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# Volume 12 (1993)

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Item The Potential Equation and Importance in Illumination Computations(Blackwell Science Ltd and the Eurographics Association, 1993) Pattanaik, S. N.; Mudur, S. P.Show more An equation adjoint to the luminance equation for describing the global illumination can be formulated using the notion of a surface potential to illuminate the region of interest. This adjoint equation which we shall call as the potential equation, is fundamental to the adjoint radiosity equation used to devise the importance driven radiosity algorithm. In this paper we first briefly derive the adjoint system of integral equations and then show that the adjoint linear equations used in the above algorithm are basically discrete formulations of the same. We also show that the importance entity of the linear equations is basically the potential function integrated over a patch. Further we prove that the linear operators in the two equations are indeed transposes of each other.Show more Item Graphic Objects: A Mathematical Abstract Model for Computer Graphics(Blackwell Science Ltd and the Eurographics Association, 1993) Torres, J.C.; Clares, B.Show more This paper defines an abstract mathematical model for graphic systems which incorporates modelling operations. The model is based on the"graphic object" concept. A graphic object is an abstraction of a graphic entity. Graphic objects are a generalization of the Fiume graphic object concept, supporting modelling operations, which give the set of graphic objects an algebraic structure of vector space and booelan algebra. Fiume visualization formalism can be applied to graphic objects. Further, product operations of graphic objects are defined, which are a generalization of solid sweep operations. Graphic object products may be used to build common 2D and 3D objects such as triangles, squares, cubes, circles and spheres from lines and arcs.Show more Item GKS-9x: Some Implementation Considerations(Blackwell Science Ltd and the Eurographics Association, 1993) Damnjanovic, L.B.; Duce, D.A.; Robinson, S.K.Show more The Graphical Kernel System (GKS) was published as an ISO standard for computer graphics programming in August 1985. GKS is now undergoing revision in ISO/IEC and at the time of writing the text of the Draft International Standard of GKS-9x was being finalized. This paper presents a way in which a key part of the new functionality in GKS-9x, namely namesets and selection criteria, can be implemented effectively.Show more Item The Determination of Form-Factors by Lookup Table(Blackwell Science Ltd and the Eurographics Association, 1993) Sun, Jizhou; Zou, L.Q.; Grimsdale, R.L.Show more Current methods for computer image synthesis using the radiosity algorithm rely heavily on the calculation of geometrical properties, known as form-factors, which quantify the energy exchange between pairs of surfaces in the environment. In this paper, a new method for form-factor calculation is presented, in which form-factors from a differential area to a finite area with various geometric orientations and dimensions can be determined by the summation of three weighted directional form-factors. The three directional form-factors are mutually independent and can be pre-calculated and stored in a lookup table accessed by two indices. Thus, the determination of the generally used complex form-factor integral is greatly simplified and mainly replaced by accessing a pre-established lookup table. Quantitative comparisons of Wallace s ray casting method and the method presented indicate that the new method can provide more accurate form-factors, which implies that the number of sample points necessary on each source patch is comparatively reduced and the process of form-factor calculation can be accelerated.Show more Item A New Paradigm for Geometric Processing(Blackwell Science Ltd and the Eurographics Association, 1993) Yamaguchi, Fujio; Niizeki, MasatoshiShow more A new paradigm for geometric processing is proposed. The paradigm can be featured by the four key phrases: (1) Totally Four Dimensional Homogeneous Processing, (2) 4 ? 4 Matrix Method and 4 ? 