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A natural extension of the conventional finite volume method into polygonal unstructured meshes for CFD application

APPLIED MATHEMATICAL MODELLING 1996年第2期20卷 170-183页

作者： Chow, P Cross, M Pericleous, K Centre for Numerical Modelling and Process Analysis School of Computing and Mathematical Science University of Greenwich LondonU.K.

A new general cell-centered solution procedure based upon the conventional control or finite volume (CV or FV) approach has been developed for numerical heat transfer and fluid flow which encompasses both structured and unstructured meshes for any kind of mixed polygon cell. Unlike conventional FV methods for structured and block structured meshes and both FV and FE methods for unstructured meshes, the irregular control volume (ICV) method does not require the shape of the element or cell to be predefined because it simply exploits the concept of fluxes across cell faces. That is, the ICV method enables meshes employing mixtures of triangular, quadrilateral, and any other higher order polygonal cells to be exploited using a single solution procedure. The ICV approach otherwise preserves all the desirable features of conventional FV procedures for a structured mesh;in the current implementation, collocation of variables at cell centers is used with a Rhie and Chow interpolation (to suppress pressure oscillation in the flow field) in the context of the SIMPLE pressure correction solution procedure. In fact all other FV structured mesh-based methods may be perceived as a subset of the ICV formulation. The new ICV formulation is benchmarked using two standard computational fluid dynamics (CFD) problems, i.e., the moving lid cavity and the natural convection driven cavity. Both cases were solved with a variety of structured and unstructured meshes, the latter exploiting mixed polygonal cell meshes. The polygonal mesh experiments show a higher degree of accuracy for equivalent meshes (in nodal density terms) using triangular or quadrilateral cells;these results may be interpreted in a manner similar to the CUPID scheme used in structured meshes for reducing numerical diffusion for flows with changing direction.

关键词： finite volume unstructured mesh computational fluid dynamics

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Experience from 10 years of student projects oriented towards graphic interaction

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COMPUTERS & GRAPHICS 1996年第3期20卷 463-471页

作者： Kjelldahl, L Sundblad, Y Numerical Analysis and Computing Science—NADA Royal Institute of Technology—KTH S-100 44 Stockholm Sweden

The characteristics of a project course (160 h of work for the students) oriented towards graphical interaction is described. Project proposals are presented to the students by researchers from different applications. The students get a high degree of freedom during the project and partly as a result of this they are enthusiastic about the work and produce very impressive prototypes which they present at the end of the course. Copyright (C) 1996 Elsevier science Ltd

关键词： Interactive computer graphics

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Closing the loop: Detection and pursuit of a moving object by a moving observer

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IMAGE AND VISION computing 1996年第4期14卷 265-275页

作者： Nordlund, P Uhlin, T Computational Vision and Active Perception Laboratory (CVAP) Department of Numerical Analysis and Computing Science Royal Institute of Technology S-100 44 Stockholm Sweden

We present an integrated system which is able to pursue a moving object and maintain the object centered in the image by controlling a robot-head. The system contains three parts: independent motion detection, tracking, and control of a robot-head. This paper focuses on the detection mechanism, and briefly discusses the tracking and control issues. The system runs continuously in time and updates the object localization at a frame-rate of 25 Hz. The moving object can be tracked, although the observer performs an unknown independent motion, involving both translation and rotation. We focus on a simple motion detection algorithm, since computational cost is of major importance for real-time systems with feedback. The algorithm is noniterative and computationally inexpensive. The running time complexity is O(n) for an image containing n pixels. The image-processing takes place on a MaxVideo200 pipeline processor, and the head control algorithm is running on a Transputer T800 network. Some offline experimental results are presented, where comparisons are made between affine and translational image motion models.

