- 标题
- 摘要
- 关键词
- 实验方案
- 产品
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Paracatadioptric camera calibration using sphere images and common self-polar triangles
摘要： Camera calibration is an indispensable step in achieving stratified three-dimensional reconstruction. In the imaging model of the unit viewing sphere of a central catadioptric camera, there exists a pair of common self-polar triangles between a pair of antipodal sphere images and the image of the great circle on the base plane, so the vertices of the common self-polar triangles can be obtained. According to the imaging characteristics of the sphere on the unit viewing sphere, the line connecting the two common poles outside the conics is a vanishing line, enabling the imaged circular points to be determined. Finally, the intrinsic parameters of the paracatadioptric camera can be found using constraints between the imaged circular points and the image of the absolute conic. Since the boundary points of the sphere image can be almost completely extracted, the curve fitting accuracy can be improved. Experiments with simulated as well as real data demonstrated that our method is feasible and effective.
关键词： Camera intrinsic parameters,Paracatadioptric camera,Imaged circular points,Sphere image,Common self-polar triangle
更新于2025-09-23 15:23:52
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A profile error evaluation method for freeform surface measured by sweep scanning on CMM
摘要： Coordinate measuring machines (CMMs) are widely used in evaluating the profile error of the freeform surfaces. Traditional trigger probes are inefficient at collecting large scale measurement sets. A CMM equipped with a sweep scanning probe dramatically improves the efficiency of inspection. However, the large scale data obtained by scanning is challenging for most existing profile error evaluation methods, which are based on iterative algorithms. This paper proposes an efficient and accurate method to evaluate the profile error of freeform surfaces. To simultaneously simplify the calculation and retain the accuracy of evaluation, a new method to extract key points from scanning data set is presented. First, the key points are defined as those measured points with regional outstanding deviations from the design surface. Second, wavelet decomposition is utilized to decompose the curves formed by the deviations of the measured points in the scanning data set, and the key points are extracted according to the decomposition results. In addition, the key points set, as a representation of the scanning data, are evaluated using Sequential Quadratic Programming (SQP) algorithm. Finally, a simulation example and an actual machined part are used to test the proposed evaluation method. The results prove that the proposed method is both accurate and efficient.
关键词： Key points extraction,Profile error evaluation,Freeform surface,Wavelet decomposition,Sweep scanning
更新于2025-09-23 15:23:52
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[IEEE 2018 15th International Conference on Control, Automation, Robotics and Vision (ICARCV) - Singapore, Singapore (2018.11.18-2018.11.21)] 2018 15th International Conference on Control, Automation, Robotics and Vision (ICARCV) - Quadrotors' Low-cost Vision-based Autonomous Landing Architecture on a Moving Platform
摘要： In this paper, a low-cost vision-based autonomous landing architecture for an unmanned aerial vehicle (UAV) on a moving platform is presented. First, a novel landing pad was designed for a monocular camera to robustly detect the pad in both high and low altitudes. In order to solve mirror effect and occasional misidentification, a 3D points cluster algorithm for relative position estimation is presented. Second, the dynamics of the quadrotor is simplified for this landing task and a PD controller is designed based on the estimated relative position. Finally, the low-cost system architecture of the quadrotor and the experiment results are both presented to show the effectiveness of the proposed method.
关键词： landing pad design,PD controller,quadrotor,moving platform,low-cost,vision-based,3D points cluster algorithm,autonomous landing
更新于2025-09-23 15:23:52
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[IEEE 2018 25th IEEE International Conference on Image Processing (ICIP) - Athens, Greece (2018.10.7-2018.10.10)] 2018 25th IEEE International Conference on Image Processing (ICIP) - Self-Calibration of Traffic Surveillance Cameras Based on Moving Vehicle Appearance and 3-D Vehicle Modeling
摘要： This paper proposes an effective and practical method for self-calibration of traffic surveillance cameras. Based on analyzing multiple moving vehicles across multiple frames, the Canny edge detector and Hough transform are first adopted to obtain orthogonal horizontal vanishing points pairs, from which corresponding vertical vanishing points are derived. Next, mean shift clustering and Laplace linear regression are employed to deal with noise and outliers during estimation of vanishing points. To overcome the unreliable estimation issues of orthogonal vanishing points pairs, we further utilize the projective line segments obtained from 3-D vehicle model to create more reliable pairs and iteratively improve the calibration results. Finally, the estimation of distribution algorithm (EDA) is also applied to relax the assumptions made on camera parameters and the moving trajectories of vehicles during the iterations. Experimental results on different datasets prove the feasibility of our proposed scheme.
