Volume 11 (1992)

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Accurate Image Generation and Interactive Image Editing with the A-buffer
(Blackwell Science Ltd and the Eurographics Association, 1992) Lau, Wing Hung; Wiseman, Neil
The A-buffer suggested by Carpenter [2] is a hidden surface removal method which deals with arbitrary overlapping edges in a pixel. In this paper, we suggest a way of increasing the subpixel resolution by storing the bitmask index rather than the bitmask. This allows much more accurate images to be generated while at the same time, minimising memory usage. It also allows zooming to reveal more information. We also suggest an enhancement to the A-buffer by allowing the creation of dynamic objects. These dynamic objects can then be edited (deleted, moved, etc.) interactively as image modification and assembly is going on. This new buffer forms part of the image composition system we have been working on.
An adaptive Discretization Method For Radiosity
(Blackwell Science Ltd and the Eurographics Association, 1992) Languenou, Eric; Bouatouch, Kadi; Tellier, Pierre
When using radiosiiy, the visual quality of the rendered images strongly depends on the method employed for discretizing the scene into patches. A too fine discretization may give rise to artifacts, while with a coarse discretization areas with high radiosity gradient may appear. To overcome these problems, the discretization must adapt to the scene. That is, the interaction between two patches must account for the distance between them as well as their surface area. In other words, surfaces far away are discretized less finely than nearby surfaces. These aspects are considered by the new adaptive discretiration method described in this paper. It performs both discretization and system resolution at each iteration of the shooting process, allowing then interactivity.
Adaptive Sampling of Area Light Sources in Ray Tracing Including Diffuse Interreflection
(Blackwell Science Ltd and the Eurographics Association, 1992) Kok, Arjan J. F.; Jansen, Frederik W.
Ray tracing algorithms that sample both the light received directly from light sources and the light received indirectly by diffuse reflection from other patches, can accurately render the global illumination in a scene and can display complex scenes with accurate shadowing. A drawback of these algorithms, however, is the high cost for sampling the direct light which is done by shadow ray testing. Although several strategies are available to reduce the number of shadow rays, still a large number of rays will be needed, in particular to sample large area light sources. An adaptive sampling strategy is proposed that reduces the number of shadow rays by using statistical information from the sampling process and by applying information from a radiosity preprocessing. A further reduction in shadow rays is obtained by applying shadow pattern coherence, i.e. reusing the adaptive sampling pattern for neighboring sampling points.
Affine Texture Mapping and Antialiasing Using Integer Arithmetic
(Blackwell Science Ltd and the Eurographics Association, 1992) Nehlig, P.; Ghazanfarpour, D.
Texture mapping techniques are very useful for generating more realistic images. However, texture compression, generally induced by geometric transformations, is at the origin of aliasing artifacts especially the well-known"moire" patterns. Two discrete affine texture mapping methods based exclusively on integer arithmetic are presented here. This original approach of discrete affine mapping is adequate for antialiasing in the case of compressed textures.
Area Sampling Buffer: Tracing Rays with Z-Buffer Hardware
(Blackwell Science Ltd and the Eurographics Association, 1992) Sung, Kelvin
An algorithm that allows the use of z-buffer hardware in assisting area sampling for a ray tracing style renderer is described. The implementation issues involved in discretizing the sampling area and light source area are discussed and solutions are proposed. The effects of the hardware-assisted ray tracing approach on frame-buffer systems and image synthesis processes are considered.
A CAD System for Color Design of a Car
(Blackwell Science Ltd and the Eurographics Association, 1992) Oshima, Tetsuya; Yuasa, Shinji; Sakanoshita, Ken-ichi; Ogata, Yoshinori
We have developed a color CAD system which enables a color designer to evaluate and create body colors of a car on a graphic display and put this system into practical use. The system has three features- generating realistic images comparable to photography to satisfy the car designer s needs, through close analysis of the physical phenomena taking place in the real environment, reproducing the desired colors very accurately on a CRT monitor, through analysis of the CRT colorimetric characteristics of reproducing the specified colors in response to the input signals, and utilizing a parallel computer to generate realistic images at a high speed and to control colors through interactive operation. Application of the CAD system reported here to the field of color design made it possible to fully evaluate and create body colors by means of computer graphics, replacing the conventional method which requires the painting of clay models or prototype cars.
Combined Direct and Inverse Kinematic Control for Articulated Figure Motion Editing
(Blackwell Science Ltd and the Eurographics Association, 1992) Boulic, Ronan; Thalmann, Daniel
A new approach for the animation of articulated figures is presented. We propose a system of articulated motion design which offers a full combination of both direct and inverse kinematic control of the joint parameters. Such an approach allows an animator to specify interactively goal-directed changes to existing sampled joint motions, resulting in a more general and expressive class of possible joint motions. The fundamental idea is to consider any desired-joint space motion as a reference model inserted into the secondary task of an inverse kinematic control scheme. This approach profits from the use of half-space Cartesian main tasks in conjunction with a parallel control of the articulated figure called the coach-trainee metaphor. In addition, a transition function is introduced so as to guarantee the continuity of the control. The resulting combined kinematic control scheme leads to a new methodology of joint-motion editing which is demonstrated through the improvement of a functional model of human walking.
