Vertical Distortion Research Articles

A lifting-wavelet-based iterative thresholding correction for atomic force microscopy images with vertical distortion

To eliminate distortion caused by vertical drift and illusory slopes in atomic force microscopy (AFM) imaging, a lifting-wavelet-based iterative thresholding correction method is proposed in this paper. This method achieves high-quality AFM imaging via line-by-line corrections for each distorted profile along the fast axis. The key to this line-by-line correction is to accurately simulate the profile distortion of each scanning row. Therefore, a data preprocessing approach is first developed to roughly filter out most of the height data that impairs the accuracy of distortion modeling. This process is implemented through an internal double-screening mechanism. A line-fitting method is adopted to preliminarily screen out the obvious specimens. Lifting wavelet analysis is then carried out to identify the base parts that are mistakenly filtered out as specimens so as to preserve most of the base profiles and provide a good basis for further distortion modeling. Next, an iterative thresholding algorithm is developed to precisely simulate the profile distortion. By utilizing the roughly screened base profile, the optimal threshold, which is used to screen out the pure bases suitable for distortion modeling, is determined through iteration with a specified error rule. On this basis, the profile distortion is accurately modeled through line fitting on the finely screened base data, and the correction is implemented by subtracting the modeling result from the distorted profile. Finally, the effectiveness of the proposed method is verified through experiments and applications.

Fully Transparent and Distortion-Free Monotonically Stretchable Substrate by Nanostructure Alignment.

Stretchable devices, garnering increasing attention as next-generation form factors, have a crucial problem in that vertical contraction occurs during stretching, causing image distortion of stretchable displays and discomfort in skin-attached devices. Previous structural strategies to mitigate vertical contraction, such as auxetic reentrants and wrinkles, suffer from the drawback that their structure becomes visible during stretching. In this study, this issue is addressed by unidirectionally aligning nanoscopic cylinders within block copolymer elastomer films. Employing a shear-rolling process at high temperatures on thick films of polystyrene-block-polyisobutylene-block-polystyrene thermoplastic elastomers, macroscopic mechanical anisotropy is achieved, resulting in completely transparent and monotonically stretchable substrates devoid of vertical or depth distortion during deformation. Significantly, the unidirectional orientation of high-modulus cylindrical nanostructures induces macroscopic mechanical anisotropy with a modulus ratio exceeding five times. While the Poisson's ratio of conventional elastic materials hovers around ≈0.5, this mechanical anisotropy minimizes vertical contraction, yielding a Poisson's ratio below 0.07. Moreover, owing to the negligible size of the nanocylinders compared to visible-light wavelengths, the substrate can be monotonically uniaxially stretched while maintaining high transmittance without introducing distortions, surface undulations, or haziness, resulting in distortion-free stretchable substrates.

Mechanically anisotropic stretchable and transparent composite substrates for distortion-free display

Developing new-form factor devices has led to the creation of foldable and rollable displays. However, next-step stretchable devices with shape changes face a new issue different from conventional displays. When the screen is stretched, screen distortion occurs due to the nature of contraction perpendicular to the stretching direction to preserve the volume. To address this issue, we focused on the composite approach with mechanical anisotropy using continuously aligned fiber fillers. In this study, we fabricated transparent mechanically anisotropic stretchable substrates with near-zero Poisson's ratio by incorporating continuous and aligned transparent ribbon arrays within a transparent, stretchable matrix. The aligned ribbon-reinforced composite substrates have mechanical anisotropy by mainly reinforcing stiffness only in the ribbon-aligned direction. When unidirectional stretchable devices in the direction perpendicular to the alignment are developed, the stiffness of the substrate contracting in the ribbon alignment direction is relatively high compared to the vertical direction, and thus the vertical displacement is diminishing, so substrates with Poisson's ratio close to 0 can be realized. Based on this approach, we realized a light-emitting device (LED) array system with near-zero vertical distortion by attaching LED arrays and printed intrinsically stretchable interconnections on our mechanically anisotropic composite substrate.

