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REVIEW 4 major objections 4 minor 89 references

Extending Foundational Monocular Depth Estimators to Fisheye Cameras with Calibration Tokens

T0 review · 4 major / 4 minor · reviewed 2026-08-05 · deepseek-v4-flash

Pith's one-line read A few trainable tokens let a frozen depth model estimate depth on fisheye images, without retraining, finetuning, or undistorting the input.

desk verdict Calibration tokens are a cheap, practical fix for fisheye depth on frozen perspective models, but the paper's zero-shot generalization to >180-degree cameras is not supported by a training set that never sees rays beyond the perspective FOV. read the letter →

arxiv 2508.04928 v5 pith:VD7KEV2A submitted 2025-08-06 cs.CV cs.AIcs.LG

classification cs.CVcs.AIcs.LG
keywords monoculardepthestimationfisheyecamerascalibrationtokensvisiontransformersself-supervisedlearningcovariateshiftKannala-Brandtdistortionfoundationalmodels
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

Foundational monocular depth estimators, trained on tens of millions of perspective images, produce poor depth maps on fisheye images because lens distortion shifts the distribution of their internal representations. The paper claims this shift can be corrected in latent space: appending a small set of trainable Calibration Tokens to each transformer encoder layer modulates fisheye embeddings back toward the perspective-image distribution, so the frozen model estimates depth accurately on fisheye images. The tokens are trained self-supervised, using only perspective images distorted by a polynomial fisheye model, with the loss computed after warping the predicted fisheye depth back into the perspective frame. With eight tokens per layer, the method adds less than one percent memory and latency, improves over prior fisheye depth baselines indoors and outdoors, and leaves perspective performance untouched since the tokens can simply be removed.

What carries the argument

Layer-wise Calibration Tokens $\Phi \in \mathbb{R}^{L \times M \times F}$: a distinct set of $M$ trainable token vectors, $F$-dimensional, appended to the patch-token sequence at each of the $L$ encoder layers of a frozen vision-transformer FMDE and discarded from the output. The attention mechanism lets these tokens modulate fisheye latent embeddings toward the perspective distribution. The training objective warps the fisheye depth prediction back to the perspective frame with the inverse synthetic distortion $T^{-1}$ and minimizes $\log(|\tilde{d} - T^{-1} \circ \hat{d}| + 1)$, so supervision comes from the FMDE's own high-fidelity perspective depth estimates.

What would settle it

Train the tokens with only mild synthetic distortions and test on KITTI-360's >180-degree fisheye images; if adding tokens then fails to improve over the frozen baseline's RMSE, the claim that the synthetic distortion range covers real fisheye cameras is falsified.

Watch

Extended reading notes

Core claim

Calibration Tokens are a set of $M$ trainable vectors appended to the patch-token sequence at every encoder layer of a vision-transformer FMDE, where they modulate the latent embeddings through attention. The paper's central discovery is that this latent-space recalibration is enough: the frozen FMDE, already capable of high-fidelity depth estimation on perspective images, also produces accurate depth on fisheye images once its embeddings are aligned to the perspective embedding distribution. The alignment is learned entirely from perspective images by synthesizing fisheye distortions with the Kannala-Brandt polynomial model, then minimizing the log-$L_1$ difference between the FMDE's perspe

Load-bearing premise

The synthetic fisheye distortions, created by randomly distorting perspective images with Kannala-Brandt polynomial coefficients in the range [-1.0, -0.01], are representative enough of real fisheye cameras that a few tokens trained on synthetic images transfer zero-shot to real fisheye images, including cameras with fields of view beyond 180 degrees.

Editorial extensions

If this is right

  • One frozen FMDE can serve both perspective and fisheye cameras: append Calibration Tokens for fisheye input, omit them for perspective input, with no change to the backbone.
  • No camera calibration parameters are needed at inference; the tokens, trained over a range of synthetic distortions, absorb the calibration information.
  • Training scale is not limited by scarce real fisheye data, since any calibrated perspective image dataset can be synthetically distorted for token training.
  • Because only tokens are trained, adapting to a newer FMDE is much cheaper than full-model finetuning, and the same token mechanism is portable to any transformer-based FMDE.
  • The method consistently improves over the frozen baseline across backbones (12% RMSE reduction for MiDaS, 17% for DepthAnything indoors, 13% for UniDepth indoors) while adding under 0.05% memory and under 1 ms of latency.

