17904994
Using omnidirectional structure from motion for registration of range images of minimal overlap
We propose a novel method of merging a series of range images with a minimal overlap between any two consecutive range images. We rigidly mount a parabolic catadioptric camera to the range scanner. Using two omniviews we are able to accurately estimate the relative displacement between two range views. The resultant motion is used for the registration of all range data to the same coordinate system. An additional perspective camera calibrated with respect to the scanner is used for texture mapping.
206590820
Structure from Motion with Known Camera Positions
The wide availability of GPS sensors is changing the landscape in the applications of structure from motion techniques for localization. In this paper, we study the problem of estimating camera orientations from multiple views, given the positions of the viewpoints in a world coordinate system and a set of point correspondences across the views. Given three or more views, the above problem has a finite number of solutions for three or more point correspondences. Given six or more views, the problem has a finite number of solutions for just two or more points. In the three-view case, we show the necessary and sufficient conditions for the three essential matrices to be consistent with a set of known baselines. We also introduce a method to recover the absolute orientations of three views in world coordinates from their essential matrices. To refine these estimates we perform a least-squares minimization on the group cross product SO(3) × SO(3) × SO(3). We report experiments on synthetic data and on data from the ICCV2005 Computer Vision Contest.
14590041
Designing Access Methods for Bitemporal Databases
By supporting the valid and transaction time dimensions, bitemporal databases represent reality more accurately than conventional databases. The authors examine the issues involved in designing efficient access methods for bitemporal databases, and propose the partial-persistence and the double-tree methodologies. The partial-persistence methodology reduces bitemporal queries to partial persistence problems for which an efficient access method is then designed. The double-tree methodology "sees" each bitemporal data object as consisting of two intervals (a valid-time and a transaction-time interval) and divides objects into two categories according to whether the right endpoint of the transaction time interval is already known. A common characteristic of both methodologies is that they take into account the properties of each time dimension. Their performance is compared with a straightforward approach that "sees" the intervals associated with a bitemporal object as composing one rectangle, which is stored in a single multidimensional access method. Given that some limited additional space is available, the experimental results show that the partial-persistence methodology provides the best overall performance, especially for transaction timeslice queries. For those applications that require ready, off-the-shelf, access methods, the double-tree methodology is a good alternative.
1885434
Cost and availability tradeoffs in replicated data concurrency control
Techniques for optimizing a static voting type algorithm are presented. Our basic optimization models are based on minimizing communications cost subject to given reliability constraints. Two models are presented; in the first model the reliability constraint is failure tolerance, while in the second it is availability. Other simpler models that are special cases of these two basic models and rise from making simplifying assumptions such as equal vote values or constant inter-site communications costs are also discussed. We describe a semi-exhaustive algorithm and efficient heuristics for solving each model. The algorithms utilize a novel signature based method for identifying equivalent vote combinations, and an efficient procedure for computing availability. Computational results for the various algorithms are also given. Finally, the optimized static algorithm was compared against the available copies method, a dynamic algorithm, to understand the relative performance of the two types of algorithms. We found that if realistic reconfiguration times are assumed, then no one type of algorithm is uniformly better. The factors that influence relative performance have been identified. 16 refs., 2 figs., 10 tabs.
11586321
SCTP vs TCP : performance comparison in MANETs
We provide a simulation-based performance comparison of SCTP (stream control transmission protocol) vs TCP (Transmission Control Protocol) in MANET (mobile ad hoc network) environments. We conclude that the behavior of SCTP and TCP in MANETs is similar, but TCP outperforms SCTP in most cases because of extra overheads present in SCTP.
8829443
Exploring the trade-off between label size and stack depth in MPLS routing
Multiprotocol label switching or MPLS technology is being increasingly deployed by several of the largest Internet service providers to solve problems such as traffic engineering and to offer IP services like virtual private networks (VPNs). In MPLS, the analysis of the packet (network layer) header is performed just once, and each packet is assigned a stack of labels, which is examined by subsequent routers when making forwarding decisions. Despite the fact that MPLS is becoming widespread on the Internet, we know essentially very little about the performance one can achieve with it, and about the intrinsic trade-offs in its use of resources. In this paper, we undertake a comprehensive study of the label size versus stack depth trade-off for MPLS routing protocols on lines and trees. We show that in addition to LSP tunneling, label stacks can also be used to dramatically reduce the number of labels required for setting up MPLS LSPs in a network. Based on this observation, we develop routing algorithms and prove lower bounds for two basic problems: (1) fixed label routing: given a fixed number of labels, we want to minimize the stack depth, and (2) fixed stack routing: given a bound on the stack depth, we want to minimize the number of labels used. Our simulation results validate our approach, demonstrating that our novel protocols enable MPLS routing on large trees with few labels and small stack sizes. Thus, our MPLS routing algorithms are applicable to a number of practical scenarios involving the provisioning of VPNs and multicast trees.
2753181
Hierarchical Quorum Consensus: A New Algorithm for Managing Replicated Data
A novel algorithm for managing replicated data is presented. The proposed method is based on organizing the copies of an object into a logical, multilevel hierarchy, and extending the quorum consensus algorithm to such an environment. Several properties of the method are derived and optimality conditions are given for minimizing the quorum size. It is shown that, given a collection of n copies of an object, the method allows a quorum to be formed with n/sup 0.63/ copies versus ((n+1)/2) copies in the case of the majority voting algorithm. Tradeoffs between the proposed method and three other quorum-based methods are discussed, and the main features of each method are highlighted. >