Composing Hierarchically Structured Images
(Blackwell Science Ltd and the Eurographics Association, 1992) Patterson, John W.; Cockton, Gilbert
This paper begins by examining the classical raster-based composition model, establishing its weakness, and developing a new composition algebra based on line drawing. It then examines the role of composition in the Hierarchical Display Model, demonstrates that the forms of composition assumed in this model are inadequate to deal correctly with the interactions of two-dimensional representations of three-dimensional objects, and shows that our composition algebra resolves this difficulty. The composition steps required can be packaged with the use of a single attribute which constrains the apparent order of composition. This attribute is associated with the object which is to be taken out of order , obviating any needsfor layers.
A Computer Model For Pinscreen Simulation: A New Animation Paradigm
(Blackwell Science Ltd and the Eurographics Association, 1992) Lopes, Pedro Faria; Gomes, Mario Rui
Actual computer image and motion synthesis paradigms are presented in order to introduce a new computer animation paradigm that simulates the traditional pinscreen animation technique. Pinscreen is a very particular and difficult device that enables a"chiaroscuro" approach to image synthesis. The paper describes the traditional pinscreen technique, the way it is operated, results and related difficulties. An extended computer model, including colour and time extensions, to simulate the pinscreen is presented. Results achieved with vector and raster approaches are discussed. Advantages and possibilities of the method when compared with the traditional method and the problems that arise from this technique will also be explained. Finally current and future work is referenced.
Computing Dynamic Changes to BSP Trees
(Blackwell Science Ltd and the Eurographics Association, 1992) Chrysanthou, Y.; Slater, M.
This paper investigates a new method for dynamically changing Binary Space Partition (BSP) trees. A BSP tree representation of a 3D polygonal scene provides an ideal data structure for rapidly performing the hidden surface computations involved in changing the viewpoint. However, BSP trees have generally been thought to be unsuitable for applications where the geometry of objects in the scene changes dynamically. The purpose of this paper is to introduce a dynamic BSP tree algorithm which does allow for such changes, and which maintains the simplicity and integrity of the BSP tree representation. The algorithm is extended to include dynamic changes to shadows. We calibrate the algorithms by transforming a range of objects in a scene, and reporting on the observed timing results.
Consistent Schemes for Addressing Surfaces when Ray Tracing Transparent CSG Objects
(Blackwell Science Ltd and the Eurographics Association, 1992) Gervautz, Michael
Constructive Solid Geometry (CSG) defines the shape of objects, i.e. the inside and the outside regions of objects, but it does not define the material properties of the space in the interior of objects. The definition is ambiguous. There are points of space lying in several primitives, which can consist of different materials. When visualizing transparent objects this ambiguity leads to strange results, which are not consistent with the CSG modelling scheme and must therefore be eliminated. This work describes how an unambiguous model can be built by asymmetric CSG-operators and how correct refraction and shading on material boundaries can be established by separating surface properties from material properties. This separation leads to a consistent view of CSG modelling also concerning materials and surfaces. Ray tracing CSG trees and the shading model are influenced by these asymmetric operators. We introduce an applied classification scheme to handle the requirements of the new operator definition.
A Cooperative Architecture for Hypermedia Editing - CoMEdiA
(Blackwell Science Ltd and the Eurographics Association, 1992) Santos, Adelino
CoMEdiA is a cooperative hypermedia editing prototype, which enables co-authors to cooperatively produce hypermedia documents. It allows co-authors to communicate their ideas, drafts, guidelines, constraints, and annotations with other co-authors in order to exchange information (remotely or face-to-face), improvepassages and modify notes until a final document is achieved. We did not concentrate on the depth but on the breadth of the features. Our efforts were on integrating and coordinating concepts from collaboration, multimedia, and hyper organization rather than on making a specialized system in any of them. Our first step was to investigate issues in group editing and cooperation. Then we started by having text in our documents, later static images, and 2D-Graphics. This procedure was adopted because we felt these were the most simple media to integrate and process.
A Development Environment for Constructing Graph-Based Editing Tools
(Blackwell Science Ltd and the Eurographics Association, 1992) Chen, M.; Townsend, P.; Wang, C.Y.
Scientists and engineers use numerous kinds of graph notations to model various objects in the real world and their relationships in an abstract pictorial form. However, developing a specific graph editing tool for each of these notations is often very costly in terms of time and resources. This paper outlines a design of a suitable development environment for constructing graph-based editing tools, based on the requirements analysis of such an environment derived from a survey of more than 60 different graph notations.