Reading between the lines

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

  • Beyond the paper: the same latent-alignment mechanism could in principle adapt FMDEs to other camera-induced covariate shifts, such as severe barrel or pincushion distortion, rolling shutter, or synthetic non-lens degradations, as long as they can be generated from perspective images; the paper tests only fisheye distortion.
  • Because supervision is the FMDE's own perspective output, the method's ceiling is the FMDE's perspective accuracy; in scenes where the FMDE misestimates perspective depth, Calibration Tokens cannot recover that error.
  • The zero-shot transfer to KITTI-360's field of view exceeding 180 degrees hints that the tokens learn something more general than the exact synthetic coefficient range used in training; a direct way to test that would be sweeping the synthetic distortion range and measuring the error curve, which the paper does not report.
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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 / 4 minor

Summary. The paper proposes Calibration Tokens, a set of trainable tokens appended to the encoder layers of frozen Transformer-based foundational monocular depth estimators (FMDEs), to adapt perspective-trained models to fisheye images. The training is self-supervised: perspective images are synthetically distorted with Kannala-Brandt parameters, the FMDE predicts depth for both the perspective and distorted views, and a log-L1 loss is computed in the perspective reference frame after undistorting the fisheye depth prediction. No real fisheye images or ground-truth depth are used for training. The method is evaluated on ScanNet++ (indoor) and KITTI-360 (outdoor) with MiDaS, DepthAnything, and UniDepth, reporting RMSE and δ1 improvements over no-token baselines and comparison to DepthAnyCamera and FoV A-Depth, while claiming a single token set works for both indoor and outdoor scenes.

Significance. If validated, the central idea is attractive and practical: a small set of trainable tokens (<0.1% parameter increase, <1% latency increase) can adapt a frozen FMDE to fisheye cameras without finetuning, while remaining backward-compatible with perspective images and not requiring camera intrinsics at inference. The paper provides code, evaluates on multiple FMDEs, and includes ablations of design choices. The main risks are evaluative rather than architectural: the alignment protocol for relative-depth models is unspecified, and the synthetic distortion distribution may not cover the extreme rays of >180-degree fisheye cameras. These issues are load-bearing for the paper's strongest claims but appear addressable with additional analysis.

major comments (4)
  1. [§4, Table 1, Supp. B.4] MiDaS and DepthAnything produce affine-invariant relative depth, but Table 1 reports RMSE against ground truth without specifying any scale/shift alignment. If raw predictions are compared directly, RMSE is not meaningful; if alignment is used, the procedure must be described. Please state exactly what alignment (per-image least-squares, global, none) was applied, and report scale-invariant metrics such as δ1 consistently. This is necessary to interpret the reported improvements.
  2. [§3.2, Eq. (1), Supp. B.3] Synthetic training data is generated by distorting perspective images, so every ray in the generated fisheye image satisfies θ < 90° relative to the optical axis. KITTI-360's fisheye cameras have FOV exceeding 180°, which includes rays with θ > 90°. Therefore the token set never encounters the geometry of the outermost fisheye regions during training, and the reported KITTI-360 improvements do not by themselves establish zero-shot generalization to >180° fisheye cameras. Please report results restricted to θ ≤ 90° versus θ > 90° (or a comparable FOV breakdown), and temper the generalization claim accordingly.
  3. [Table 1, KITTI-360 row] For DepthAnything on KITTI-360, RMSE improves from 2.214 to 2.043 but δ1 drops from 0.839 to 0.810. The text states that Calibration Tokens 'consistently improve accuracy' and that both MiDaS and DepthAnything improve on RMSE, but the δ1 decrease is not discussed. This is a direct inconsistency in the stated claim. Please address whether this is an alignment artifact or a real trade-off, and avoid claiming consistent improvement across all metrics.
  4. [§4.3, Table 4] The ablation study validates the design choices—LogL1 loss and layer-wise tokens—directly on the same real test sets (ScanNet++ and KITTI-360) that are used for final reporting, without a held-out validation split. This selection-on-test-set inflates the apparent advantage of the chosen configuration. Please add a validation split or explicitly acknowledge this as a limitation, and report the ablated variants on that split.
minor comments (4)
  1. [Eq. (2)] The formula for φ is written as arctan((y − cx)/(x − cy)); the roles of x, y and cx, cy appear swapped. Please correct the notation.
  2. [§5] The Discussion uses 'FMEs' instead of 'FMDEs' in the first sentence.
  3. [Supp. B.4] The text says the authors 'measure relative depth estimation performance to mitigate the gap introduced by fisheye images,' but the reported metrics RMSE and δ1 are defined in absolute/linear depth space. Please clarify how relative depth is handled in the evaluation.
  4. [Table 1, DepthAnyCamera rows] The rows for DepthAnyCamera trained on the 'Mix 200K' set show very large RMSE values, but no training details (optimizer, iterations, resolution) are given. Since this baseline is the main comparison, please include training hyperparameters or clarify whether the same protocol was used.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the training objective and the evaluation are decoupled, and the core improvement is measured against external fisheye ground truth.