4 Determinant Method, (3) Integer Arithmetic with Adaptive Data-Length Control Technique, (4) Systematic Paradigm Structures Based on the Duality.Show more Item Fast Spheres(Blackwell Science Ltd and the Eurographics Association, 1993) Patterson, John W.Show more A new method for generating sphere-like images, using parabolic surfaces delimited by Bresenham s circle- generation algorithm, is presented. In many cases the resultant images are indistinguishable from spheres illuminated from a given single direction. The main form of the algorithm uses first and second integer differences to minimise computation and uses typically not more than one floating-point calculation per sphere. Two variants of the algorithm are given, one optimized for the special case of the light-source being behind the view-point, and one in which values are calculated for a z-buffer hidden-surface algorithm at the same time as the pixel values. The difference formulae can be exploited by differencing hardware or digital signal processors although very little arithmetic, typically five low-weight integer operations, including address calculation operations, is required per pixel on a conventional architecture.Show more Item Quality Control of an Interpolation Method for Discontinuous Parametric Surfaces(Blackwell Science Ltd and the Eurographics Association, 1993) Seron, F.J.; Torrens, J.J; Magallon, J.A.Show more In this paper we study a finite element interpolation method for fitting discontinuous parametric surfaces when the data points are the nodes of a curvilinear grid. Quality control of the interpolating surfaces is also considered, focussing on the display of isophotes and reflection lines using ray tracing techniques. Finally, graphical and numerical examples are given.Show more Item Modeling of Skylight and Rendering of Outdoor Scenes(Blackwell Science Ltd and the Eurographics Association, 1993) Tadamura, Katsumi; Nakamae, Eihachiro; Kaneda, Kazufumi; Baba, Masashi; Yamashita, Hideo and Nishita, TomoyukiShow more Photorealistic animated images are extremely effective for pre-evaluating visual impact of city renewal and construction of tall buildings. In order to generate a photorealistic image not only the direct sunlight but also skylight must be considered.This paper proposes a method of high-fidelity image generation for photorealistic outdoor scenes based on the following ideas: * 1The intensity distribution of skylight taking account of scattering and absorption due to particles in the atmosphere which coincides with CIE standard skylight luminance functions is sought, and realistic images considering about spectrum distribution of skylight for any altitude of the sun can be easily and accurately displayed. * 2A rectangular parallelepiped with a specialized distribution of intensity simulating the skylight is introduced for efficient calculation of illumination due to skylight, and by employing a graphics hardware calculation of the skylight illuminance taking into account shadow effects is obtained with high efficiency- these techniques can be used to generate sequences of images, making animations possible at far lower calculation cost than previous methods.Show more Item A Multiprocessor Implementation of Radiosity(Blackwell Science Ltd and the Eurographics Association, 1993) Ng, Adelene; Slater, MelShow more This paper describes a multi-processor implementation of form factor computation in the radiosity method. Form factors are computed using the ray casting method and the algorithm is enhanced with Binary Space Partition (BSP) Trees and bounding boxes. Experimental results are presented together with a discussion of load balance efficiency.Show more Item The ARGOSI Classification Scheme for Graphics and Networking Applications(Blackwell Science Ltd and the Eurographics Association, 1993) Sutcliffe, D.C.; Gallop, J.R.; Maybury, R.; Day, R.A.; Duce?, D.A.; Reynolds?, G.J.Show more ARGOSI is a project concerned with the relationship between standards for graphics and networking, as rejected in its full title: Applications Related Graphics and OSI Standards Integration. It is a collaborative project between eleven European organizations and is funded under the Esprit-Il programme of the Commission of the European Community.In order to achieve integration of graphics and OSI standards and to meet the requirements of applications, it is necessary to understand the needs of applications to transfer graphical information and the networking services required to support this. A major part of the ARGOSI project has been to derive a classification framework for applications based on a set of metrics describing their use of graphics and networking. Once classified, a study is to be made to determine the set of graphics and OSI standards that will satisfy the requirements of each group of applications or to identify deficiencies or omissions in the set of current standards that prevent the requirements being met. The classification scheme is believed to be a major stepforward in the field.Show more Item 3D Computational Morphology(Blackwell Science Ltd and the Eurographics Association, 1993) Veltkamp, Remco C.Show more Computational Morphology is the analysis of form by computational means. This discipline typically uses techniques from Computational Geometry and Computer Aided Geometric Design. The present paper is more specifically about the construction and manipulation of closed object boundaries through a set of scattered points in 3D. Original results are developed in three stages of computational morphology: * impose a geometrical structure on the set of points- * construct a polyhedral boundary surface from this geometrical structure- * build