关键词： normal velocity real-time system tracking

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Dynamic fixation and active perception

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INTERNATIONAL JOURNAL OF COMPUTER VISION 1996年第2期17卷 113-135页

作者： Pahlavan, K Uhlin, T Eklundh, JO Computational Vision and Active Perception Laboratory (CVAP) Department of Numerical Analysis and Computing Science Royal Institute of Technology S-100 44 Stockholm Sweden

Fixation is the link between the physical environment and the visual observer, both of which can be dynamic. That is, dynamic fixation serves the task of preserving a reference point in the world, despite relative motion. In this respect, fixation is dynamical in two senses: in response to voluntary changes of fixation point or attentive cues-gaze shiftings, and in response to the desire to compensate for the retinal slip-gaze holding. The work presented here, addresses the vergence movement and preservation of binocular fixation during smooth pursuit. This movement is a crucial component of fixation. The two vergence processes, disparity vergence and accommodative vergence, are described;a novel algorithm for robust disparity vergence and an active approach for blur detection and depth from defocus are presented. The main characteristics of the disparity vergence technique are the simplicity of the algorithm, the influence of both left and right images in the course of fixation and the agreement with the fixation model of primates. The major characteristic of the suggested algorithm for blur detection is its active approach which makes it suitable for achieving qualitative and reasonable depth estimations without unrealistic assumptions about the structures in the images. The paper also covers the integration of the two processes disparity vergence and accommodation vergence which are in turn accomplished by an integration of the disparity and blur stimuli. This integration is accounted for in both static and dynamic experiments.

关键词： Computer vision

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Active fixation for scene exploration

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INTERNATIONAL JOURNAL OF COMPUTER VISION 1996年第2期17卷 137-162页

作者： Brunnstrom, K Eklundh, JO Uhlin, T Computational Vision and Active Perception Laboratory (CVAP) Department of Numerical Analysis and Computing Science KTH (Royal Institute of Technology) S-100 44 Stockholm Sweden

It is well-known that active selection of fixation points in humans is highly context and task dependent. It is therefore likely that successful computational processes for fixation in active vision should be so too. We are considering active fixation in the context of recognition of man-made objects characterized by their shapes. In this situation the qualitative shape and type of observed junctions play an important role. The fixations are driven by a grouping strategy, which forms sets of connected junctions separated from the surrounding at depth discontinuities. We have furthermore developed a methodology for rapid active detection and classification of junctions by selection of fixation points. The approach is based on direct computations from image data and allows integration of stereo and accommodation cues with luminance information. This work form a part of an effort to perform active recognition of generic objects, in the spirit of Malik and Biederman, but on real imagery rather than on line-drawings.

关键词： active vision Active detection Junction Device Component fixation image data Fixation Recognition (Psychology) Accommodation Line drawings Shape Ocular Accommodation

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Machine vision research at CVAP: An introduction

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INTERNATIONAL JOURNAL OF COMPUTER VISION 1996年第2期17卷 107-112页

作者： Eklundh, JO Computational Vision and Active Perception Laboratory (CVAP) Department of Numerical Analysis and Computing Science KTH (Royal Institute of Technology) S-100 44 Stockholm Sweden

This paper deals with the problems on the recovery and shape and motion. It also focuses on what is representing shape. In addition, it touches a briefly on the geometry for computer vision which concerns surface modeling. However, this paper goes beyond the need to model geometry and shape for computer vision, in that is also aims at developing mathematically well-founded computational environments for studying geometry by itself.

关键词： Computer vision

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Projectively invariant decomposition and recognition of planar shapes

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INTERNATIONAL JOURNAL OF COMPUTER VISION 1996年第2期17卷 193-209页

作者： Carlsson, S Computational Vision and Active Perception Laboratory (CVAP) Department of Numerical Analysis and Computing Science KTH (Royal Institute of Technology) S-100 44 Stockholm Sweden

An algorithm is presented for computing a decomposition of planar shapes into convex subparts represented by ellipses, The method is invariant to projective transformations of the shape, and thus the conic primitives can be used for matching and definition of invariants in the same way as points and lines. The method works for arbitrary planar shapes admitting at least four distinct tangents and it is based on finding ellipses with four points of contact to the given shape. The cross ratio computed from the four points on the ellipse can then be used as a projectively invariant index. It is demonstrated that a given shape has a unique parameter-free decomposition into a finite set of ellipses with unit cross ratio. For a given shape, each pair of ellipses can be used to compute two independent projective invariants. The set of invariants computed for each ellipse pair can be used as indexes to a hash table from which model hypothesis can be generated Examples of shape decomposition and recognition are given for synthetic shapes and shapes extracted from grey level images of real objects using edge detection.