Informing directed energy deposition strategies through understanding the evolution of residual stress

Residual stress is a critical concern in directed energy deposition (DED) as it impacts the functional performance of components, in terms of load-bearing capacity and corrosion behavior. In this paper, an analytical model is developed to characterize the temperature distribution, residual stress profiles, and geometric distortions in DED-built components, which enables efficient planning of the DED strategies. The model combines a finite difference method to predict the temperature history, modified Green's functions to quantify thermal stress, and a radial return method to update plastic stress, which features the deposition of materials with heat source movement. The predictions are validated against the temperature, residual stress, and distortion profiles reported in the literature. In the fabrication of multiple-layer thin-wall structures, the model demonstrates a maximum prediction error of 18.2% for the peak temperature at the middle of the deposit-substrate interface and 17.7% for the vertical distortion of the deposit-substrate system. Simulation results from the model provide insights into understanding stress evolution in the deposition and offer guidance in the selection of linear energy density parameters (heat source power and traverse speed) and process planning parameters (idle time and deposition patterns) when designing DED strategies for thin-wall structures. This approach can be implemented in an efficient workflow to optimize the DED technologies for specified residual stress profiles within a component geometry regardless of heat source or material type.

Deformation analysis and protection measures of existing metro tunnels effected by river channel excavation in soft soils

The excavation of river channel in soft soils has an inevitable influence on adjacent metro tunnels. This paper investigates the effect of river channel excavation on the underlying metro tunnels and corresponding protective measures based on an engineering project in Fuzhou city, China. A three-dimensional finite element model is developed to calculate the vertical and horizontal displacements and diametrical distortion of tunnels under different excavation sequences and soil reinforcement measures. Real-time monitoring of the vertical and horizontal displacements and diametrical distortion of tunnels is carried out and analyzed in detail. The results show that the vertical displacements and diametrical distortion of tunnels are primarily caused by the excavation of the river directly above the tunnels, while the horizontal displacements are induced by the excavation of the river next to the tunnels. A combination of cement slurry with a portal form and concrete with a plate form is recommended to reinforce the bottom soil of rivers and protect the operational tunnels. The accuracy of the calculation model and the effectiveness of the protection measure are confirmed by using on-site measurements. The findings can provide useful reference for the safety assessment and protection measures of similar projects.

Propriétés remarquables de la réfraction astronomique dans une atmosphère à symétrie sphérique

The general theoretical properties and various approximate formulas for the angle χ S of astronomical refraction at S are presented, beginning with its fundamental expression by the refraction integral and immediately leading to the Simpson formula. Assuming a spherically symmetric atmosphere, many additional results show up, starting with Oriani’s “theorem” and Bouguer’s equation. From these, an usual form of the refraction integral is derived. A fundamental approximation follows to deduce the Laplace formula, then the series expansions that generalize it but are divergent for an isothermal atmosphere—because of the approximation made. Two results, free from any approximation, exist and are presented: Biot’s and Biot–Sang–Meyer–Fraser–White’s theorems. It leads to an approximate link with extinction, as found by Laplace, and an approximation of the refraction χ Sh for points on the astronomical horizon. We compute the chromatic effects on χ S , χ Sh —and (in a document associated with this article) on various characteristics of optical ducts considered in air—and an original expression of the “local vertical angular distortion coefficient” on the horizon (in Annexe B).

A Study of the Correlation between the Bulkiness of peri-Substituents and the Distortion of a Naphthalene Ring.

