REVIEW 4 major objections 5 minor 27 references
Noise Analysis and Modeling of the PMD Flexx2 Depth Camera for Robotic Applications
T0 review · 4 major / 5 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read A two-part Gaussian model reproduces the PMD Flexx2 depth camera's noise, with axial noise matching to 0.015 nats KL divergence.
desk verdict Useful first characterization of Flexx2 noise, undermined by in-sample 'validation' but worth engaging. 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 object is the axial-noise formula $\sigma_z(z, \theta_y) = a + b z + c z^2 + d z^n \frac{\theta_y^2}{(\pi/2 - \theta_y)^2}$, with coefficients fitted per mode and the exponent $n$ optimized to 2.7, together with a single per-mode lateral-noise standard deviation $\sigma_x$ set at the 90th percentile of the measured data. This formula turns raw pixel measurements into a closed-form relationship between noise magnitude, depth, and viewing angle, which is what a simulator needs to draw new depth readings.
What would settle it
A reader could aim a PMD Flexx2 at surfaces with different colors and reflectances—black, metal, or dark wood—at the same distances and angles, and compare the per-pixel noise histograms to the model's predictions; if the measured axial standard deviations deviate from the fitted formula beyond the reported KL divergence, the model does not generalize.
Extended reading notes
Core claim
The central claim is that the non-systematic depth noise of the PMD Flexx2 can be represented well enough by a two-part Gaussian model. Axial noise follows a Gaussian whose standard deviation grows with distance and with incidence angle, described by the fitted formula $\sigma_z(z, \theta_y) = a + b z + c z^2 + d z^n \frac{\theta_y^2}{(\pi/2 - \theta_y)^2}$ with exponent $n = 2.7$; the model achieves a low average KL divergence of 0.015 nats, meaning the fitted distribution is close to the measured histogram. Lateral noise, although not itself Gaussian, is modeled conservatively as a fixed Gaussian per mode with standard deviations of 0.864, 1.098, and 1.649 pixels for the three tested modes, giving a 0.868-nats average KL divergence. The purpose is to provide parameters that a depth-camera simulator can use to reproduce the Flexx2's noise statistics, which the authors argue is a step toward closing the sim-to-real gap in learning-based robot control.
Load-bearing premise
The noise statistics measured on a single white painted wooden cabinet at one indoor lighting condition, using a fitted plane as the reference surface, are representative of the noise the Flexx2 produces in robotic deployments on other surfaces, materials, and lighting.
Editorial extensions
If this is right
- Depth-camera simulators can plug in the fitted coefficients to generate realistic axial noise for distances up to 1.8 m and incidence angles up to 60°, covering typical indoor robot perception ranges.
- The comparison across modes suggests that Mode 5 at 30 fps is the most precise for oblique viewing, while Mode 9 at 30 fps is best when looking straight at a surface, a fact that can guide mode selection on a robot.
- Because lateral noise is set at the 90th percentile, simulated depth edges will be jittery more often than in reality, giving a conservative stress test for perception algorithms.
- The KL-divergence numbers give the community a quantitative yardstick for how well Gaussian models approximate this sensor, and the same protocol can be repeated for other depth cameras.
Reading between the lines
- The 0.015-nats axial fit may not survive contact with darker or specular surfaces, so for simulators targeting varied environments the fitted coefficients would likely need to be re-estimated per material class.
- The same measurement and fitting template could be applied to other time-of-flight cameras with only the model constants changed, making the paper a reusable recipe for sensor-noise characterization.
- If lateral noise truly is heavy-tailed rather than Gaussian, a mixture or normalizing-flow model might cut the 0.868-nats divergence, though the conservative 90th-percentile choice already serves the stated goal of safe simulation.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper presents an empirical characterization of non-systematic depth noise of the PMD Flexx2 time-of-flight camera for three operating modes. The authors collected 300-frame sequences of a white planar cabinet at distances from 0.4 m to 1.8 m and incidence angles from 0° to 60°, fitted a plane to the measured depth, and separated the noise into axial (depth) and lateral (edge) components. Axial noise is modeled as Gaussian with standard deviation σz(z, θ) = a + b·z + c·z² + d·z^n·θ²/(π/2−θ)², with n = 2.7 and per-mode coefficients fitted by minimizing mean squared error. Lateral noise is modeled as Gaussian with per-mode standard deviation equal to the 90th percentile of the measured edge noise. The paper reports average KL divergences of 0.015 nats (axial) and 0.868 nats (lateral) and claims that these results validate the models for use in robot simulators.