关键词： horizon line,EDA optimization,traffic camera self-calibration,3-D vehicle modeling,vanishing points
更新于2025-09-23 15:23:52
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Geometric accuracy of remote sensing images over oceans: The use of global offshore platforms
摘要： The geometric accuracy of tens of millions of scenes of medium-resolution remote sensing (RS) images collected in the past 45 years has been systematically evaluated for land scenes, but the accuracy of ocean scenes is poorly known due to the lack of ground control points (GCPs). In this study, the locations of offshore platforms are first derived from time-series of Landsat-8 OLI images, and are then used as offshore reference points to systematically assess the geometric performance of RS images covering offshore oil/gas development areas. An inventory of 16,131 offshore platforms at the global scale is established, and then a novel method using the position-invariant characteristic of offshore platforms and the coherent characteristic of the geometric shift among tie-points (i.e. between sensed points from to-be-assessed images and the corresponding OLI-derived reference points) is developed for assessing the geometric accuracy of Landsat and other RS images. The method has been applied to 112,935 Landsat scenes (~1.87% of the entire archive) over oceans. The results indicate an optimal performance of Landsat OLI images (both pre-collection and Collection-1) but a less reliable performance of Landsat TM/ETM+ L1TP images. Approximately 50% of TM L1GS and ETM+ L1GT images have at least 2 pixels of geometric error. The new reference points inventory and the developed method were also applied to many other low-resolution and finer-resolution imagery (e.g. VIIRS Night-fire product, Terra/Aqua MODIS active fire product, ENVISAT ASAR, ALOS-1 PALSAR, Sentinel-1 SAR, Sentinel-2 MSI, the National Agriculture Imagery Program (NAIP) aerial images, and images from several Chinese satellites), and a quantitative description of the geometric accuracy of these sensors is also presented. The findings suggest that the new offshore reference point inventory is probably useful to help establish more robust offshore GCPs for U.S. Geological Survey (USGS) GCP library and further improve the ongoing USGS Global GCP improvement plan and European Space Agency Global Reference Image plan.
关键词： Offshore platforms,Remote sensing images,Landsat,Geometric accuracy,Ground control points
更新于2025-09-23 15:23:52
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Exceptional Points of Degeneracy Induced by Linear Time-Periodic Variation
摘要： We present a general theory of exceptional points of degeneracy (EPD) in periodically time-variant systems. We show that even a single resonator with a time-periodic component is able to develop EPDs, contrary to parity-time- (PT) symmetric systems that require two coupled resonators. An EPD is a special point in a system parameter space at which two or more eigenmodes coalesce in both their eigenvalues and eigenvectors into a single degenerate eigenmode. We demonstrate the conditions for EPDs to exist when they are directly induced by time-periodic variation of a system without loss and gain elements. We also show that a single resonator system with zero time-average loss-gain exhibits EPDs with purely real resonance frequencies, yet the resonator energy grows algebraically in time since energy is injected into the system from the time-variation mechanism. Although the introduced concept and formalism are general for any time-periodic system, here, we focus on the occurrence of EPDs in a single LC resonator with time-periodic modulation. These findings have significant importance in various electromagnetic and photonic systems and pave the way for many applications, such as sensors, amplifiers, and modulators. We show a potential application of this time-varying EPD as a highly sensitive sensor.
关键词： exceptional points of degeneracy,time-periodic systems,sensors,modulators,LC resonator,amplifiers
更新于2025-09-23 15:23:52
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Minimally asymmetric state conversion around exceptional singularities in a specialty optical microcavity
摘要： Exploiting scattering matrix (S-matrix) formalism, a unique route for manipulating the exceptional points (EPs) has been reported in a trilayer open optical microcavity with non-uniform real refractive index. The boundaries of the intermediate index layer are appropriately tapered. The cavity is partially pumped by transverse distribution of a balanced gain-loss profile. With strict restriction of balanced gain-loss, controlling infinitesimally small variation of gain-coefficient with simultaneous tapering parameter an EP has been encountered; which is being encircled in cavity parameter space to study dynamics of the coupled states around the EP in the context of robust selective flip-of-state phenomena with lower-asymmetry in state conversion efficiencies. Such a specialty trilayer microcavity with tunable width is reported for the first time; which turns out to be a specific approach to achieve maximum efficiency in conversion with minimum asymmetry. Considering suitable state-of-the-art techniques, this exclusive proposal may open up a fabrication feasible platform for device level implementation of system control exploiting EPs.