A systematic study on the distortion of a naphthalene ring was performed using steric repulsion between peri-substituents at the 1- and 8-positions. The introduction of bromo groups into the methyl groups of the 1,8-dimethylnaphthalene enhanced the steric repulsion to distort the naphthalene ring. X-ray crystallography revealed that 1,8-bis(bromomethyl)naphthalene had a vertical distortion with a 11.0° dihedral angle (α) between peri-substituents which disturbed the coplanarity of the naphthalene ring. On the other hand, the dihedral angle of 1,8-bis(dibromomethyl)naphthalene was smaller (α = 8.3°) despite the bulkier substituents. In this case, horizontal distortion of the naphthalene ring increased. These distortions should non-electronically activate the naphthalene framework. In order to evaluate their reactivity, nitration and hydrogenation were carried out; however, the 1,8-bis(dibromomethyl)naphthalene was intact under the employed conditions. A DFT calculation suggested that the inertness of the 1,8-bis(dibromomethyl)naphthalene is presumably due to the negative hyperconjugation of the (dibromo)methyl group.

Thermo-metallurgical-mechanical modeling of FG titanium-matrix composites in powder bed fusion

The build-up of residual stress plays a crucial role in the formation quality of as-built parts fabricated by powder bed fusion (PBF). However, the comprehensive understanding of residual stress and distortion fields in PBF-printed functionally graded titanium matrix composites (FGTMCs) has not been achieved. Furthermore, the effect of metallurgical transformation on thermo-mechanical behavior of these materials during the PBF remains unclear. In this paper, considering the phase transition effect due to thermal evolution in the PBF process, a thermo-metallurgical-mechanical coupled model of printed FGTMCs is developed. The equivalent temperature-dependent elastic modulus and heat conductivity of FGTMCs are calculated based on the homogenization method. The differential quadrature method is first employed to efficiently predict the temperature, residual stress, and distortion distributions of PBF-printed FGTMCs. The accuracy of the proposed model is verified by a comparison with predictions in the literature. Furthermore, the effect of crucial process and geometrical parameters on the thermo-mechanical behaviors of FGTMCs is analyzed. The results indicate that the residual stress and vertical distortion of FGTMCs increase as the laser penetration, energy densities, and power index increase, while decreasing significantly with the increased ceramic content, preheating temperature, and layer thickness. These results provide valuable insights for guiding the process optimization of additively manufactured FGTMCs.

Position and Attitude Determination in Urban Canyon with Tightly Coupled Sensor Fusion and a Prediction-Based GNSS Cycle Slip Detection Using Low-Cost Instruments.

We present a position and attitude estimation algorithm of moving platforms based on the tightly coupled sensor fusion of low-cost multi baseline GNSS, inertial, magnetic and barometric observations obtained by low-cost sensors and affordable dual-frequency GNSS receivers. The sensor fusion algorithm is realized by an Extended Kalman Filter and estimates the states including GNSS receiver inter-channel biases, integer ambiguities and non-GNSS receiver biases. Tightly coupled sensor fusion increases the reliability of the position and attitude solution in challenging environments such as urban canyons by utilizing the inertial observations in case of GNSS outage. Moreover, GNSS observations can be efficiently used to mitigate IMU sensor drifts. Standard GNSS cycle slips detection methods, such as the application of triple differences or linear combinations such as Melbourne-Wübbena combination and the phase ionospheric residual extended TurboEdit method. However, these techniques are not well suited for the localization in quickly changing environments such as urban canyons. We present a new method of tightly coupled sensor fusion supported by a prediction based cycle slip detection technique, applied to a GNSS setup using three antennas leading to multiple moving baselines on the platform. Thus, not only the GNSS signal properties but also the dynamics of the moving platform are considered in the cycle slip detection. The developed algorithm is tested in an open-sky validation measurement and two sets of measurement in an urban canyon area. The sensor fusion algorithm processes the data sets using the proposed prediction-based cycle slip method, the loss-of-lock indicator-based, and for comparison, the Melbourne-Wübbena and the TurboEdit cycle slip detection methods are also included. The obtained position and attitude estimation results are compared to the internal solution of raw data source GNSS receivers and to the observations of a high-accuracy GNSS/INS unit including a fiber optic gyro. The validation test confirms the proper cycle slip detection in an ideal environment. The more challenging urban canyon test results show the reliability and the accuracy of the proposed method. In the case of the second urban canyon test, the proposed method improved the integer ambiguity resolution success rate by 19% and these results show the lowest horizontal and vertical coordinate distortion in comparison of the linear combination and the loss-of-lock-based cycle slip methods.