Significance. If the claims were fully supported, the paper would provide the first published noise model for the Flexx2 and a practical recipe for injecting realistic depth noise into robot simulators. The experimental protocol is clearly described, the model is simple and interpretable, and the authors correctly separate axial and lateral noise following established literature. The paper also acknowledges the single-surface limitation in its conclusion. However, the validation evidence is in-sample only, the 0° anomaly is handled inconsistently, and no code or data are released, which limits reproducibility. These issues must be resolved before the validation claim can be accepted as stated.
major comments (4)
- [Section V-A, Eq. (1), Tables III–IV] The KL divergence values in Table IV are computed on the same measurement grid used to fit the coefficients a, b, c, d, and n in Eq. (1). No held-out subset, cross-validation, or separate measurement session is described. The reported average of 0.015 nats is therefore an in-sample goodness-of-fit measure, not an out-of-sample validation. The abstract and conclusion state that the results 'validate our noise models'; this is not supported by the evidence presented. Please either perform an out-of-sample evaluation (e.g., leave-one-distance-out or a new data collection) or explicitly reframe the KL numbers as fit-quality metrics rather than validation.
- [Section V-A, Table IV] The text states that the 0° anomaly is ignored for noise modeling ('we ignore this anomaly'), yet Table IV reports KL values for 0° rows and the overall average of 0.015 nats includes them. Mixing these extrapolated points with the in-sample fits is misleading. Excluding the 0° rows, the average over the remaining 12 entries is approximately 0.0095 nats, which is a different quantitative claim. Please recompute the average excluding 0° and report both values, or justify why 0° data are included in the validation average if they were not used for fitting.
- [Section V-B, Table V] The lateral-noise model sets σx to the 90th percentile of the measured data and then evaluates the model with a KL divergence computed on the same data. This is not an independent validation; it only shows that a Gaussian with that σx reproduces the central part of the empirical distribution from which the parameter was derived. Additionally, matching the 90th percentile does not by itself make the model conservative for the tails if the empirical distribution is heavier-tailed than Gaussian. Please provide a quantile–quantile plot or exceedance-probability comparison to support the 'conservatively modeled' claim, and separate the fitting step from the evaluation step.
- [Section III-B and Section VI] All data come from a single white painted wooden cabinet at one indoor lighting condition. The conclusion acknowledges this limitation, but the abstract and introduction motivate the model for robotic applications and sim-to-real transfer. As presented, the fitted coefficients in Table III cannot be shown to generalize to other surface colors, materials, or lighting conditions. Please either add measurements for additional surfaces and lighting conditions or clearly restrict the contribution in the abstract to the tested condition, rather than claiming a general Flexx2 noise model for robotic applications.
minor comments (5)
- [Section V-A, Table IV] Please specify exactly how the KL divergence was computed, including histogram binning, number of bins, and whether pixels from all 300 frames were pooled; otherwise the reported numbers are not reproducible.
- [Section III-B] Please state whether the camera's exposure or integration time was fixed across measurements or controlled by auto-exposure; ToF noise depends strongly on integration time, and the text mentions auto exposure only in connection with the 0° anomaly.
- [Section IV] The lateral-noise standard deviation is reported in pixels; please state how this maps to world units or to the resolution of a simulated depth image, since simulator noise models typically require a spatial scale.
- [Table III] The coefficient row header appears as 'aaa bbb ccc' in the published text; please correct this to the coefficient names a, b, c, d.
- [Section V-B] The claim that a KL divergence of 0.868 nats is 'satisfactory' needs a comparison baseline or tolerance; without one, the reader cannot interpret whether this value is acceptable for the intended simulation use.
Circularity Check
The reported KL-based validation is computed on the same data used to fit the noise-model parameters, so the accuracy claims are in-sample goodness-of-fit rather than independent prediction.
-
fitted input called prediction
[Section V-A (Equation 1, Tables III and IV)]
"By minimizing the mean squared error loss to fit the model to the measured data, we obtained the coefficients a, b, c, d. ... The quality of the resulting model is validated by computing the average pixel-wise KL divergence between the measured axial noise and the proposed model ... using the selected pixels from all recorded images."
The KL divergence is evaluated on the same measurements whose standard deviations were used to fit a, b, c, d and to select n=2.7. The reported 0.015 nats therefore measures how well the fitted Gaussian reproduces the training data, not how well the model predicts new distances, angles, surfaces, or lighting conditions. Calling this 'validation' reduces the accuracy claim to an in-sample goodness-of-fit: the fitted coefficients already force the modeled sigma_z curve to match the measured sigma_z curve on the same grid.
