EG1987

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3D Models in Computer-Assisted Chemistry
(Eurographics Association, 1987) Roch, Michel; Cretegny, Isabelle; Weber, Jacques
Novel and efficient algorithms have been developed in order to display on a raster scan device some basic 3D models used in computer-assisted chemistry. They allow, among other things, the construction and manipulation of the geometrical structure, or shape, of chemical species containing several hundreds of atoms, both as simple skeleted and sophisticated 3D space filling models with Gouraud shading and hiddensurface treatment. In addition, the technique of image storing is used in order to simultaneously display the molecular model and both 2D (colorfilled contour levels) and 3D (solid model) representations of chemical properties of the compound. The images thus generated allow an immediate perception of the major features of the species under study, which provides the chemist with an invaluable tool in computer-assisted chemistry. Some simple examples, which illustrate the potential of these 3D models for discussing chemical reactivity, are presented. Finally, the algorithms and procedures used for constructing the 3D images are briefly described and discussed. *Correspondence address: Dr J. Weber, Laboratory of Computational Chemistry, University of Geneva, 30 quai Ernest Ansermet, 1211 Geneva 4, Switzerland
Active-Ray Tracing for 3D Medical Imaging
(Eurographics Association, 1987) Trousset, Yves; Schmitt, Francis
Large quantities of three-dimensional (3D) information are produced by 3D medical imaging techniques such as CT and MRI, in the form of digital volumes (DV). We propose a new approach to help a human observer to visualize these volumes. This approach is based on the idea that, in order to achieve true user-interaction, segmentation and visualization should not be treated separately. This is achieved by introducing active ray tracing, a method inspired by ray tracing but working directely on unsegmented digital volumes. Segmentation takes place as the active rays penetrate through the volume.
An Algorithm for 3D Scan-Conversion of Polygons
(Eurographics Association, 1987) Kaufman, Arie
A three-dimensional (3D) scan-conversion algorithm, that scanconverts 3D planar polygons into their discrete voxel-map representation within a Cubic Frame Buffer (CFB), is presented. The algorithm, which is a variation of a 2D scan-line filling algorithm, is incremental and uses only simple operations like additions and testy inside the inner loops. The algorithm performs scan-conversion with computational complexity which is linear in the number of voxels written to the CFB. The paper also presents an algorithm that scan-converts polygons clipped to the CFB boundaries with no added time complexity. An all-integer decision mechanism which makes the inner-most loop of the algorithm more efficient is discussed too. All the algorithms guarantee lack of 6-connected "tunnels" in the converted polygons. The algorithms have been implemented as part of the 3D geometry processor of the CUBE Architecture, which is a voxel-based system for 3D graphics. These algorithms allow the' CUBE system to generate the essential primitive polygon within the CFB from a 3D geometric model.
An Approach to the Formal Specification of Configurable Models of Graphics Systems
(Eurographics Association, 1987) Arnold, D.B.; Duce, D.A.; Reynolds, G.J
This paper describes a general framework for the formal specification of modular graphics systems. The approach is illustrated by an example taken from the Graphical Kernel System (GKS) and uses the Z specification notation.
An Automatic Interpolation Method of Grayvalued Images Utilizing Density Contour Lines
(Eurographics Association, 1987) Agui, Takeshi; Saito, Minoru; Nakajima, Masayuki; Arai, Yukihiro
This paper presents an automatic method of interpolation or inbetweening for gray valued images, which have been difficult to be processed automatically. The method consists of three parts. The first i s the extraction of density contour lines from a pair of gray valued key frame images, the second i s the interpolation of the contour lines and the l a s t is the generation of in-between gray valued images. In the second part, the correlation of corresponding contour lines of the key frames i s carried out automatically utilizing some kinds of similarity functions. The method is applied to a pair of pictures of a plaster head photographed slightly different directions and generates a very smooth rotating animation picture of the p l a s t e r head. This method has much applicability to computer animation and motion picture analysis. Stereo pair images are available from key frame pictures of objects photographed from any direction. Interpolation of color images is also possible.
Bezier Patches with Local Shape Control Parameters
(Eurographics Association, 1987) Schmitt, Francis J. M.; Du, Wen-Hui
In this paper, we introduce three types of shape control parmeters (tension, bias, and continuity) into the modelling of free-form surfaces composed of bicubic Bernstein-Bezier patches. This is an extension to surfaces of the three control parameters introduced in the local interpolatory spline curves by Kochanck and Bartels [12]. We first analyze the general conditions for guaranteeing the G' continuity of the surface at corners where three, four, or five patches meet. The analysis of the continuity constraints reveals the remaining degrees of freedom which can be exploited for the control of the surface shape. The shape control parameters arc then introduced. Experimental results are provided. They show the efficiency of these parameters for an easy and intuitive control of the surface shape.
The Clockworks: An Object-Oriented Computer Animation System
(Eurographics Association, 1987) Breen, David E.; Getto, Phillip H.; Apodaca, Anthony A.; Schmidt, Daniel G.; Sarachan, Brion D.