Spatial Self-Phase Modulation in Graphene-Oxide Monolayer

The spatial self-phase modulation (SSPM) of the optical field revealed the magnitude and polarity of nonlinear refraction coefficients of the graphene-oxide (GO) atomic layers in an aqueous base solution with a resonant excitation using a chopped quasi-static laser at 532 nm. The SSPM of the optical field as a result of the intrinsic nonlinear refraction coefficient of GO atomic layers and the spatial distribution of intensity displayed the concentric diffraction rings at the far field due to the coherent superposition of transverse wave vectors. The number of concentric rings as a function of the applied intensity revealed the nonlinear refraction coefficient of GO which was estimated to be ~–6.65 × 10−12 m2/W for the laser-excitation duration of ~0.32 s, where the negative polarity of nonlinear refraction coefficient was confirmed with the interference image profile of SSPM. The upper and vertical distortion of concentric rings at the far field at the longer laser-excitation duration of ~0.8 s indicates the distortion of the coherent superposition of transverse wave vectors due to the localized thermal vortex of GO in the aqueous solution that offers novel platforms of thermal metrology based on localized optical nonlinearity and temperature-sensitive all-optical switching.

Accuracy and stability of computer-aided customized lingual fixed retainer: a pilot study

BackgroundWith advances in digital technology, new types of lingual fixed retainers are being developed. However, there are few studies that quantitatively evaluate the accuracy and stability of lingual fixed retainers. The aim of this study was to assess the accuracy and stability of two types of computer-aided customized lingual fixed retainers and a conventional lingual fixed retainer.MethodsA total of 10 maxillary and 10 mandibular duplicated dental models were selected, and then, three types of retainers were fabricated on the canine-to-canine area for each model. To evaluate accuracy, wire clearance at interproximal area (WCI) was measured using superimposition analysis. Initial flatness deformation was also measured for vertical distortion of retainers. Lateral width, anteroposterior length, and flatness deformation were measured at three-time points for stability assessment. Thermocycling was used to induce 6 months of time flow.ResultsThe custom-bent group showed significantly higher WCI than the custom-cut and manual groups in the maxillary arch (P = 0.002). The custom-cut group showed significantly less flatness deformation, which was followed by the custom-bent and manual groups in both the maxillary and mandibular arch (P < 0.001). There was no significant difference in stability between the three retainer groups during 5100 cycles of thermocycling (corresponding to 6-month period).ConclusionsSince there was no difference in stability between the three groups, it is recommended to use custom-cut type retainers in light of accuracy. However, accuracy and stability are not the only factors to consider when selecting type of retainers. Because each retainer has advantages and disadvantages, the type of retainers should be decided in consideration of the clinical environment.

Cost-Effective Design of a Miniaturized Zoom Lens for a Capsule Endoscope.

This paper presents a miniaturized design of a 2× zoom lens for application to a one-megapixel image sensor in a capsule endoscope. The zoom lens is composed of four lenses, including three plastic aspheric lenses and one glass spherical lens, and adopts a three-lens group design. This capsule endoscope is mainly for observation of the small intestine, which has a radius of about 12.5 mm. The height of the object is thus set to 12.5 mm. The object surface is designed to be curved surface with a radius of curvature of 15 mm. The focal length of the zoom lens ranges from 1.064 mm to 2.039 mm, and the full angle of view ranges from 60° to 143°, the f-number is F/2.8-F/3.5, the zoom lens is 11.6 mm in length, and the maximum effective diameter of the zoom lens is 6 mm. The zoom lens design is divided into six segments, corresponding to the different magnifications from Zoom 1 to Zoom 6. The magnification ratios are -0.0845, -0.0984, -0.1150, -0.1317, -0.1482, and -0.1690, respectively. Comparing the positions from Zoom 1 to Zoom 6, the maximum optical distortion is -14.89% for the Zoom 1 and 1.45% for the Zoom 6. The maximum vertical video distortion is 8.19% for Zoom 1 and 1.00% for Zoom 6. At a 1.0 field of view, the minimum relative illuminance is 71.8% at a magnification of M = -0.1317. Finally, we perform the tolerance analysis and lens resolution analysis at different zooming positions. Our design can obtain high-quality images for capsule endoscope.

