Volume 07 (1988)
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Item EUROGRAPHICS General Assembly(Blackwell Publishing Ltd and the Eurographics Association, 1988)Item Pseudo Ordering of CSG-Trees(Eurographics Association, 1988) Cottingham, Marion S.Using Constructive Solid Geometry (CSG) methods, it is usual for primitive object representations to be stored at the leaf nodes of binary trees. The major part of the work involved in generating an image of the object is finding what surface is visible at each pixel in the screen. Using conventional rendering methods this can be simplified by ordering the primitives by their screen positions and by their depths. Using ray tracing techniques this can be achieved by testing if rays intersect with primitives, the number of these intersection tests can be reduced by ordering. However it is not generally possible to order data (in any one direction) in CSG-trees where intersection or difference operators are involved. This paper describes a method that enables 'local' three-way ordering of the data contained in CSG-trees that can be used with either conventional scan-line rendering methods or ray tracing techniques. This is achieved by the introduction of underlying data structures that dynamically change throughout the image generation step. Using this method, the primitive/polygon visible at a particular pixel can usually be accessed directly via pointers.Item Report on the Third Eurographics Workshop on Graphics Hardware(Blackwell Publishing Ltd and the Eurographics Association, 1988) Blake, E.H.Item Experience with Chisl, a Configurable Hierarchical Interface Specification Language(Blackwell Publishing Ltd and the Eurographics Association, 1988) Wood, C.A.; Gray, P.D.; Kilgour, A.C.This paper describes the design, implementation and initial evaluation of a graphical dialogue specification language and its interpreter. Features of the system include a hierarchical dialogue structure with the capability to export options to lower levels of the hierarchy, the use of a restricted form of production rule approach to option selection in each unit of the dialogue, and suspended time editing (allowing modification of the dialogue specification during execution without loss of context).Item Form-Factors for General Environments(Eurographics Association, 1988) Shao, Ping-Pine; Peng, Qun-Sheng; Liang, You-DoneA new algorithm, based on the hemi-cube formulation, which calculates the form-factors required by the solution of the rendering equation, is presented. The concept of form-factors of the standard radiosity method is developed, In particular, the concept of specular form-factors is proposed. These new form-factors are also purely geometric terms describing the transfer of energy from one surface to another within a non-diffuse environment. The new form-factor is evaluated by numerical integrations based on the hemi-cube algorithm. The equations of the effective diffuse radiosity are presented and solved by numerical methods. The extension fully develops the characteristic of radiosity method and successfully solves the view-independent energy transfer in a general environment.Item Eurographics Workshop on Conformance Testing of the Computer Graphics Interface (CGI)(Blackwell Publishing Ltd and the Eurographics Association, 1988)Item GRAFLOG: Understanding Drawings through Natural Language(Blackwell Publishing Ltd and the Eurographics Association, 1988) Pineda, Luis A.; Klein, Ewan; Lee, JohnThis paper describes an experimental interactive graphics interface, GRAFLOG, in which drawings receive linguistic interpretations. It is possible to emulate linguistic interaction in situations where graphics is thought to be necessary. The paper presents examples of such a kind of dialogue and the architecture of the implementation. The paper explains how representations of drawings can be constructed by treating graphical symbols as"objects", and how a parallel linguistic interpretation for these symbols can be constructed. It highlights the relevance of"deictic expressions" and"spatial prepositions" in building the interface mechanisms between these two kinds of representations. Lastly, it shows how a reasoning component is constructed for making deductions from premises that are found in both the graphical and linguistic domains. Using GRAFLOG, it is possible to represent knowledge through words and pictures. GRAFLOG is implemented, using an object oriented programming style, in PROLOG and GKS.Item GKS Binding to Occam 2(Blackwell Publishing Ltd and the Eurographics Association, 1988)Item Theoretical Developments on Polygonal Approximation of Parametric Surfaces for Ray Tracing(Blackwell Publishing Ltd and the Eurographics Association, 1988) Bouatouch, KadiSome theoretical extensions are brought to Koparkar and Mudur s method which deals with a polygonal approximation of parametric surfaces using potential extrema. The proposed extensions allow the determination of both the existence and the equation of a curve solution of potential extrema. Solutions are given to solve the crack problem and to avoid the artificats due to an inexact ray-surface intersection point near the silhouette or on the higher curvature regions. Moreover, two methods of ray tracing surfaces are proposed.Item Planar Convex Hull Algorithms in Theory and Practice(Blackwell Publishing Ltd and the Eurographics Association, 1988) Day, A. M.Sequential and parallel planar convex hull algorithms, their applications and some of the problems encountered on implementations are described. Details of Pascal implementations are given for three of the sequential algorithms: Graham s, Floyd-Eddy and the Approximation method. The programs are compared experimentally.Item A Simple Spectral Approach to Stochastic Modelling for Natural Objects(Eurographics Association, 1988) Anjyo, Ken-ichiStochastic modeling has been widely used in computer graphics to depict natural objects or phenomena. Various techniques are available, depending on what object is to be represented and what extent of realism to be achieved. This paper presents a simple approach to stochastic spectral synthesis for producing a large range of natural objects and scenes at low computational cost. By considering a wide class of power spectra, the method can provide twodimensional stochastic models for various objects, involving terrains, clouds and sea waves. Moreover, some functional operations and parameters for the models are introduced, which can make the models more flexible in describing a three-dimensional complex scene.Item An Expert System for Polyhedra Modelling(Eurographics Association, 1988) Martin, Philippe; Martin, DominiqueVery often we wish to construct shapes according to some criteria, which are properties we want the object to possess. In this case explicit construction becomes an inadapted way of