Facial hexahedral mesh transferring by volumetric mapping based on harmonic fields

Hexahedral mesh has obvious mechanical advantages over tetrahedral mesh, but it is no trivial task to generate hexahedral mesh for complex object shapes such as individual faces. This paper presents a novel method to generate patient-specific hexahedral meshes of facial soft tissue models, based on a volumetric cross-parameterization mapping from a standard hexahedral mesh to the individual model. The volumetric parameterization is constructed based on triple of the volumetric harmonic fields, which are adapted to be as close to mutually orthogonal as possible, to achieve some quasi-conformal effect. In addition, some piecewise constraints on the harmonic fields are added to ensure anatomical feature correspondence. Experimental results show that our approach works efficiently for facial soft tissue modeling, avoids element flipping and preserves mesh element angles to a significant extent.     

Complexity and design of QoS routing algorithms in wireless mesh networks

Quality of service (QoS) provisioning in wireless mesh networks (WMNs) is an open issue to support emerging multimedia services. In this paper, we study the problem of QoS provisioning in terms of end-to-end bandwidth allocation in WMNs. It is challenging due to interferences in the networks. We consider widely used interference models and show that except a few special cases, the problem of finding a feasible path is NP-complete under the models. We propose a k-shortest path based algorithmic framework to solve this problem. We also consider the problem of optimizing network performance by on-line dynamic routing, and adapt commonly used conventional QoS routing metrics to be used in WMNs. We find the optimal solutions for these problems through formulating them as optimization models. A model is developed to check the existence of a feasible path and another to find the optimal path for a demand; moreover, an on-line optimal QoS routing algorithm is developed. Comparing the algorithms implemented by the proposed framework with the optimization models shows that our solution can find existing feasible paths with high probability, efficiently optimizes path lengths, and has a comparable performance to the optimal QoS routing algorithm. Furthermore, our results show that contrary to wireline networks, minimizing resource consumption should be preferred over load distribution even in lightly loaded WMNs.     

Route Maintenance in IEEE 802.11 wireless mesh networks

In this paper, we propose a novel Route Maintenance scheme for IEEE 802.11 wireless mesh networks. Despite lack of mobility and energy constraints, reactive routing protocols such as AODV and DSR suffer from frequent route breakages in 802.11 based infrastructure wireless mesh networks. In these networks, if any intermediate node fails to successfully transmit a packet to the next hop node after a certain number of retransmissions, the link layer reports a transmission problem to the network layer. Reactive routing protocols systematically consider this as a link breakage (and therefore a route breakage). Transmission failures can be caused by a number of factors e.g. interference or noise and can be transient in nature. Frequent route breakages result in significant performance degradation. The proposed mechanism considers multiple factors to differentiate between links with transient transmission problems from those links which have permanent transmission problems and takes a coherent decision on link breakage. The proposed mechanism is implemented in AODV for single-radio single-channel mesh network and an extension is incorporated in multi-radio multi-channel scenarios. Simulation results show substantial performance improvement compared to classical AODV and local route repair schemes.     

Extra back-off flow control in multi-hop wireless networks

CSMA is the predominant distributed access protocol for wireless mesh networks. Originally designed for single-hop settings, CSMA can exhibit severe performance problems in multi-hop networks in terms of stability and end-to-end throughput. To ensure a smoother flow of packets, we examine an enhancement referred to as Extra Back-off (EB) flow control. In this enhanced scheme a node remains silent for a certain extra back-off time (imposed on top of the usual back-off time that is part of CSMA) after it has transmitted a packet, to give both the downstream and upstream neighbors the opportunity to transmit. EB flow control entails only a small modification to CSMA, preserving its distributed character. In order to examine the performance of EB flow control, we analyze a novel class of Markov models at the interface between classical tandem queues and interacting particle systems. The results demonstrate that EB flow control provides an effective mechanism for improving the end-to-end throughput performance.     

立体视觉测程研究进展

立体视觉测程技术正逐渐成为移动机器人在非结构化环境中导航定位的主要实现方法.本文对立体视觉测程技术在机器人导航应用中的现状进行分析,总结目前立体视觉测程技术的主要研究内容和方法,评述各种技术方法的优缺点,最后对立体视觉测程的发展进行展望.     

基于几何特征和力学特性的自适应网格生成算法

为获得适合有限元分析的满意网格划分,提出了平面域的基于几何特征和力学特性相结合的自适应网络生成方法,实现了应力集中区的网格局部加密及平稳变密度的网格自动剖分,通过实例表明本方法实用性强、效果良好。     

用于半自动视频对象提取的自适应网格图像分割

随着MPEG－4标准的发展和基于内容的视频处理研究,视频对象平面（VOP）的有效产生成为一个关键问题。本文提出一种基于区域的自适应网格彩色图像分割方法,可用于获得半自动视频对象跟踪和提取所需的初始VOP。该方法利用CIE L＊a*b＊色彩空间的特征量,对视频序列的第一帧进行三角形网格的分裂与合并。对MPEG－4标准测试序列的分割实验取得了较好的结果。     

电涡流检测系统仿真分析的前处理技术

基于推进波前网格生成方法,文中提出了一种适合于是流检测系统的有限元分析的可靠的自适应网格生成方法,并以此方法成功地划盼了电涡流检测系统的有限元网络,并给出了仿真分析结果。     

多孔平面的快速边界元划分

在 3D VL SI互连寄生电容的边界元素法计算中 ,多孔平面的边界元划分是十分困难的问题 .文中提出一种快速划分多孔平面边界元的方法 ,它可高效处理非正交几何边界形状 ,形成规则的梯形元 .与全局扫描线法相比 ,有较高的划分速度、计算速度与精度     

非结构化对等网络资源定位机制研究

非结构化对等网络是当今P2P的主流,资源搜索定位机制是其核心问题,Gnutella为代表的非结构化对等网络采用基于BFS和DFS的定位机制。在分析非结构化对等网络特征的基础上,提出了改进搜索定位机制的思路和方法。