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1997 Technical Reports

CS-97-35
Title Adaptive Protocols for Negotiating Non-Deterministic Choice over Synchronous Channels
Authors Erik D. Demaine
Abstract In this paper, we propose several deadlock-free protocols for implementing the generalized alternative construct, where a process non-deterministically chooses between sending or receiving among various synchronous channels. We consider general many-to-many channels and examine in detail the special case of fan (many-to-one and one-to-many) channels, which are common and can be implemented much more efficiently. We propose a protocol that achieves an optimal number of message cycles per user-level communication, significantly improving on previous results. We propose several other ``less aggressive'' protocols, which may be more suitable for some applications and networks, and demonstrate how to adaptively switch between them and modify protocol parameters. Finally, we show how to maintain dynamic membership of channels, while avoiding race conditions that would prevent garbage collection of processes.
Comments A version of this paper has been submitted to the 1st Merged Symposium of the 12th International Parallel Processing Symposium and 9th Symposium on Parallel and Distributed Processing, Orlando, Florida, March 30-April 3, 1998.
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CS-97-32
Title Penalty Methods for American Options With Stochastic Volatility
Authors R. Zvan, P.A. Forsyth and K.R. Vetzal
Abstract The American early exercise constraint can be viewed as transforming the two dimensional stochastic volatility option pricing PDE into a differential algebraic equation (DAE). Several methods are described for forcing the algebraic constraint by using a penalty source term in the discrete equations. The resulting nonlinear algebraic equations are solved using an approximate Newton iteration. The solution of the Jacobian is obtained using an incomplete LU (ILU) preconditioned PCG method. Some example computations are presented for option pricing problems based on a stochastic volatility model, including an exotic American chooser option written on a put and call with discrete double knockout barriers and discrete dividends.
Date October 1997
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CS-97-31
Title The Suffix-Signature method for Searching for Phrases in Text
Authors M. Zhou
Abstract Finding all occurrences of a given word in a large static text is a well-studied problem. Most solutions, however, are not well-suited for phrase-searching. In this thesis, we investigate a new algorithm to find all occurrences of a given phrase in a large, static text, based on the data structure known as a suffix array. Using this algorithm, phrases of bounded length can be found with expected search time of one disk access to the text and one disk access to an index. To achieve this performance for phrases of up to five words in length requires an index having total size of approximately 120% of the size of the text. The algorithm guarantees a worst case search performance of two disk accesses to the text per phrase search.

The method augments a suffix array with a parallel signature array, so that every indexed phrase has an associated signature. To search for a phrase, we search a block of the index in memory to locate matching signatures. Then we read one or two phrases corresponding to matching signatures from disk and compare them to the target phrase to filter out false matches.

We present theoretical properties of the data structure and algorithm derived from a suitable model. The theoretical results have been validated through experimentation with actual data ranging in size from 6Mb to 550Mb and also including actual query patterns derived from logs of searches on the World Wide Web. These experiments show that the approach is applicable in practice to a variety of texts and realistic phrase searches.
Date October 1997
Comments This report is a reprint of a thesis that embodies the results of research done in partial fulfillment of the requirements for the degree of Doctor of Philosophy of Mathematics in Computer Science at the University of Waterloo.
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CS-97-30
Title The Use of 3D Sound as a Navigational Aid in Virtual Environments
Authors R. Guenther
Abstract Because virtual environments are less visually rich than real-world environments, careful consideration must be given to their design, otherwise much of the potential for virtual reality will be lost. One important design criteria is to make certain that adequate navigational cues are incorporated into a complex virtual world. The usability of virtual environments will be decreased if users often become lost or disoriented, as is frequently the case now.

This thesis investigates the use of spatialized sound as a navigational aid for people using virtual reality systems. An experiment was performed to determine if incorporating spatialized sound into a virtual environment a) helps people find specific objects in the environment, and b) in uences the rate at which people acquire spatial knowledge.