关键词： Computer vision

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Direct computation of shape cues using scale-adapted spatial derivative operators

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INTERNATIONAL JOURNAL OF COMPUTER VISION 1996年第2期17卷 163-191页

作者： Garding, J Lindeberg, T Computational Vision and Active Perception Laboratory (CVAP) Department of Numerical Analysis and Computing Science KTH (Royal Institute of Technology) S-100 44 Stockholm Sweden

This paper addresses the problem of computing cues to the three-dimensional structure of surfaces in the world directly from the local structure of the brightness pattern of either a single monocular image or a binocular image pair. It is shown that starting from Gaussian derivatives of order up to two at a range of scales in scale-space, local estimates of (i) surface orientation from monocular texture foreshortening, (ii) surface orientation from monocular texture gradients, and (iii) surface orientation from the binocular disparity gradient can be computed without iteration or search, and by using essentially the same basic mechanism. The methodology is based on a multi-scale descriptor of image structure called the windowed second moment matrix, which is computed with adaptive selection of both scale levels and spatial positions. Notably, this descriptor comprises two scale parameters;a local scale parameter describing the amount of smoothing used in derivative computations, and an integration scale parameter determining over how large a region in space the statistics of regional descriptors is accumulated. Experimental results for both synthetic and natural images are presented, and the relation with models of biological vision is briefly discussed.

关键词： Computer vision

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Hypothesis testing in perfect phylogeny for a bounded number of characters 13th

Hypothesis testing in perfect phylogeny for a bounded number...

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13th Annual Symposium on Theoretical Aspects of Computer science, STACS 1996

作者： Lagergren, Jens Department of Numerical Analysis and Computing Science The Royal Institute of Technology StockholmS-100 44 Sweden

ISBN: (纸本)9783540609223

We introduce the hypothesis testing problem (HTP). In HTP the input is a family of species F and a hypothesis, i.e., a tree where the leaves are labeled with species from some subfamily of F. The problem is to decide whether there is a perfect phylogeny for F which agrees with the hypothesis. We show that HTP can be solved in O(m 2 r m |F|(|F|+mr)) time, where m is the number of characters and r is the maximum number of states on any character. We obtain an O(m 3 r m+1 + |F|m 2) algorithm for the perfect phylogeny problem (PPP), as well. The fastest previously known algorithm for PPP, with fixed m, has running time O(m m+1 r m+1 + |F|m 2) [10]. We also consider several variations of HTP which we either show to be solvable in polynomial time or NP-complete. © 1996, Springer Verlag. All rights reserved.

关键词： Biology

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A computational model of depth-based attention

A computational model of depth-based attention

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13th International Conference on Pattern Recognition, ICPR 1996

作者： Maki, Atsuto Nordlund, Peter Eklundh, Jan-Olof Department of Numerical Analysis and Computing Science Royal Institute of Technology S-100 44 Stockholm Sweden

ISBN: (纸本)081867282X

We present a computational model for attention. It consists of an early parallel stage with preattentive cues followed by a later serial stage, where the cues are integrated. We base the model on disparity image flow and motion. As one of the several possibilities we choose a depth-based criterion to integrate these cues, in such a way that the attention is maintained to the closest moving object. We demonstrate the technique by experiments in which a moving observer selectively mask our different moving objects in real scenes. © 1996 IEEE.

关键词： Computation theory

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