Measuring the vertical response of the Galactic disc to an infalling satellite

ABSTRACT Using N-body simulations of the Milky Way interacting with a satellite similar to the Sagittarius dwarf galaxy, we quantitatively analyse the vertical response of the Galactic disc to the satellite’s repeated impacts. We approximate the vertical distortion of the Galactic disc as the sum of the first three Fourier azimuthal terms m = 0, 1, and 2, and observe their evolution in different dynamical regimes of interaction. After the first interaction, the m = 0 term manifests itself as outgoing ring-like vertical distortions. The m = 1 term (S-shape warp) is prograde when the impacts of the satellite are more frequent, or in general close to an interaction, whereas it is slowly retrograde in the most quiescent phases. The m = 2 term is typically prograde, and close to an interaction it couples with the m = 1 term. Finally, we find that the vertical response of the disc can be recovered in an unbiased way using the instantaneous positions and velocities of stars in a limited volume of the Galactic disc, analogous to real data, and that the measured vertical pattern speeds have a constraining power in the context of a Milky Way–satellite interaction.

Simulation based calculation of ship motions in extreme seas with a body-exact strip theory approach

For all design phases of naval vessels, the fidelity of seakeeping calculations in extreme seas is open to discussion due to the inadequacy of the linear theory of ship motions. Currently the computer-generated time series of ship responses and wave height (the real time computer experiments) are utilized to calculate the distribution of the vertical distortion, shear force and bending moment by means of “ship hydroelasticity theory”. Inspired by these studies a simulation based calculation of symmetric ship motions is performed in long crested irregular head seas, in addition with a body-exact strip theory approach. The scope of this study is limited to the ship motions only. Verification is achieved utilizing the spectral analysis procedure which contains the discrete Fourier transform (DFT) and the smoothing algorithms. The results are compared with the experimental data, and the ANSYS AQWA software results. The simulation results provide adequate data for the extreme responses. This state-of-the-art method in addition with a “body-exact strip theory approach” ensures the consistent assessment of the seakeeping performance in extreme sea condition. As a result, it is evaluated that this calculation method can be used in the design stages of naval platforms.

Three-dimensional positional accuracy of intraoral and laboratory implant scan bodies