The results show that the addition of spatialized sound to a virtual environment improved the ability of people to find objects, although the addition of sound did not increase the amount of configurational knowledge a person was able to acquire. In fact, it appears that the addition of sound may have suppressed the development of configurational knowledge.
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CS-97-29
Title A Parametric Hybrid Triangular Bezier Patch
Authors M.F. Davidchuk
Abstract The triangular Bezier patch presents a natural primitive for modelling surfaces of arbitrary topology, but does not enjoy the same widespread use as the tensor product patch. Existing triangular Bezier patch interpolation schemes that are designed to fit surfaces to parametric data produce interpolants that suffer from noticeable visual flaws. Patch boundaries are often visible as creases in the resulting surface.

Interpolation of functional data is simpler than interpolation of parametric data. However schemes such as the Clough-Tocher technique have many of the same visual flaws as parametric schemes. Foley and Opitz introduced an improvement over Clough-Tocher. Central to the Foley-Opitz scheme is the cross boundary construction that produces quadratically varying cross boundary derivatives.

My work involves attempting to extend Foley's scheme to the parametric setting. Modifying this scheme requires the formulation of an extension to the standard rational blend technique. In addition to blending interior control points, boundary control points must also be blended to incorporate Foley's cross boundary construction, which relies on the natural parameterization of the functional setting.

When interpolating irregularly scattered data and when increasing the tessellation of the data mesh, the new scheme shows improvement over representative parametric data fitting schemes. The quality of the resulting surfaces depended largely on the correlation between the normals and the triangles of the underlying mesh.
Date September 1997
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CS-97-28
Title A Technique for Finding and Verifying Speed-Dependences in Gate Circuits
Authors R. Negulescu
Abstract Before sizing the delays of the components in a circuit, it is often necessary to find delay constraints that would ensure the correctness of the circuit, and to verify the sufficiency of these constraints. We formalize constraints of a certain type as processes in a metric-free model. The circuit specification and components are also represented as processes, to be coupled and compared with the constraint processes. We use a BDD-based tool to verify a circuit together with a set of constraints, and to find hints as to which constraints are needed. We illustrate this technique on a circuit that uses extended isochronic forks.
Date July 1997
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CS-97-27
Title PDE Methods for Pricing Barrier Options
Authors R. Zvan, K.R. Vetzal and P.A. Forsyth
Abstract The paper presents an implicit method for solving PDE models of contingent claims proces with general algebraic constraints on the solution. Examples of constraints include barriers and early exercise features. In this unified framework, barrier options with or without American-style features can be handled in the same way. Either contiuously or discretely monitored barriers can be accommodated, as can time-varying barriers. The underlying asset may pay out either a constant dividend yield or a discrete dollar dividend. The use of the implicit method leads to convergence in fewer time steps compared to explicit schemes. This paper also discusses extending the basic methodology to the valuation of two asset barrier options and the incorporation of automatic time stepping.
Date July 1997
Report PDE Methods for Pricing Barrier Options (PDF) Compressed PostScript:
PDE Methods for Pricing Barrier Options (PS.Z)
CS-97-26
Title A Structured Text ADT for Object-Relational Databases
Authors L.J. Brown, M.P.Consens, I.J. Davis, C.R. Palmer and F.W. Tompa
Abstract There is a growing need, both for use within corporate intranets and within the rapidly evolving World Wide Web, to develop tools that are able to retrieve relevant textual information rapidly, to present textual information in a meaningful way, and to integrate textual information with related data retrieved from other sources.

This paper introduces a model for structured text and presents a small set of operations that may be applied against this model. Using these operations structured text may be selected, marked, fragmented, and transformed into relations for use in relational and object oriented database systems.