a hierarchy of polyhedral approximations together with localization information-The economic advantage of this approach is that there is no dependency on any specific data source. It can be used for various types of data sources or when the source is unknown.Show more Item ANIMA: An Interactive Tool for Scientific Data Animation(Blackwell Science Ltd and the Eurographics Association, 1993) Moltedo, Laura; Morigi, SerenaShow more This paper describes an animation interactive system that produces image sequences used to study the numerical results of scientific simulations. The fundamental goal of the system is to provide an easy method for observing and exploring scientific computation during its space-time evolution. To do that it is necessary to combine simulation processes, algorithmic and key frame animation with geometric transformations, light effects and camera movements. ANIMA allows the researcher to use animation without special programming, with the aid of a graphic user interface. Modelling, rendering, motion control and synchronization of graphic objects are interactively specified by the animator in order to define an animation script. The system development methodology follows an object-oriented technique using modular programming, abstract data type definition, and graphic metaphor with user interface menus.Show more Item Pictorial Editing by Shape Matching Techniques(Blackwell Science Ltd and the Eurographics Association, 1993) Ventura, A. Della; Ongaro, P.; Schettini, R.Show more A number ofpainting and retouching packages operate much like the artist s traditional canvas andpalette, with electronic toolsfunctioning like their studio counterparts. These programs, while offering an ample set of tools for creating an image, suffer from an intrinsic limitation as regards its modification, which lies in the fact that changes to the shape of objects and those requiring some kind ofpattern recognition are generally difficult and cannot be accomplished automatically. This paper deals with an original methodfor providing a pictorial editor with the"search-and-replace" facility, that works rather like text substitution in a word processor. The user defines the search (model) and the replacement (target) patterns by example, i.e. by showing the system the patterns taken from the image or an existing catalogue. The editor then searchesfor objects that match the model s pattern and replaces them with the target one in an automatic or user-controlled mode. The method is based on a model-driven matching technique, capable of measuring the similarity of objects that are partially occluded or transformed by translation, rotation, or change of scale. Salient features of the method are its robustness and the limited number of parameters needed to adapt the search procedure to different application contexts. A search-and-replace function used for pattern-editing in the field of textile design, is presented as a working example.Show more Item Photo-Realistic Imaging of Digital Terrains(Blackwell Science Ltd and the Eurographics Association, 1993) Cohen, Daniel; Shaked, AmitShow more We present a method for the generation of photo-realistic images of views over terrain datasets by mapping a digital aerial photograph on a perspective projection of a digital elevation map. We use high resolution for both digital maps to increase the quality and the realism of the image at the cost of the overhead of processing very large data bases. In the core of this paper we present an accelerated ray casting technique based on a new algorithm of traversing a pyramidal data structure. Unlike other known traversal techniques, the cost of a single step of the algorithm consists of a few additions, shifts and comparisons only.Show more Item Variational Surface Design and Surface Interrogation(Blackwell Science Ltd and the Eurographics Association, 1993) Hagen, H.; Hahmann, St.; Bonneau, G.-P.Show more The generation of technical smooth surfaces from a mesh of three-dimensional data points is an important problem in geometric modelling. In this publication we give a survey of some new techniques based on a calculus of variation approach. Apart from the pure construction of these surfaces, the analysis of their quality is equally important in the design and manufacturing process. Generalized focal surfaces are presented here as a new surface interrogation tool.Show more Item Extracting contour lines from a hierarchical surface model(Blackwell Science Ltd and the Eurographics Association, 1993) De Floriani, Leila; Mirra, Daniela; Puppo, EnricoShow more The Hierarchical Triangulated Irregular Network (HTIN) is a structure for representing 2?