full rationale

The paper's derivation chain is self-contained and not circular. Calibration Tokens are trained by synthesizing fisheye distortions from perspective images (Kannala-Brandt model, Eq. 1) and minimizing a self-supervised consistency loss (Eq. 3) between the FMDE's perspective output and the re-projected fisheye output. The use of the FMDE's own perspective estimates as pseudo-targets is a form of knowledge distillation, not a circular reduction, because the evaluation is performed on real fisheye images (ScanNet++, KITTI-360) with external ground truth that was never used in training. The citation of AugUndo [69] is inspirational rather than load-bearing, and the method does not depend on any unverified claim from the authors' prior work. The main caveats are (a) the ablation in Tab. 4 selects design choices (layer-wise tokens, LogL1) on the same test benchmarks used for the final claims, which can inflate results through selection bias, and (b) the synthetic fisheye training distribution, generated from perspective images, likely cannot cover rays with theta > 90 degrees, so generalization to FOV > 180 cameras like KITTI-360 is not fully established. Both are correctness/generalization concerns, not examples of a prediction being equivalent to its inputs by construction. Hence no circular step is identified.

Assumptions & free parameters 2 free parameters · 3 assumptions · 1 invented entities

The method's free parameters are the token count and the synthetic distortion range, both chosen by hand. The central assumptions are that the synthetic distortion model covers real fisheye lenses, that encoder-only modulation suffices, and that the FMDE's perspective outputs are reliable training targets. No external entities are postulated beyond the token mechanism itself.

free parameters (2)
  • Number of calibration tokens per layer (M) = 8
    Chosen by hand; not swept in the paper, and it directly controls the capacity of the adaptation mechanism.
  • Kannala-Brandt distortion coefficient sampling range = [-1.0, -0.01]
    Range used to synthesize fisheye images during training (Supp. B.3); determines the distribution the tokens are trained on and is not justified from real fisheye cameras.
assumptions (3)
  • domain assumption The Kannala-Brandt polynomial model (Eq. 1) can represent real fisheye distortions sufficiently well for training tokens that transfer to real cameras.
    Invoked at Sec 3.2 when synthesizing fisheye images; if real fisheye lenses, especially FOV > 180 degrees, are far from this model, the training signal is misaligned.
  • domain assumption Appending trainable tokens and modulating attention in the encoder is sufficient to transform fisheye embeddings into perspective-like embeddings; the decoder needs no modification.
    Sec 3.1 states tokens are only applied to encoder layers; if the decoder also encodes camera-specific bias, the alignment is incomplete.
  • domain assumption The FMDE's perspective depth estimates are high-fidelity enough to serve as training targets (pseudo ground truth).
    Loss Eq. 3 uses the FMDE's perspective output as target; systematic FMDE errors on perspective images will be reproduced in the fisheye adaptation.
invented entities (1)
  • Calibration Tokens
    purpose: Trainable parameter vectors appended to each encoder layer of a frozen transformer-based FMDE to shift fisheye image embeddings toward the perspective embedding distribution.
    The tokens are a new mechanism proposed by this paper; there is no independent falsifiable prediction outside the paper's own experiments. They are internal parameters, not a new physical entity.