The extended functionality has been accepted for inclusion within the SQL/MM standard, and a prototype database engine that supports SQL with extensions to incorporate the proposed text operations has been implemented. This prototype serves as a proof of concept intended to address industrial concerns, and it demonstrates the power of the proposed abstract data type for structured text.
Date July 1997
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A Structured Text ADT for Object-Relational Databases (PS.Z)
CS-97-24
Title Preconditioning Methods for Very Ill-conditioned Three-dimensional Linear Elasticity Problems
Authors E. Graham and P.A. Forsyth
Abstract Finite element models of linear elasticity arise in many application areas of structural analysis. Solving the resulting system of equations accounts for a large portion of the total cost for large, three-dimensional models, for which direct methods can be prohibitively expensive. Preconditioned conjugate gradient (PCG) methods are used to solve difficult problems with small $(\ll 1)$ average element aspect ratios. Incomplete LU (ILU) factorisations based on a drop tolerance parameter are used to form the preconditioning matrices. Various new techniques known as reduction techniques are examined. Combinations of these reduction techniques result in highly effective preconditioners for problems with very poor aspect ratios. Standard and hierarchical triquadratic basis functions are used on hexahedral elements, and test problems comprising a variety of geometries with up to 50,000 degrees of freedom are considered. Manteuffel's method of perturbing the stiffness matrix to ensure positive pivots occur during factorisation is used, and its effects on the convergence of the preconditioned system are discussed.
Date July 1997
Comments We have submitted the paper (25 pages) to the International Journal for Numerical Methods in Engineering.
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CS-97-23
Title A Finite Element Approach to the Pricing of Discrete Lookbacks with Stochastic Volatility
Authors P.A. Forsyth, K. Vetzal and R. Zvan
Abstract Finite element methods are described for valuing lookback options under stochastic volatility. Particular attention is paid to the method for handling the boundary equations. For some boundaries, the equations reduce to first order hyperbolic equations which must be discretized to ensure that outgoing waves are correctly modelled. Some example computations show that for certain choices of parameters, the option price computed for a lookback under stochastic volatility can differ from the price under the usual constant volatility assumption by as much as 35\% (i.e. \$7.30 compared with \$5.45 for an at-the-money put), even though the models are calibrated so as to produce exactly the same price for an at-the-money vanilla European option with the same time remaining until expiry.
Date July 1997
Comments Submitted to: Appl. Math Finance (July, 1997)
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A Finite Element Approach to the Pricing of Discrete Lookbacks with Stochastic Volatility (GZIP)
CS-97-22
Title Computing Extreme Origami Bases
Authors Erik D. Demaine and Martin L. Demaine
Abstract In this paper, we examine extreme origami bases that fold the boundary of a polygonal sheet of paper to a common plane, generalizing the Husimi and Meguro molecules used in origami design. This also solves the folding-and-cutting problem, where we want to make a specified polygon by one complete straight cut after arbitrary folding of a square sheet of paper. We present an algorithm to construct the crease pattern of an extreme base for paper in the shape of an arbitrary simple polygon. It is based on the straight skeleton, a variant on the medial axis. For convex polygons, we describe exactly how the folding process can be performed. This proves that the folding can even be done with paper that is rigid except at the creases, and allows animation of the folding process. Such descriptions have not been achieved before for an infinite class of origamis.
Date December 1997
Comments We are extending this work to cutting out non-convex polygons and more generally arrangements of line segments.
Report Computing Extreme Origami Bases (PDF) Compressed PostScript:
Computing Extreme Origami Bases (PS.Z)
CS-97-19
Title Using Ray Tracing for Site Specific Indoor Radio Signal Strength Analysis
Authors M. Nidd
Abstract As wireless networks are installed at more locations, it will become more important that they be organised in an efficient layout. This requires prediction tools that use site-specific information to give better results than the traditional statistical models. While counting the number of walls between the transmitter and receiver can provide a rough approximation, more comprehensive techniques can make better predictions.