The Clockworks is an object-oriented computer animation system developed at RPl's Center for Interactive Computer Graphics (CICG). The Clockworks has the ability to model and graphically simulate complex 3-D engineering processes, Its interactive capabilities also allow it be used as a design tool. Object-oriented concepts have been utilized in developing its high level architecture and its low level implementation. The Clockworks is defined as a collection of objects which communicate with the user and each other via messages. The actual implementation involved the creation of an object-oriented programming methodology using C and Unix. The complete system provides a rich research environment for exploring modelling, scripting and rendering. It also provides an interactive environment for design and a simulation environment for visual analysis of complex interacting structures.
COLOUR SECTION
(Eurographics Association, 1987) -
Conformal Texture Mapping
(Eurographics Association, 1987) Fiume, E.; Fournier, A.; Canale, V.
A new class of geometric mappings is introduced to computer graphics, and the utility of this class is illustrated by applying it to texture mapping. When mapping a texture onto a surface such as a polygon, the entire texture can rarely be mapped without some clipping or non-linear transformation. Is it possible to map a texture bijectively to an arbitrary polygon such that the entire texture is mapped? This paper presents a solution to this problem. A new class of mapping techniques based on conformal mapping is proposed. The technique allows one to construct a continuous, bijective map from a polygonal texture space (e.g., a square) to an arbitrary convex polygon. The resulting map is texture-independent. The theory and .an implementation of conformal texture mapping is discussed, and several simple filtering techniques to support it are outlined. Conformal mapping extends the range of geometric mapping techniques, and is pertinent to many areas of computer graphics. Other examples of the potential utility of conformal mappings are also discussed.
Designing a System to Provide Graphical User Interfaces: The THESEUS Approach
(Eurographics Association, 1987) Huebner, W.; Lux-Mulders, G.; Muth, M.
This paper presents the design of THESEUS, a system for programming graphical user interfaces especially targetted at the area of software engineering. Starting with the special needs and requirements that arise in this area, the design alternatives for such a system - abstraction level of the programming interface, division of control and dialogue specification - are discussed. The decisions that resulted in the layer model of THESEUS are substantiated. THESEUS offers an application-oriented programming interface supporting a user interface with multiple windowing, graphics and modem interaction techniques. The dialogue control mechanisms,' event driven input management and the definition of hierarchical object-oriented output are described in detail. Experience with a first implementation is also presented.
Display of Solid Models with a Multi-Processor System
(Eurographics Association, 1987) Jansen, Frederik W.; Sutherland, Robert J.
There is a growing need for fast high-quality display of solid objects. Recently-developed custom-VLSI hardware offers high-speed display but lacks the flexibility and processing power at the pixel level to sustain the calculations and filter operations needed for realistic reflection models, texture mapping and anti-aliasing. Multi-processor systems built with general-purpose components of relatively high capacity and moderate cost provide an alternative; they also offer higher performance than single processor systems but retain their flexibility. A CSG hidden-surface algorithm based on depth-buffering and image subdivision is presented which is suitable for use with such a multi-processor system.
Dynamic Management of 3D Scenes
(Eurographics Association, 1987) Hegron, Gerard
When simulating a moving observer or sensor in a 3D scene which contains a large number of objects, only the objects lying in the field of view or of interaction, named the local data base, have to be taken into account in order to decrease, at each step, computation time necessary for either image generation or various processings. This paper presents an algorithm which achieves the dynamic management of the local data base, that is to say which manages the set of objects which enter or leave the field of view of an observer (camera, sensor) during its displacement. This method consists, broadly, of the following two steps: a binary space partitioning of the 3D space is performed off-line from the object bounding boxes by means of planes perpendicular to the X, Y and Z axes and a subregion adjacency graph is created; the dynamic management of the local data base is achieved on-line by modelling the bounded bearing volume of the sensor by a cube of R "radius" (half-edge), and by using the adjacency graph, and inclusion and intersection criteria in order to exploit spatial and temporal coherences between each displacement. When the scene data base is very large, the dynamic management of the RAM memory can be done simultaneously by using this method reasoning from the bounding boxes of disjoined sub - scenes.
Efficient and Consistent Algorithms for Determining the Containment of Points in Polygons and Polyhedra
(Eurographics Association, 1987) Chen, Min; Townsend, Teeter
Algorithms are presented for the determination of whether a given point E² in is interior to, exterior to or on an arbitrary polygonal boundary and for the determination of whether a point in E ³ is interior to, exterior to or on a simple polyhedral boundary. The algorithms are based on the principle of using binary coded coordinate systems and parity counting of the number of intersections of the polygon or polyhedron boundary with an infinite vector. The amount of floating-point arithmetic, including arithmetical and comparative operations has been reduced to a minimum making the algorithms very suitable for implementation in either low-level language software or by hardware. Performance of the algorithms is compared with a number of others taken from the literature and a considerable increase in efficiency is apparent. The algorithms are also shown to be consistent.
Experiences in Implementing Postscript
(Eurographics Association, 1987) Goswell, Crispin A. A.