关键词： Microcavity devices,Mode-converters,Exceptional points,Avoided resonance crossing,Optical resonators
更新于2025-09-23 15:23:52
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Multiple topological transitions in twisted bilayer graphene near the first magic angle
摘要： Recent experiments have observed strongly correlated physics in twisted bilayer graphene (TBG) at very small angles, along with nearly flat electron bands at certain fillings. A good starting point in understanding the physics is a continuum model (CM) proposed by Lopes dos Santos et al. [Phys. Rev. Lett. 99, 256802 (2007)] and Bistritzer et al. [Proc. Natl. Acad. Sci. USA 108, 12233 (2011)] for TBG at small twist angles, which successfully predicts the bandwidth reduction of the middle two bands of TBG near the first magic angle θ0 = 1.05°. In this paper, we analyze the symmetries of the CM and investigate the low energy flat band structure in the entire moiré Brillouin zone near θ0. Instead of observing flat bands at only one 'magic' angle, we notice that the bands remain almost flat within a small range around θ0, where multiple topological transitions occur. The topological transitions are caused by the creation and annihilation of Dirac points at either K, K', or Γ points or along the high symmetry lines in the moiré Brillouin zone. We trace the evolution of the Dirac points, which are very sensitive to the twist angle, and find that there are several processes transporting Dirac points from Γ to K and K'. At the Γ point, the lowest energy levels of the CM are doubly degenerate for some range of twisting angle around θ0, suggesting that the physics is not described by any two-band model. Based on this observation, we propose an effective six-band model (up to second order in quasimomentum) near the Γ point with the full symmetries of the CM, which we argue is the minimal model that explains the motion of the Dirac points around Γ as the twist angle is varied. By fitting the coefficients from the numerical results, we show that this six-band model captures the important physics over a wide range of angles near the first 'magic' angle.
关键词： flat bands,twisted bilayer graphene,continuum model,magic angle,topological transitions,Dirac points
更新于2025-09-23 15:22:29
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[IEEE 2018 37th Chinese Control Conference (CCC) - Wuhan (2018.7.25-2018.7.27)] 2018 37th Chinese Control Conference (CCC) - LIDAR-based Vehicle Recognition with Global Cylindrical-coordinate Histogram Descriptor
摘要： Vehicle Recognition is crucial for an Autonomous Land Vehicle (ALV) navigation in urban environments. In this paper, we proposed a novel Global Cylindrical Coordination Histogram Descriptor(GCCHD) to recognize vehicles in urban environments. This description is created for the centre point of each object. In order to achieve independence with respect to the rotation around z axis, a global reference frame is introduced in GCCHD, and all the three-dimensional points in the cylindrical support region are projected into the three-dimensional histogram according to their three cylindrical coordinates. Experiments on the public Sydney Urban Object dataset and the dataset we prepare and label manually verify the performance of the GCCHD in vehicle recognition.
关键词： Vehicle Recognition,three-dimensional points,GCCHD,Autonomous Land Vehicle
更新于2025-09-23 15:22:29
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[IEEE 2018 12th International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials) - Espoo, Finland (2018.8.27-2018.9.1)] 2018 12th International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials) - General Conditions to Realize Exceptional Points of Degeneracy and Applications
摘要： We explore the emergence of exceptional points of degeneracy (EPDs) that denote a coalescence of multiple eigenmodes in coupled-mode structures. We establish the general conditions that lead to observing EPDs of different orders in various coupled systems, either uniform (non-periodic) or periodic. EPDs can be found in lossless systems as well as in systems with gain-loss balance. Also, we provide examples of potential applications of structures with EPDs that help improve the performance of devices, like sensors, oscillators, etc., at microwave and optical frequencies.
关键词： exceptional points of degeneracy,optical frequencies,microwave frequencies,EPDs,gain-loss balance,oscillators,lossless systems,coupled-mode structures,sensors
更新于2025-09-23 15:21:21