An Efficient Adaptive Algorithm for Constructing the Convex Differences Tree of a Simple Polygon
(Blackwell Science Ltd and the Eurographics Association, 1992) Rappoport, Ari
The convex differences tree (CDT) representation of a simple polygon is useful in computer graphics, computer vision, computer aided design and robotics. The root of the tree contains the convex hull of the polygon and there is a child node recursively representing every connectivity component of the set difference between the convex hull and the polygon. We give an O(n log K + K log2 n) time algorithm for constructing the CDT, where n is the number of polygon vertices and K is the number of nodes in the CDT. The algorithm is adaptive to a complexity measure defined on its output while still being worst case efficient. For simply shaped polygons, where K is a constant, the algorithm is linear. In the worst case K = O(n) and the complexity is O(n log2 n). We also give an O(n log n) algorithm which is an application of the recently introduced compact interval tree data structure.
Equidistant Smoothing of Polyhedra with Arbitrary Topologies
(Blackwell Science Ltd and the Eurographics Association, 1992) Ma, Lizhuang; Liang, Youdong; Peng, Qunsheng
Smoothing of polyhedron with arbitrary topology is an important issue in CAGD and CAD/CAM, but so far it is deemed to be difficult to smooth the complex corners of a polyhedron. In this paper, the concept of distance surfaces of a surface and a solid is introduced, and the incisive properties of such surfaces are addressed which provide a theoretical foundation for modifying a general corner. The method is based on making constricted volume and the maximum distance the volume can be constricted is given too. It is shown that by the proposed method in this paper any polyhedron can be G1 smoothed with quadraic and, sometimes toroidal surfaces. The new approach is suitable for engineering design and NC machining. The associated algorithm based on the classification theorem of corners is simple, fast and robust.
Fast Generation and Surface Structuring Methods for Terrain and Other Natural Phenomena
(Blackwell Science Ltd and the Eurographics Association, 1992) Koh, Eng-Kiat; Hearn, D. D.
Fractal geometry has proven to be a powerful tool for modeling natural phenomena. Using discrete approximations to fractional Brownian motion over a finite grid plane, computer graphics terrain-rendering algorithms are able to generate highly realistic topographical displays. Similar procedures can be applied to model other natural phenomena, such as clouds and water. Two important considerations in these algorithms are computational efficiency and the ability to control macroscopic surface features. Here we introduce a technique for structuring surface features so as to conform to a specified"elevation" envelope. We also present methods for implementing this technique using a recursive random midpoint-displacement procedure.
Formal Specification in the Revision of GKS: An Illustrative Example
(Blackwell Science Ltd and the Eurographics Association, 1992) Duce, D.A.; Damnjanovic, L.B.
The first ISO/IEC standard for computer graphics, the Graphical Kernel System (GKS) was published in August 1985. In accordance with ISO/IEC procedures, GKS is now being reviewed and revised. This paper describes how formal specification techniques are being used by the authors to analyse key parts of proposals being made for changes to the framework of GKS to bring the standard into line with the requirements of applications and the operating environment likely to be found in the mid-1990 s.
The Fractal Nature of an Ecological Model
(Blackwell Science Ltd and the Eurographics Association, 1992) Bennan, Wang; Yin, Shi
With the help of computer graphics, the chaotic behaviour of an ecological model in the complex plane has been investigated. Convergence maps that show multi-fractal characteristics (science) and beautiful patterns (art) are presented.
Fractals and Solid Modeling
(Blackwell Science Ltd and the Eurographics Association, 1992) Groller, Eduard
Trying to combine fractal geometry and solid modeling seems to be a contradiction in itself, In this paper a new type of 3D objects is presented that accomplishes this combination in a specific way. Objects with a fractal macro structure and a 3D solid micro structure can be specified and rendered efficiently by using context free, attribute, geometric grammars. This new object type can be incorporated into the CSG-modeling technique (Constructive Solid Geometry) in two ways: a) using CSG for the specification of the micro structure of the new object type, b) using these fractal like objects as a new type of primitive in the CSG model. Ray tracing is used for generating high quality images of these geometrically complex objects.
A Functional Approach to the Visual Simulation of Gaseous Turbulence
(Blackwell Science Ltd and the Eurographics Association, 1992) Sakas, Georgios; Westermann, Rudiger
This paper presents a functional method for the visual simulation of 2-D or 3-D turbulent gaseous motion by using time-varying fractals. The used function incorporates results from the"spectral theory of turbulence", thereby providing a physics-based approach adapted to the needs of computer graphics. The involved turbulence function is band-limited, continuous, differentiable, anisotrop, and smooth, provides different fractal dimensions along each axis, may be evaluated locally with different parameters, and requires only minimal storage space, thus supporting an implementation on large parallel processing networks with small nodes. Inhomogeneity in the form of local disturbances of the turbulence field may also be easily considered. The parameters used to describe turbulent motion are rather intuitive, so that they may be utilized easily by users. Examples for modeling different types of clouds and fire are given.

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