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Cite this review

Pith. "Pith review of Extending Foundational Monocular Depth Estimators to Fisheye Cameras with Calibration Tokens." pith.science (2026). https://pith.science/paper/VD7KEV2A

@misc{pith2026250804928,
  author       = {Pith},
  title        = {Pith review of: Extending Foundational Monocular Depth Estimators to Fisheye Cameras with Calibration Tokens},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/VD7KEV2A}},
  note         = {Machine review of arXiv:2508.04928}
}
read the original abstract

We propose a method to extend foundational monocular depth estimators (FMDEs), trained on perspective images, to fisheye images. Despite being trained on tens of millions of images, FMDEs are susceptible to the covariate shift introduced by changes in camera calibration (intrinsic, distortion) parameters, leading to erroneous depth estimates. Our method aligns the distribution of latent embeddings encoding fisheye images to those of perspective images, enabling the reuse of FMDEs for fisheye cameras without retraining or finetuning. To this end, we introduce a set of Calibration Tokens as a light-weight adaptation mechanism that modulates the latent embeddings for alignment. By exploiting the already expressive latent space of FMDEs, we posit that modulating their embeddings avoids the negative impact of artifacts and loss introduced in conventional recalibration or map projection to a canonical reference frame in the image space. Our method is self-supervised and does not require fisheye images but leverages publicly available large-scale perspective image datasets. This is done by recalibrating perspective images to fisheye images, and enforcing consistency between their estimates during training. We evaluate our approach with several FMDEs, on both indoors and outdoors, where we consistently improve over state-of-the-art methods using a single set of tokens for both. Code available at: https://github.com/JungHeeKim29/calibration-token; https://github.com/Suchisrit/CalibrationTokens.

Figures

Figures reproduced from arXiv: 2508.04928 by the authors.

Figure 1
Figure 1. Foundational monocular depth estimators fail on fisheye images. Despite being trained on large-scale datasets, foundational monocular depth estimators (FMDEs) models pro￾duces erroneous outputs. The inaccurate, blurry estimates are caused by a covariate shift that stem from fisheye distortion. eye or other wide-angle cameras, which allows for wider coverage of the 3D environment. However, images cap￾tured by these c… view at source ↗
Figure 2
Figure 2. Inference on different cameras. Calibration Tokens enable foundational monocular depth estimators to adapt to fisheye images while maintaining performance on perspective images. induce artificial distortion on perspective images to create pairs of perspective and synthetic fisheye images with di￾verse fisheye distortions. However, rather than doing the same in the output space, we undistort the fisheye depth maps to… view at source ↗
Figure 3
Figure 3. Overview of our method. We introduce a set of trainable Calibration Tokens, which is appended to the input sequence of the fisheye image tokens. The Calibration Tokens are trained to adapt the model to produce accurate depth maps for images with various fisheye distortions. A unique fisheye calibration token is appended to the input of each new layer of the encoder. 3.1. Extending FMDEs with Calibration Tokens Speci… view at source ↗
Figures from the paper (6 more)
Figure 4
Figure 4. Figure 4: Comparison on ScanNet++(Indoor) [77] and KITTI-360 [36] dataset. Qualitative comparison results on ScanNet++ and KITTI-360 datasets. Here, +C. T. indicates prediction results by appending Calibration Tokens to patch embeddings of the model located above. Calibration To…
Figure 6
Figure 6. Figure 6: Comparison of attention maps for single and multi￾layer tokens. We visualize the attention map of Calibration To￾kens to the encoded patch embeddings. Calibration Tokens attend highly to distorted border regions: “Single Token” (top) has little effect in most layers du…
Figure 8
Figure 8. Figure 8: Validation on LogL1 loss. We evaluate the effectiveness of our LogL1 loss by comparing a single-layer token baseline with an additional LogL1 loss. Incorporating LogL1 loss helps model to mitigate artifacts in the highlighted border regions of fisheye images, leading t…
Figure 9
Figure 9. Figure 9: 3D reconstruction result of UniDepth predictions on ScanNet++ dataset. results [PITH_FULL_IMAGE:figures/full_fig_p014_9.png]
Figure 10
Figure 10. Figure 10: Additional comparison results on ScanNet++ dataset [PITH_FULL_IMAGE:figures/full_fig_p016_10.png]
Figure 11
Figure 11. Figure 11: Additional comparison results on KITTI-360 dataset [PITH_FULL_IMAGE:figures/full_fig_p017_11.png]

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Pith tools

Reviewed August 5, 2026 · model on record in the stance chip above.