. The evaluation code of this work has been published on https://github.com/BaymaxShao/EndoMUST.
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Reviewed August 6, 2026 · model on record in the stance chip above.
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