This report explores ray tracing as a method for predicting signal strength in an indoor wireless radio environment. It presents a dynamic ``cone tracing'' algorithm that offers a reasonable compromise between execution time and accuracy. This technique bypasses the duplication of effort present in similar previous attempts to analyze the signal strength in an entire room, instead of just at a single receiver location. The more complete view gives the planner seemingly continuous information about the region of interest, providing more information about the areas of poor coverage, and allowing for more informed decisions to be made in attempting corrections. This improved quality is achieved without the sacrificing response time.
Date April 1997
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CS-97-18
Title NP-hardness of some map labeling problems
Authors C. Iturriaga and A. Lubiw
Abstract One of the most challenging tasks of cartographic map lettering is the optimal placement of the region information on a map. We show the NP-hardness of two formulations of this task: the Elastic Labelling Problem and Sliding Labels Problem. In the elastic labelling problem we are given a set of Elastic Rectangles as labels, each associated with a point in the plane. An elastic rectangle has a specified area but its width and height may vary. The problem then is to choose the height and width of each label, and the corner of the label to place at the associated point, so that no two labels overlap. This problem is known to be NP-hard even when there is no elasticity (just because of the choice of the corners). We show that the problem remains NP-hard when we have elasticity but no choice about which corner of the label to use — we call this the One-corner Elastic Labelling Problem. In the sliding labels problem each label has fixed dimension but any boundary point (not just a corner) of the label can be placed at the associated point in the plane. This problem is known to be NP-hard. We show that it remains NP-hard when only points on the left or right boundary of the label can be placed at the associated point — we call this the Left-right Sliding Labels Problem. A companion paper will give algorithms for special cases of the elastic labelling problem and the sliding labels problem.
Report NP-hardness of some map labeling problems (PDF) Compressed PostScript:
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CS-97-17
Title Montana Smart pointers, They're Smart and They're Pointers
Authors J. Hamilton
Abstract The Montana C++ programming environment provides an API interface to the compiler, which allows the compilation process to be extended through programmer-supplied tools. This paper investigates the feasibility of that interface, using smart pointers as an example. Smart pointers are a powerful feature of the C++ language that enable a variety of applications, such as garbage collection, persistence, and distributed objects. However, while smart pointers can be used in much the same way as built-in pointers, they are not interchangeable. Using the Montana API, smart pointer functionality can be introduced for built-in pointers, thus enabling built-in pointers that act like smart pointers. We provide an overview of the Montana programming environment and describes how smart pointers can be implemented using the Montana API.
Date July 1997
Comments Submitted to: 1997 USENIX COOTS (accepted)
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CS-97-16
Title Operation Data Storage Unification
Authors J.R. Hamilton
Abstract We see a world where personal system software is self-installing, self-configuring, self-personalizing, self-administering, and self-healing. In this world, personal system data is visible in a single name space regardless of owning application and data location, supports uniform non-procedural query, transactions, and security, is automatically backed up, is recoverable, and supports portable and disconnected operation.

This paper focuses on how database technology can be applied to help achieve many of these goals. We propose an integrated storage manager, the Virtual Object File System (VOFS), which supports structured files, unstructured files, and relational access. VOFS also supports disconnected operations by caching and replicating all data stored via any of the supported persistence APIs. We show how the combination of support for storage Integration and disconnected and portable operation found in VOFS can substantially reduce administrative costs and support more compelling client applications.

The VOFS system can be extended to support alternative caching systems, new access methods based upon past navigational paths and temporal access, and different software and data billing schemes.
Date July 1997
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CS-97-15
Title A Coordinate Free Geometry ADT
Authors S. Mann, N. Litke and A. DeRose
Abstract An algebra for geometric reasoning is developed that is amenable to software implementation. The features of the algebra are chosen to support geometric programming of the variety found in computer graphics and computer aided geometric design applications. The implementation of the algebra in C++ is described, and several examples illustrating the use of this software are given.
Date June 1997
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CS-97-11
Title Delay-Insensitivity and Semi-Modularity
Authors J.A. Brzozowski and H. Zhang
Abstract A network is delay-dense if, of each pair of adjacent components in the network, at least one is a delay. In this paper, we prove that a delay-dense asynchronous network is delay-insensitive if and only if its behaviour is semi-modular. We consider autonomous networks, i.e., networks without external inputs, whose components can be any sequential machines of the Moore type. A formal model for this type of networks and their behaviours is established. We define delay-insensitivity of networks using the notion of bisimulation. The definition of semi-modularity is generalized to reflect non-determinism in network behaviours.

Keywords: asynchronous, bisimulation, delay-dense, delay-insensitive, delay extension, isochronic, module, network, semi-modular, speed-independent
Date March 1997
Comments Also available here.
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Delay-Insensitivity and Semi-Modularity (PS.Z)
  CS-97-04  
Title Planar Mesh Refinement Cannot be Both Local and Regular
Authors J.F. Buss and R.B. Simpson 
Abstract We show that two desirable properties for planar mesh refinement techniques are incompatible. Mesh refinement is a common technique for adaptive error control in generating unstructured planar triangular meshes for piecewise polynomial representations of data. Local refinements are modifications of the mesh that involve a fixed maximum amount of computation independent of the number of triangles in the mesh. Regular meshes are meshes for which every interior vertex has degree 6.