working. One would wish to obtain the result just by giving the desired properties. This is the approach we try to develop here, limiting the problem, for a start, to a special class of solids: the polyhedra. The aim of this study is to demonstrate the general principles which lead to a definition of solids through their properties, and not through an explicit construction. This leads us to the idea that giving a set of properties is a means to model solids, which allows us to talk about declarative modelling.Item Eurographics Competition Crossword The Results(Blackwell Publishing Ltd and the Eurographics Association, 1988)Item Theory and Practice of Geometric Modeling Cal for Papers(Blackwell Publishing Ltd and the Eurographics Association, 1988)Item Liberation from Rectangles: A Tiling Method for Dynamic Modification of Objects on Raster Displays(Eurographics Association, 1988) Slater, Mel; Davison, Allan; Smith, MarkWhen graphics objects (also called segments) are used on raster display hardware, problems arise because of the mismatch between the high level requirements of object manipulation, and the low level representation from which the image is refreshed. This paper describes a tiling algorithm which provides a solution to the problems of damage repair and hit detection. In particular methods for handling object priority are discussed and experimental results showing the performance of the algorithm for a number of cases are presented.Item A Evalution of Some Three-Color Tiling Patterns(Eurographics Association, 1988) Alt, Paul; Cordonnier, VincentSome new technologies for information display use discrete arrays of cells or picture elements. Each cell displays a single color and the colors do not overlap as they do on a CRT screen. Colored cells are distributed on the two dimension display surface by repeating a basic model called a pattern. Patterns can be evaluated by observing images displayed using them. However, such approaches usually do not give numerical results and depend strongly on the selected images and the observers. In this paper, we suggest and examine some pattern evaluation criteria which relate only to the pattern and its geometrical aspect: Fidelity of color location, the quality of white area and the fidelity of shapes used for lines drawing or characters. Numerical and geometrical approaches are proposed to evaluate the quality of several patterns. The result is a classification of the test patterns. Some of them seem to be better than frequently used patterns. In addition, comments are offered on future pattern design.Item P.A. Firby and D.J. Stone"Interference in Computer Graphics"(Blackwell Publishing Ltd and the Eurographics Association, 1988)Item The Use of Digital Signal Processors in Computer Graphics(Blackwell Publishing Ltd and the Eurographics Association, 1988) Bland, D.; Bonnet, T.The geometric processing stage in a computer graphic system is generally realised in VLSI or dedicated hardware, this paper describes the use of a Digital Signal Processor (DSP). By using a programmable DSP the hardware design effort is displaced into software. Using a single DSP in a co-processor configuration the speed of the vertex processing stage within the geometric processor is increased by a factor of five compared to a high performance microprocessor. The use of DSPs in a multi-processor system is also discussed.Item The Calculus of the Non-Exact Perspective Projection - Scene-Shifting for Computer Animation(Eurographics Association, 1988) Hofmann, Georg RainerThis paper shows the principle way to apply the, scene-shiftin technique of the classical film to computer animation. calculus is presented which is a modification of the well-known classical calculus of the perspective projection. However,, a given perspective view (which may be a frame in a computer animated film) of a three-dunensiona! scene does not change homogenous1 when the position of the viewer (i.e. the eyepoint, the location of the camera ... ) changes. So the subject of this paper is to ask for these parts of the frame with only little changes within a tolerance E. Based on the theorems of the presented calculus, a computer animated film as a sequence of single frames may no longer be computed frame by frame, one frame independent from another, and every frame passing through the whole visualization pipeline of the rendering system. In any frame, parts of the preceeding frame may be inserted, if necessary with two-dimensional modifications: scaling, shifting. This will decrease the computing costs for that frame significantly, since for these arts of the frame no new perspective transformahon and rendering is required. It is a shortening of the visualization pipeline. For the sake of the compactness of this paper we will give no detailed proofs of the theorems presented in here. The interested readers may obtain these information directly from the author.Item A Compositional Semantics for Graphics(Eurographics Association, 1988) Pineda, Luis A.In this paper a theory for developing "intelligent" interactive graphic systems is detailed. The Fregean compositionality principle is enunciated for graphical representations. Geometrical symbols and relations receive semantic interpretations which are expressed as first order relations in the first order logical language. These interpretations are introduced with the help of deictic expressions. Deictic expressions constitute one associative mechanism between analogical representational systems used in graphics, and functional representational systems commonly used in AI applications. Interpretations of graphical symbols and geometrical relations between them constitute an ontology upon which complex linguistic interpretations are assigned to graphics. A concept of graphical grammar is introduced. Frege's compositionality principle, and the notion of graphical grammar lead to a concept of meaningful drawing. The graphical grammar constitutes a second associative mechanism between the two representational systems that have been mentioned. The truth conditions for relations in the graphical grammar are computed through geometrical knowledge. Computational geometry algorithms are associated with the high level representational system by means of the graphical grammar. Semantic interpretations of graphics are useful in carrying out natural language-like dialogue about graphical representations. Dialogues refer to true facts in particular interactive states, A notion of interactive state as a function of time and situation is then developed. One example of how this theory can be used in linking interactive graphics with AI applications is given. In the example, the semantic interpretation of a geographical map is constructed. This theory has been tested with an experimental program called GRAFLOG. The program is implemented in PROLOG and GKS.