-
fitted input called prediction
[Section V-B (Table V)]
"We consider the noise to be independent of distance and incidence angle, conservatively setting the standard deviation sigma_x for each mode at the 90th percentile of the collected data ... The model is validated using the same pixel-wise KL divergence metric as used for axial noise."
sigma_x is read off the same lateral-noise measurements that are then used to compute the KL divergence. The Gaussian with this sigma_x is compared against the exact distribution from which the 90th percentile was taken, so the reported KL values (0.896, 0.773, and 0.935 nats) are fit diagnostics on the training data. They do not test generalization to other configurations or materials; the 'conservative estimate' claim is therefore not independently validated.
full rationale
The paper contains no load-bearing self-citation and no imported uniqueness theorem; the axial and lateral model coefficients are genuinely estimated from new Flexx2 measurements. However, the central validation claim is circular in the pattern of 'fitted input called prediction.' The axial coefficients are optimized on the measured data, and the KL divergence that 'validates' the model is computed on those same measurements. The lateral model is even more explicit: sigma_x is set to the 90th percentile of the collected data and then assessed by KL on that same collected data. In both cases the reported numbers are in-sample fit quality metrics, not out-of-sample predictive validation. The paper's own conclusion concedes the model is based on a single surface type, which is a generalization limitation rather than a circularity, but it compounds the in-sample validation problem. Likewise, the 0-degree data are excluded from fitting ('we ignore this anomaly') yet appear in Table IV and are included in the reported averages; this is an inconsistency in the validation protocol, not itself a circular step. Overall, the models may be reasonable empirical fits, but the abstract and conclusion's statement that 'these results validate our noise models' overstates what an in-sample KL computation can establish.
Assumptions & free parameters
free parameters (3)
- Axial model coefficients a, b, c, d (per mode) =
Mode 5 30fps: 0.002362, -0.001041, 0.000753, 0.000185; Mode 5 60fps: 0.002209, -0.000793, 0.001418, 0.000370; Mode 9…
- Exponent n in axial model =
2.7
- Lateral noise sigma_x per mode =
0.864, 1.098, 1.649 pixels
assumptions (5)
- domain assumption Axial depth noise is Gaussian per pixel and stationary across the 300 captured frames.
- ad hoc to paper Lateral noise is modeled as Gaussian per pixel even though its measured distribution deviates from Gaussian.
- domain assumption The white painted cabinet surface is flat and the fitted plane is the true reference surface.
- ad hoc to paper The 0-degree anomaly is excluded from model fitting.
- domain assumption Noise characteristics generalize from the single measured surface to other surfaces and environments.
Cite this review
Pith. "Pith review of Noise Analysis and Modeling of the PMD Flexx2 Depth Camera for Robotic Applications." pith.science (2026). https://pith.science/paper/DK3M7ODJ
@misc{pith2026241215040,
author = {Pith},
title = {Pith review of: Noise Analysis and Modeling of the PMD Flexx2 Depth Camera for Robotic Applications},
year = {2026},
howpublished = {\url{https://pith.science/paper/DK3M7ODJ}},
note = {Machine review of arXiv:2412.15040}
}
read the original abstract
Time of Flight ToF cameras renowned for their ability to capture realtime 3D information have become indispensable for agile mobile robotics These cameras utilize light signals to accurately measure distances enabling robots to navigate complex environments with precision Innovative depth cameras characterized by their compact size and lightweight design such as the recently released PMD Flexx2 are particularly suited for mobile robots Capable of achieving high frame rates while capturing depth information this innovative sensor is suitable for tasks such as robot navigation and terrain mapping Operating on the ToF measurement principle the sensor offers multiple benefits over classic stereobased depth cameras However the depth images produced by the camera are subject to noise from multiple sources complicating their simulation This paper proposes an accurate quantification and modeling of the nonsystematic noise of the PMD Flexx2 We propose models for both axial and lateral noise across various camera modes assuming Gaussian distributions Axial noise modeled as a function of distance and incidence angle demonstrated a low average KullbackLeibler KL divergence of 0015 nats reflecting precise noise characterization Lateral noise deviating from a Gaussian distribution was modeled conservatively yielding a satisfactory KL divergence of 0868 nats These results validate our noise models crucial for accurately simulating sensor behavior in virtual environments and reducing the simtoreal gap in learningbased control approaches