Statement of problemIn the implant digital workflow, scan bodies provide the 3D position of digital implants in the virtual dental arch. However, limited evidence is available on scan body accuracy, selection, and usage. PurposeThe purpose of this in vitro study was to evaluate the 3D positional accuracy of 4 intraoral and 6 laboratory scan body systems to the implants and laboratory replicas of an implant system under various torque magnitudes. Material and methodsTen test groups comprising 4 intraoral (I): Medentika L-Series (MS), Straumann CARES Mono (SM), Core 3D (CO), Straumann RC (SS); and 6 laboratory (L): Nobel Procera Pos Locator (NP), Sirona InPost (SR), Amann Girrbach (AG), Straumann CARES Mono (SM), Core 3D (CO), Straumann RC (SS) scan bodies were derived from 7 scan body systems. Of these, 3 systems (SM, CO, SS) are used for both intraoral and laboratory applications. The scan bodies were tested on Straumann Bone Level Regular CrossFit implants or laboratory replicas. Eight test groups allowed for the variation of torque application (5, 10, and 15 Ncm), while 2 test groups (NP, SR) were hand positioned only. Prefabricated metal abutments (ME) for both implants and laboratory replicas served as controls. A coordinate measuring machine measured four 3D positional accuracy variables: vertical linear distortion (dz), 2D tolerance displacement (dr), global linear distortion (dR), and scan body height discrepancy (ΔH) (n=10). The data were analyzed with 2-way analysis of variance tests and post hoc analysis with Tukey tests (α=.05). ResultsFor both intraoral and laboratory test groups, 2-way ANOVA found that the system had a significant effect on all distortion variables (P<.001), while torque magnitude had a significant effect only on dz and ΔH (P<.001). Overall, mean dz ranged from 5 ±12 μm for L-AG at 15 Ncm to 23 ±14 μm for L-AG at 5 Ncm. Mean dr ranged from 5 ±4 μm for I-SM at 15 Ncm to 73 ±41 μm for L-SS at 10 Ncm, and mean dR ranged from 11 ±6 μm for I-SM at 10 Ncm to 74 ±41 μm for L-SS at 10 Ncm. Mean ΔH ranged from -5 ±10 μm for I-SM at 15 Ncm to 23 ±14 μm for L-AG at 5 Ncm. Among intraoral test groups, for dz and ΔH, all the test groups except for I-SM at 15 Ncm and I-MS at 10 and 15 Ncm were significantly more positive than the control (P<.001). For dr, I-SS at 5, 10, and 15 Ncm was significantly different from the control (P<.001). For dR, only I-SS at 5 Ncm was significantly different from the control (P<.001). Among laboratory test groups, for dz and ΔH, L-AG at 5 Ncm and L-CO at 15 Ncm were significantly more positive than the control (P<.001). For dr, L-SS at 10 and 15 Ncm were significantly different from the control (P<.001). For dR, only L-SS at 10 Ncm was significantly different from the control (P<.001). Intraoral and laboratory systems show comparable 3D positional accuracy. ConclusionsOverall, I-SS and L-SS were the least accurate. The system tested had a significant effect on 3D positional accuracy, while torque magnitude had no consistent effect across all systems.

A micro–macro modelling methodology for the analysis of infilled frames

A macro-modelling approach is proposed for representing the masonry infill walls in a non-linear analysis of infilled frame structures. The infill wall is modelled by equivalent struts. The paper presents a combination of an experimental testing method and analytical method to determine the constitutive law of the equivalent struts model. Thus, the model is based on the physical behaviour of the infill wall, rather than on empirical values that came from experimental or numerical tests of infilled frames. Upon requirement, the model can be developed to horizontal or vertical distortion of the infill wall. The strut model was incorporated in a detailed, micro non-linear two-dimensional finite element analysis and validated against reinforced concrete frames infilled by hollow concrete blocks and autoclaved aerated concrete blocks subjected to loss of supporting column scenario. The validation procedure showed that the model captures not only the stiffness and the resistance of a reinforced concrete infilled frame but also the failure mode and the pattern of cracking of the frame.

Attenuation Coefficient Guided Two-Stage Network for Underwater Image Restoration

Underwater images suffer from severe color casts, low contrast and blurriness, which are caused by scattering and absorption when light propagates through water. However, existing deep learning methods treat the restoration process as a whole and do not fully consider the underwater physical distortion process. Thus, they cannot adequately tackle both absorption and scattering, leading to poor restoration results. To address this problem, we propose a novel two-stage network for underwater image restoration (UIR), which divides the restoration process into two parts viz. horizontal and vertical distortion restoration. In the first stage, a model-based network is proposed to handle horizontal distortion by directly embedding the underwater physical model into the network. The attenuation coefficient, as a feature representation in characterizing water type information, is first estimated to guide the accurate estimation of the parameters in the physical model. For the second stage, to tackle vertical distortion and reconstruct the clear underwater image, we put forth a novel attenuation coefficient prior attention block (ACPAB) to adaptively recalibrate the RGB channel-wise feature maps of the image suffering from the vertical distortion. Experiments on both synthetic dataset and real-world underwater images demonstrate that our method can effectively tackle scattering and absorption compared with several state-of-the-art methods.

