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71.
Justin Solomon Mirela Ben‐Chen Adrian Butscher Leonidas Guibas 《Computer Graphics Forum》2011,30(5):1543-1552
Cartoon animation, image warping, and several other tasks in two‐dimensional computer graphics reduce to the formulation of a reasonable model for planar deformation. A deformation is a map from a given shape to a new one, and its quality is determined by the type of distortion it introduces. In many applications, a desirable map is as isometric as possible. Finding such deformations, however, is a nonlinear problem, and most of the existing solutions approach it by minimizing a nonlinear energy. Such methods are not guaranteed to converge to a global optimum and often suffer from robustness issues. We propose a new approach based on approximate Killing vector fields (AKVFs), first introduced in shape processing. AKVFs generate near‐isometric deformations, which can be motivated as direction fields minimizing an “as‐rigid‐as‐possible” (ARAP) energy to first order. We first solve for an AKVF on the domain given user constraints via a linear optimization problem and then use this AKVF as the initial velocity field of the deformation. In this way, we transfer the inherent nonlinearity of the deformation problem to finding trajectories for each point of the domain having the given initial velocities. We show that a specific class of trajectories — the set of logarithmic spirals — is especially suited for this task both in practice and through its relationship to linear holomorphic vector fields. We demonstrate the effectiveness of our method for planar deformation by comparing it with existing state‐of‐the‐art deformation methods. 相似文献
72.
Given a set of points in the plane, and a sweep-line as a tool, what is best way to move the points to a target point using a sequence of sweeps? In a sweep, the sweep-line is placed at a start position somewhere in the plane, then moved orthogonally and continuously to another parallel end position, and then lifted from the plane. The cost of a sequence of sweeps is the total length of the sweeps. Another parameter of interest is the number of sweeps. Four variants are discussed, depending on whether the target is a hole or a pile, and whether the target is specified or freely selected by the algorithm. Here we present a ratio 4/π≈1.27 approximation algorithm in the length measure, which performs at most four sweeps. We also prove that, for the two constrained variants, there are sets of n points for which any sequence of minimum cost requires 3n/2?O(1) sweeps. 相似文献
73.
New tight bounds are presented on the minimum length of planar straight line graphs connecting n given points in the plane and having convex faces. Specifically, we show that the minimum length of a convex Steiner partition
for n points in the plane is at most O(log n/log log n) times longer than a Euclidean minimum spanning tree (EMST), and this bound is the best possible. Without Steiner points,
the corresponding bound is known to be Θ(log n), attained for n vertices of a pseudo-triangle. We also show that the minimum length convex Steiner partition of n points along a pseudo-triangle is at most O(log log n) times longer than an EMST, and this bound is also the best possible. Our methods are constructive and lead to O(nlog n) time algorithms for computing convex Steiner partitions having O(n) Steiner points and weight within the above worst-case bounds in both cases. 相似文献
74.
Geometric fusion for a hand-held 3D sensor 总被引:2,自引:0,他引:2
Abstract. This article presents a geometric fusion algorithm developed for the reconstruction of 3D surface models from hand-held sensor
data. Hand-held systems allow full 3D movement of the sensor to capture the shape of complex objects. Techniques previously
developed for reconstruction from conventional 2.5D range image data cannot be applied to hand-held sensor data. A geometric
fusion algorithm is introduced to integrate the measured 3D points from a hand-held sensor into a single continuous surface.
The new geometric fusion algorithm is based on the normal-volume representation of a triangle, which enables incremental transformation of an arbitrary mesh into an implicit volumetric field
function. This system is demonstrated for reconstruction of surface models from both hand-held sensor data and conventional
2.5D range images.
Received: 30 August 1999 / Accepted: 21 January 2000 相似文献
75.
The ability to analyze the effectiveness of agent reward structures is critical to the successful design of multiagent learning
algorithms. Though final system performance is the best indicator of the suitability of a given reward structure, it is often
preferable to analyze the reward properties that lead to good system behavior (i.e., properties promoting coordination among
the agents and providing agents with strong signal to noise ratios). This step is particularly helpful in continuous, dynamic,
stochastic domains ill-suited to simple table backup schemes commonly used in TD(λ)/Q-learning where the effectiveness of
the reward structure is difficult to distinguish from the effectiveness of the chosen learning algorithm. In this paper, we
present a new reward evaluation method that provides a visualization of the tradeoff between the level of coordination among
the agents and the difficulty of the learning problem each agent faces. This method is independent of the learning algorithm
and is only a function of the problem domain and the agents’ reward structure. We use this reward property visualization method
to determine an effective reward without performing extensive simulations. We then test this method in both a static and a
dynamic multi-rover learning domain where the agents have continuous state spaces and take noisy actions (e.g., the agents’
movement decisions are not always carried out properly). Our results show that in the more difficult dynamic domain, the reward
efficiency visualization method provides a two order of magnitude speedup in selecting good rewards, compared to running a
full simulation. In addition, this method facilitates the design and analysis of new rewards tailored to the observational
limitations of the domain, providing rewards that combine the best properties of traditional rewards. 相似文献
76.