-dimensional surfaces at different levels of detail through piecewise-linear approximations based on triangulations of the surface domain. In this paper, we present two algorithms that allow extracting a representation of the surface and contour lines at a given level of detail, directly from the HTIN.Show more Item Landscapes Synthesis Achieved through Erosion and Deposition Process Simulation(Blackwell Science Ltd and the Eurographics Association, 1993) Roudier, P.; Peroche, B.; Perrin, M.Show more This paper describes an original approach to terrain evolution in landscapes synthesis. In order to create some realistic landforms, we simulate geologically contrasted terrains and apply to them deterministic erosion processes. This allows us to relate the erosion on any point of the landsurface to local geological parameters. Any height field may be chosen as an initial topographic surface. Small perturbations may be introduced to avoid unpleasant regularities. A 3D model defines the geological parameters of each point according to its elevation. Our method is iterative: at each step, rock removal and possible alluvial deposition are computed at each point of the landsurface. The available erosion laws simulate mechanical erosion, chemical dissolution and alluvial deposition. At the end of each iteration, a new landsurface and the corresponding river network are created. Landsurfaces can be visualized at the final stage by two rendering algorithms including natural textures mapping. The stream network and the ridges may also be visualized.Show more Item The Generation of Straight Lines on Hexagonal Grids(Blackwell Science Ltd and the Eurographics Association, 1993) Yong-Kui, LiuShow more The best disposition of a discrete set of points on the plane can be reached if the points are on a hexagonal grid. This paper describes an algorithm for the generation of straight lines on hexagonal grids. It uses only integer arithmetic.Show more Item Storage Support for Multidimensional Discrete Data in Multimedia Databases(Blackwell Science Ltd and the Eurographics Association, 1993) Furtado, Paula; Teixeira, Jose CarlosShow more Multidimensional discrete data (MDD) is enormously important for multimedia applications since diverse types of such data are used simultaneously by any multimedia application. However, multidimensional discrete data is not adequately supported by most existing database systems.In this paper we discuss the major management requirements imposed by such data on multimedia database management systems both at the conceptual level and at the internal level. At the internal level, the spatial structure of the data must also be preserved and special emphasis must be given to the efficient spatial access to subrectangles of very large data of arbitrary dimensionality.Our major contribution is the presentation of a Storage Manager for this kind of data which preserves the spatial nature of the data at the internal level of the database management system and provides efficient spatial access. This Storage Manager is based on an innovative combination of the subdivision of data in multidimensional tiles and of a spatial index to access those tiles. Furthermore, it supports advanced registration data.The lower level modules of the Storage Manager are responsible for the access to the data stored in secondary storage. The implementation of these modules can rely on any secondary storage access system to access data on disk. The Storage Manager was implemented using the functionality of a relational database system. The implementation of this relational Storage Manager is presented.Show more Item PRIP - A Parallel Raster Image Processor(Blackwell Science Ltd and the Eurographics Association, 1993) Van Campenhout, Jan M.; Lasure, RoelandShow more Like colour video displays and laser printers, laser photoplotters are raster scan devices. For such devices, the pixel stream representing the image must be generated in real time, and in the (scan line) order required. However, the typical size of the images photoplotters produce is an order of magnitude higher than that of video displays and laser printers, precluding the use of full-size bitmap memories. These requirements pose particular implementation problems for the raster image processor generating the pixel stream.The parallel RIP system presented here is aimed at high-resolution laser photoplotters, and features a largely scalable performance ranging from 40 to several hundred megapixels per second. It is built with standard components such as graphics microprocessors and VRAM memories. Its architecture is that of a distributed memory multiprocessor system with a global ring-like topology. And most importantly, it can be programmed using the traditional sequential programming paradigm. Only minor additions are needed to sequential graphical algorithms to be executable on the system with an arbitrary number of processors.A prototype 8-processor PRIP system has been built and tested generating printed circuit board images for a direct imaging photoplotter. The prototype exhibits a near-linear speedup with respect to a monoprocessor solution. Architectural simulations indicate that the system can be expanded to well over 10 processors.Show more