A predictive model for in situ distortion correction in laser powder bed fusion using laser shock peen forming

Described is a hybrid metal additive manufacturing (AM) method that integrates in situ laser shock peen (LSP) forming with laser powder bed fusion (PBF) to mitigate vertical distortions during part builds. LSP has recently been proposed to reduce tensile residual stresses during selective laser melting (SLM). The effects of LSP on part distortion, however, have not been rigorously examined. It is proposed here that SLM can be integrated with in situ LSP forming to reduce distortion of the upper surface of parts during or after printing. To study the distortion correction capability, a 2-stage computational framework is created, which includes physics-based models of the SLM process and LSP treatment. Stage 1 includes thermomechanical SLM simulation to predict surface geometry and is applied to model four 50-μm layers of a 316L part having a 4 mm × 4 mm footprint. Stage 2 of the framework includes an elastic-plastic thermomechanical shock-wave simulation to predict LSP surface treatment forming effects. Surface distortion is examined for varying laser spot size, overlap, and part temperatures from 300 to 500 K, using a nanosecond-pulsed infrared laser. For the 316L SLM sample, the upper surface is predicted to have $\sim $ 9-μm vertical distortion on the 200-μm 4-layer build. With a 2-μm allowable distortion, only 44.13% of the surface initially conforms. After one LSP forming treatment at 300 K, conformance improves to 84.75%. After a third LSP forming, with 50% laser power-density increase, surface conformance increases to 91%, demonstrating potential of the hybrid AM-LSP process in reducing finish-machining.

Secrecy Performance Analysis of Air-to-Ground Communication With UAV Jitter and Multiple Random Walking Eavesdroppers

Flexible mobility and random jitter are two unique features of UAV communication platforms. Although advantages of mobility have been extensively explored, the random jitter of UAV platforms, caused by airflow and body vibrations, has been rarely studied. This work aims to answer the two fundamental questions: i) how to analyze the secrecy performance when considering UAV jitter and ii) can this inherent characteristic of UAV be exploited to enhance secrecy? Detailed, we study the modeling and analysis of UAV jitter on the secrecy performance in an air-to-ground (A2G) wiretap system with multiple non-colluding eavesdroppers, where a UAV-mounted transmitter equipped with directional antennas illuminates ground terminals in a finite area. Random waypoint model is applied to characterize the mobility of eavesdroppers. To be specific, by modeling UAV jitter in both horizontal azimuth and vertical elevation, distortion and shift of UAV illumination area are analyzed. Further, beam-illumination probability of a randomly located ground terminal is obtained. Following which, a tractable framework for analyzing the secrecy coverage probability (SCP) and ergodic secrecy capacity (ESC) is developed. Expressions for SCP and ESC are derived with characterizations for the signal-to-noise ratio received at legitimate receiver and eavesdroppers. Finally, extensive simulations are provided to validate the theoretical analysis. This is the first work to find that UAV jitter can be exploited to enhanced secrecy performance of A2G wiretap system with appropriate UAV height and beamwidth.

OmniPhotos

Virtual reality headsets are becoming increasingly popular, yet it remains difficult for casual users to capture immersive 360° VR panoramas. State-of-the-art approaches require capture times of usually far more than a minute and are often limited in their supported range of head motion. We introduce OmniPhotos, a novel approach for quickly and casually capturing high-quality 360° panoramas with motion parallax. Our approach requires a single sweep with a consumer 360° video camera as input, which takes less than 3 seconds to capture with a rotating selfie stick or 10 seconds handheld. This is the fastest capture time for any VR photography approach supporting motion parallax by an order of magnitude. We improve the visual rendering quality of our OmniPhotos by alleviating vertical distortion using a novel deformable proxy geometry, which we fit to a sparse 3D reconstruction of captured scenes. In addition, the 360° input views significantly expand the available viewing area, and thus the range of motion, compared to previous approaches. We have captured more than 50 OmniPhotos and show video results for a large variety of scenes. We will make our code available.