Figures
Reference graph
Works this paper leans on
-
[1]
Kinect™ and intel realsense™ d435 comparison: A preliminary study for motion analysis,
J. D. Mejia-Trujillo, Y . J. Castano-Pino, A. Navarro, J. D. Arango- Paredes, D. Rinc ´on, J. Valderrama, B. Munoz, and J. L. Orozco, “Kinect™ and intel realsense™ d435 comparison: A preliminary study for motion analysis,” in 2019 IEEE International Conference on E-health Networking, Application & Services (HealthCom) . IEEE, 2019, pp. 1–4
work page 2019
-
[2]
Study on the use of microsoft kinect for robotics applications,
R. A. El-laithy, J. Huang, and M. Yeh, “Study on the use of microsoft kinect for robotics applications,” in Proceedings of the 2012 IEEE/ION Position, Location and Navigation Symposium . IEEE, 2012, pp. 1280– 1288
work page 2012
-
[3]
Starleth: A compliant quadrupedal robot for fast, efficient, and versatile locomotion,
M. Hutter, C. Gehring, M. Bloesch, M. A. Hoepflinger, C. D. Remy, and R. Siegwart, “Starleth: A compliant quadrupedal robot for fast, efficient, and versatile locomotion,” in Adaptive mobile robotics. World Scientific, 2012, pp. 483–490
work page 2012
-
[4]
Evaluation of depth sensors for robotic applications,
A. M. Pinto, P. Costa, A. P. Moreira, L. F. Rocha, G. Veiga, and E. Moreira, “Evaluation of depth sensors for robotic applications,” in 2015 IEEE international conference on autonomous robot systems and competitions. IEEE, 2015, pp. 139–143
work page 2015
-
[5]
S. Zennaro, “Evaluation of microsoft kinect 360 and microsoft kinect one for robotics and computer vision applications,” 2014
work page 2014
-
[6]
Learning robust perceptive locomotion for quadrupedal robots in the wild,
T. Miki, J. Lee, J. Hwangbo, L. Wellhausen, V . Koltun, and M. Hutter, “Learning robust perceptive locomotion for quadrupedal robots in the wild,” Science Robotics , vol. 7, no. 62, p. eabk2822, 2022
2022
-
[7]
Accuracy of the microsoft kinect sensor for measuring movement in people with parkinson’s disease,
B. Galna, G. Barry, D. Jackson, D. Mhiripiri, P. Olivier, and L. Rochester, “Accuracy of the microsoft kinect sensor for measuring movement in people with parkinson’s disease,” Gait & posture , vol. 39, no. 4, pp. 1062–1068, 2014
work page 2014
-
[8]
G. Halmetschlager-Funek, M. Suchi, M. Kampel, and M. Vincze, “An empirical evaluation of ten depth cameras: Bias, precision, lateral noise, different lighting conditions and materials, and multiple sensor setups in indoor environments,” IEEE Robotics & Automation Magazine , vol. 26, no. 1, pp. 67–77, 2019
work page 2019
Show all 27 references
-
[9]
Analysis and noise modeling of the intel realsense d435 for mobile robots,
M. S. Ahn, H. Chae, D. Noh, H. Nam, and D. Hong, “Analysis and noise modeling of the intel realsense d435 for mobile robots,” in 2019 16th International Conference on Ubiquitous Robots (UR) . IEEE, 2019, pp. 707–711
2019
-
[10]
Kinect v2 for mobile robot navigation: Evaluation and modeling,
P. Fankhauser, M. Bloesch, D. Rodriguez, R. Kaestner, M. Hutter, and R. Siegwart, “Kinect v2 for mobile robot navigation: Evaluation and modeling,” in 2015 international conference on advanced robotics (ICAR). IEEE, 2015, pp. 388–394
2015
-
[11]
Accuracy and resolution of kinect depth data for indoor mapping applications,
K. Khoshelham and S. O. Elberink, “Accuracy and resolution of kinect depth data for indoor mapping applications,” sensors, vol. 12, no. 2, pp. 1437–1454, 2012
2012
-
[12]
Comparison of kinect v1 and v2 depth images in terms of accuracy and precision,
O. Wasenm ¨uller and D. Stricker, “Comparison of kinect v1 and v2 depth images in terms of accuracy and precision,” in Computer Vision–ACCV 2016 Workshops: ACCV 2016 International Workshops, Taipei, Taiwan, November 20-24, 2016, Revised Selected Papers, Part II 13 . Springer, ...
2016
-
[13]
Performance analysis of the pmd camboard picoflexx time-of-flight camera for markerless motion capture applications,
S. Pasinetti, M. M. Hassan, J. Eberhardt, M. Lancini, F. Docchio, and G. Sansoni, “Performance analysis of the pmd camboard picoflexx time-of-flight camera for markerless motion capture applications,” IEEE Transactions on Instrumentation and Measurement , vol. 68, no. 11, pp. ...
2019
-
[14]
Pmd flexx2 3d time-of-flight camera,
PMD Technologies, “Pmd flexx2 3d time-of-flight camera,” Available: https://3d.pmdtec.com/en/3d-cameras/flexx2/, 2024, [Online; accessed 12-April-2024]
2024
-
[15]
What can we learn from depth camera sensor noise?