This paper describes an implementation of POSTSCRIPT for previewing use on workstations with high resolution bitmapped displays. It discusses implementation of storage management, area fill and line drawing, imaging, fonts and font caching. Treatment of bezier curves and dashing is explained. Area fill involves both the even-odd and non-zero winding number rules, so implementation of both of these using trapezoid decomposition is described. There is also a discussion of an object-oriented implementation of polymorphism in POSTSCRIPT. This results in a flexible and user extensible system. Error handling is discussed, because its implementation has a considerable effect on the design of the virtual machine. Various hints are given on improving performance, including caching fonts on disk, treating thin lines specially and magnifying images. Most displays have only two intensities, so there is some discussion of half-toning for gray level simulation, and the extensive options that POSTSCRIPT supports for half-tone design. Finally there is a brief discussion of porting experiences with the interpreter. It is written in the C language and runs on the UNIX operating system, on Perqs, Suns and WhiteChapels.
Extended Octtrees, between CSG Trees and Boundary Representations
(Eurographics Association, 1987) Navazo, Isabel; Fontdecaba, Josep; Brunet, Pere
Besides the most widely used models in the Geometric Modeling Systems, Constructive Solid Geometry (CSG) and Boundary Representations (BR), Octtrees have appeared as an alternative representation scheme which is particularly well suited for the solid boolean operation algorithms. Extended Octtrees, which incorporates three additional node types containing part of the surface of the object, are much more compact and allow the exact representation of plane faced objects, while supporting also low complexity algorithms for boolean operations. Due to the limitations of the two main models, CSG and Boundary Representation , a number of Hybrid Systems have appeared, which support both schemes and perform every operation in the most suitable model. However, algorithms for the boundary evaluation of CSG trees are complex, and at the moment little is known on algorithms for the inverse conversion, from BR to CSG. In the present paper, the use of the Extended Octtree model as an intermediate tool in the conversions between. CSG trees and BR is studied. In the case of CSG trees build from primitives with plane faces, an algorithm for the conversion from the CSG model to the Extended Octtree representation is presented. Its complexity is linear with respect to the numbe of nodes in the Octtree. The use of this algorithm in model-to-model conversions is discussed, together with the BR to Extended Octtree and Extended Octtree to BR conversion algorithms, that present also linear complexity with respect to the total number of nodes.
Extraction and Organization of Form Features into a Structured Boundary Model
(Eurographics Association, 1987) Falcidieno, Bianca; Giannini, Franca
A method is presented for the automatic identification and extraction of feature information from the solid model of an object. The procedure consists of recognizing shape features, extracting those features as solid volumes and arranging them in a hierarchical structure. In this hierarchical model the main shape of the object is represented at the highest levels of abstraction, while form features are described at lower levels of specification. The system is divided into three modules: feature recognition, feature extraction and feature organization. The recognition step works on a face-based representation of solid objects, called Face Adjacency Hypergraph [1] and it takes advantage of the Kyprianou's method [12]. In the extraction phase every recognized form feature is completed with dummy entities to form a feasible object and in the organization step the completed features are arranged in a hierarchical graph, called Structured Face Adjacency Hypergraph, which is a modification of a model defined in a previous work [1].
A Fast Antialiasing Method with a Z-Buffer
(Eurographics Association, 1987) Ghazanfarpour, Djamchid; Peroche, Bernard
This paper describes a new antialiasing method when a z-buffer is used. Good quality antialiasing is achieved in spite of unavoidable approximations. Compared to an ordinary z-buffer, only a little extra memory space and processing time are required. In particular, this fast method can be beneficial when used on a raster graphics system equipped with a hardware-implemented z-buffer. Our approach, however, offers appropriate treatment for certain problems left unsolved by previous methods.
A Fast Ray Tracing Algorithm Using Space Indexing Techniques
(Eurographics Association, 1987) Qunsheng, Peng; Zhu, Yining; Liang, Youdong
A fast ray tracing algorithm is presented. Spatial coherency is exploited by adopting a linear octree data structure which corresponds to an adaptive partitioning of space. A ray strides over a number of empty regions aligning on its way and intersects the desired objects directly, Efficiency of the algorithm is achieved by decreasing the number of regions that the ray must be checked with, by reducing the computations involved in skipping an empty region and by performing a binary search to find the next region. An efficient algorithm based on linear programming for mapping the whole environment into a sorted linear octree is also described. Only the terminal nodes containing boundary surfaces of objects are explicitly represented, which not only shortens the searching process but also leads to a considerable saving on storage space.
A Fast Voxel Traversal Algorithm for Ray Tracing
(Eurographics Association, 1987) Amanatides, John; Woo, Andrew
A fast and simple voxel traversal algorithm through a 3D space partition is introduced. Going from one voxel to its neighbour requires only two floating point comparisons and one floating point addition. Also, multiple ray intersections with objects that are in more than one voxel are eliminated.
Generative Scene Modelling
(Eurographics Association, 1987) Beyer, Thomas; Friedell, Mark
We describe a new theory of scene modelling that includes generative processes as integral components of scene descriptions. A single, uniform treatment of generative processes replaces the variety of ad hoc mechanisms commonly in use. We discuss the implementation of the theory in an experimental modelling system and show that L-systems, fractals, and particle systems, as well as many specialized generative processes, become comparatively easy to define and use.

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