The Stanford Event Heap has been shown to provide appropriate support for constructing interactive workspace applications.
Given this success it is natural to consider the Event Heap as a platform to support other classes of Ubiquitous Computing
applications. In this paper we argue that the distributed, spontaneous nature of these applications places additional demands
on the Event Heap that require extensions to both the engineering and API. Suitable extensions are described and their use
to support a typical Ubicomp application is discussed.
相似文献
Oliver Storz (Corresponding author)Email: |
Adrian FridayEmail: |
Nigel DaviesEmail: |
77.
Pulsed laser technique application to liquid and gaseous flows and the scattering power of seed materials 总被引:1,自引:0,他引:1
Mie scattering computations have been performed for light scattered by small particles from a pulsed sheet of laser illumination and collected and imaged by a camera lens. From these computations the smallest particles that can be photographed in various fluid measurement situations, including air and water, have been determined in terms of system parameters such as laser power, light sheet geometry, f/No., and photographic film properties. The particle scattering requirements of the individual particle image mode and the speckle mode are compared. 相似文献
78.
Adrian D. Werner Qi Zhang Lijuan Xue Brian D. Smerdon Xianghu Li Xinjun Zhu Lei Yu Ling Li 《Water Resources Management》2013,27(2):507-533
The state of groundwater systems worldwide is presently not well defined, and in particular there is little context for agencies responsible for managing water resources to evaluate occurrences of groundwater depletion against other cases globally. In this study, an initial inventory of groundwater depletion problems is compiled and ranked to identify the world’s most critical cases, i.e. situations of groundwater mega-depletion. The ranking is based on an indexed approach that considers overdraft, drawdown and subsidence, plus the importance of the resources in terms of population-dependency and rates of extraction. The five most highly ranked depleted aquifers of the world include the shallow aquifers of the Hai River Plain (China), the Altiplano region (Spain), the Mexico Basin (Mexico), the Huang River basin (China) and the California Central Valley (USA). An abridged account of modelling to assess drawdown is described for the Hai River Plain, revealing that despite recharge in the order of 13,000 GL/yr, an overdraft of about 8,000 GL/yr is occurring to support the vast population of the region. This has led to up to 100 m of drawdown in places and reports of subsidence of several metres. The Hai River situation demonstrates that falling water levels may not act to alleviate pumping stresses; a symptom of unchecked extraction and an exemplary illustration of the tragedy of the commons. The causal factors leading to mega-depletion are varying across the globe and each mega-depletion case contains unique elements, although population appears to be an important factor. 相似文献
79.
Spirkl FM Kunz S Schweinberger FF Farnbacher AN Schröter R Heiz U 《The Review of scientific instruments》2012,83(1):013114
The separation of ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES) is usually performed by a time-of-flight (ToF) separation using pre-set ToF for both types of signal. In this work, we present a new, improved ex situ signal separation method for the separation of MIES and UPS for every single measurement. Signal separation issues due to changes of system parameters can be overcome by changing the ToF separation and therefore allowing for the application of a wider range of measuring conditions. The method also enables to identify and achieve separation of the two signals without any time consuming calibration and the use of any special material for the calibration. Furthermore, changes made to the discharge source are described that enable to operate an existing MIES/UPS source over a broader range of conditions. This allows for tuning of the yield of UV photons and metastable rare gas atoms leading to an improved signal to noise ratio. First results of this improved setup are well in agreement with spectra reported in literature and show increased resolution and higher signal intensities for both MIE and UP spectra compared to the previous, non-optimized setup. 相似文献
80.
The paper goes systematically through the problem of load selection, negotiating load supply details with the electric utility, electrical supply contract completion, and the design specification, construction, and maintenance of electrical substation facilities as applied to pipeline pump and compressor station. The ``simple' type of substation is discussed, although component specifications and selection normally are the same regardless of substation complexity. An up-to-date list of applicable United States and Canadian Standards as well as a detailed list of required tests to be carried out on individual components are included in the Appendix. 相似文献