A. Haider and H. Hel-Or, “What can we learn from depth camera sensor noise?” Sensors, vol. 22, no. 14, 2022. [Online]. Available: https://www.mdpi.com/1424-8220/22/14/5448
2022
-
[16]
Depth image denoising using nuclear norm and learning graph model,
C. Yan, Z. Li, Y . Zhang, Y . Liu, X. Ji, and Y . Zhang, “Depth image denoising using nuclear norm and learning graph model,” ACM Trans- actions on Multimedia Computing, Communications, and Applications (TOMM), vol. 16, no. 4, pp. 1–17, 2020
2020
-
[17]
Characterizations of noise in kinect depth images: A review,
T. Mallick, P. P. Das, and A. K. Majumdar, “Characterizations of noise in kinect depth images: A review,” IEEE Sensors journal , vol. 14, no. 6, pp. 1731–1740, 2014
2014
-
[18]
Modeling kinect sensor noise for improved 3d reconstruction and tracking,
C. V . Nguyen, S. Izadi, and D. Lovell, “Modeling kinect sensor noise for improved 3d reconstruction and tracking,” in 2012 second interna- tional conference on 3D imaging, modeling, processing, visualization & transmission. IEEE, 2012, pp. 524–530
2012
-
[19]
Walk these ways: Tuning robot control for generalization with multiplicity of behavior,
G. B. Margolis and P. Agrawal, “Walk these ways: Tuning robot control for generalization with multiplicity of behavior,” in Conference on Robot Learning. PMLR, 2023, pp. 22–31
2023
-
[20]
Learning quadrupedal locomotion over challenging terrain,
J. Lee, J. Hwangbo, L. Wellhausen, V . Koltun, and M. Hutter, “Learning quadrupedal locomotion over challenging terrain,” Science robotics , vol. 5, no. 47, p. eabc5986, 2020
2020
-
[21]
Navigating to objects in the real world,
T. Gervet, S. Chintala, D. Batra, J. Malik, and D. S. Chaplot, “Navigating to objects in the real world,” Science Robotics , vol. 8, no. 79, p. eadf6991, 2023
2023
-
[22]
Low-cost commodity depth sensor comparison and accuracy analysis,
T. Breuer, C. Bodensteiner, and M. Arens, “Low-cost commodity depth sensor comparison and accuracy analysis,” in Electro-Optical Remote Sensing, Photonic Technologies, and Applications VIII; and Military Applications in Hyperspectral Imaging and High Spatial Resolution Sensing...
2014
-
[23]
First experiences with kinect v2 sensor for close range 3d modelling,
E. Lachat, H. Macher, M.-A. Mittet, T. Landes, and P. Grussenmeyer, “First experiences with kinect v2 sensor for close range 3d modelling,” in ICIAP 2015 Workshops: BioF or , CTMR, RHEUMA, ISCA, MADiMa, SBMI, and QoEM (2015-09-07 to 2015-09-08: Genoa, Italy) , vol. 40, 2015
2015
-
[24]
metrological qualification of the intel d400™ active stereoscopy cam- eras,
S. Giancola, M. Valenti, R. Sala, S. Giancola, M. Valenti, and R. Sala, “metrological qualification of the intel d400™ active stereoscopy cam- eras,” A Survey on 3D Cameras: Metrological Comparison of Time-of- Flight, Structured-Light and Active Stereoscopy Technologies, pp. 7...
2018
-
[25]
3d sensors for sewer inspection: A quantitative review and analysis,
C. H. Bahnsen, A. S. Johansen, M. P. Philipsen, J. W. Henriksen, K. Nasrollahi, and T. B. Moeslund, “3d sensors for sewer inspection: A quantitative review and analysis,” Sensors, vol. 21, no. 7, p. 2553, 2021
2021
-
[26]
Evaluation of low-cost depth cameras for agricultural applications,
I. C. Condotta, T. M. Brown-Brandl, S. K. Pitla, J. P. Stinn, and K. O. Silva-Miranda, “Evaluation of low-cost depth cameras for agricultural applications,” Computers and Electronics in Agriculture , vol. 173, p. 105394, 2020
2020
-
[27]
Noise flow: Noise modeling with conditional normalizing flows,
A. Abdelhamed, M. A. Brubaker, and M. S. Brown, “Noise flow: Noise modeling with conditional normalizing flows,” in Proceedings of the IEEE/CVF International Conference on Computer Vision , 2019, pp. 3165–3173
2019
Reviewed August 11, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.