At least for some simple model meshing problems, optimal meshes are known to be regular, hence it would be desirable to have a refinement technique that, if applied to a regular mesh, produced a larger regular mesh. We call such a technique a regular refinement. In this paper, we prove that no refinement technique can be both local and regular. Our results also have implications for non-local refinement techniques such as Delaunay insertion or Rivara's refinement.
Date January 1997
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CS-97-03
Title A Comparison of 2D and 3D Interfaces for Editing Surfaces Reconstructed from Contours
Authors J.F. Waterhouse
Abstract The last decade of computer technology has seen the proliferation of computer graphics applications. As technology advances, there is a growing fascination with three-dimensional (3D) object representations that likely comes from their greater ability to match "real life" than their two-dimensional (2D) counterparts. Unfortunately, the benefits of 3D editing are not without a price. Most techniques for manipulating objects in a 3D environment are developed for conventional hardware configurations that use 2D input devices and CRT displays. The difficulties lie in mapping 3D spatial relationships to 2D displays, and in mapping 2D user input to 3D object manipulation. This mapping problem is somewhat mitigated by adding constraints to the degrees of freedom in the manipulation task. 3D surfaces that have been reconstructed from contours are interesting to consider as targets of 3D interaction because they provide an inherent constraint on manipulation: point motion is restricted to a plane.

As part of my research, I implemented an interactive contour editor to edit 3D surfaces that were reconstructed from planar contours. More precisely, the editor is a tool for visualising a surface derived from a set of serial sections, and for removing deformations from this surface. It was designed specifically to remove artefacts from medical images of arteries.

I used the interface from my editor in an experiment that tested whether users were faster and more accurate at manipulating surfaces in a 2D environment or a 3D environment. At the outset of this study, I predicted that 2D would be better for editing deformations of a 2D nature. That prediction was borne out by my experimental results. I had also hoped that 3D would be superior as an editing environment for correcting deformations of a 3D nature. However, the 2D character of the data had a stronger effect on performance than did the 3D character of the deformation. Despite the inherent constraints in the surfaces, participants were faster at editing in 2D for all types of deformations, while maintaining a consistent accuracy between 2D and 3D. Participants did perceive a 3D environment to be better than a 2D environment for manipulating a group of points that spanned multiple contours, although this was not reflected in the quantitative results. The intuitive preference for 3D in this situation leads me to believe that it is worth continuing the search for a natural and effective interface for editing surfaces in a 3D environment.
Date January 1997
Report A Comparison of 2D and 3D Interfaces for Editing Surfaces Reconstructed from Contours (PDF) Compressed PostScript:
A Comparison of 2D and 3D Interfaces for Editing Surfaces Reconstructed from Contours (PS.Z)
CS-97-02
Title Response Time Properties of Some Asynchronous Circuits
Authors J. Ebergen and R. Berks
Abstract We discuss response time properties of linear arrays of cells with various handshake communication behaviours. The response times of a linear array are the delays between requests and succeeding acknowledgments for the first cell. We derive simple formulas for the worst-case response time and amortized response time of linear arrays using a general variable-delay model, where delays may vary between a lower and upper bound. The properties are independent of any particular implementation of the cells of the network.
Date May 1997
Report Response Time Properties of Some Asynchronous Circuits (PDF) Compressed PostScript:
Response Time Properties of Some Asynchronous Circuits (PS.Z)
CS-97-01
Title Decomposition of Boolean Functions Specified by Cubes
Authors J.A. Brzozowski and T. Luba
Abstract We study the problem of decomposing a Boolean function into a set of functions with fewer arguments. This problem has considerable practical importance in VLSI. For example, for digital circuits designed with field-programmable gate arrays, it is necessary to express Boolean functions in terms of `smaller' functions that fit the cells of the array. The decomposition problem is old, and well understood when the function to be decomposed is specified by a truth table listing the function's minterms, or has one output only. However, modern design tools, such as Berkeley's Espresso, handle functions with many outputs and represent them by Boolean cubes rather than minterms, for reasons of efficiency. In this paper we develop a decomposition theory for multiple-output, partially specified Boolean functions represented by cubes. The theory uses ternary algebra and generalized set systems. KEYWORDS: blanket, Boolean function, cube, decomposition, disjunctive, don't care, multiple-output, set system, ternary algebra, type fr
Date January 1997
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