EG1988 Proceedings (Technical Papers)

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Generation of High-Quality Curve and Surface with Smoothly Varying Curvature
(Eurographics Association, 1988) Higashi, Masatake; Kaneko, Kohji; Hosaka, Mamoru
A method for generating curves and surfaces which satisfy constraints of tangent directions and curvature at arbitrary points and which have smoothly varying curvature is introduced. The method is geometrical determination of a Bezier polygon, and quite different from conventional methods such as minimizing the integral of the square of curvature value or the second derivative. This method enables following things : generation of smooth curves by explicitly indicating the characteristic properties of the curves generation of smooth surfaces having not only smooth boundaries but also cross tangent and curvature which change smoothly along the boundary curve, high accurate approximation of intersection curves of surfaces, by satisfying a tangent direction, an osculating plane. and curvature constrained by the surface equations.
Application Profiles for Computer Graphics Standards - A Touch of Realism
(Eurographics Association, 1988) Mumford, Anne M.
Computer graphics standards have become widely used and are now a fact of life. We have reached the stage of reviewing and extending the current standards. There is an important move within the user community which needs to be taken into consideration in these processes. This is the development of application profiles within user groups to make their use of the graphics standards easier and to ensure portability. This paper suggests that this move is important and cannot be ignored by users of standards or those involved with the current standards work.
Fast Algorithm for Polygon Clipping with 3D Windows
(Eurographics Association, 1988) Burkert, Andreas; Noll, Stefan
Many applications which use 3D graphics, need 3D geometric modeling and hence polygon clipping against 3D Windows or general volumes. The disadvantages of existing clipping algorithms are that they are 2D, slow or produce incorrect results. This paper describes a new 3D clipping algorithm for concave polygons with one contour. The new algorithm is an extension to the algorithm of Liang and Barsky: The necessary condition for the appearance of a corner of the clipping rectangle in the output polygon is replaced by a necessary and sufficient condition and then extended to the three dimensional case. Another advantage of the presented algorithm is the the removal of degenerated edges. The algorithm is useful for implementing the draft international standards GKS-3D and PHIGS and also for 3D Window Systems.
A New Algorithm for Converting Boundary Representation to Octree
(Eurographics Association, 1988) Tang, Zesheng; Lu, Shengkai
Conversion algorithms between different object representations have become Increasingly important In soIid modeling system. In this paper, an algorithm for converting boundary representation of 3D object to octree structure is presented. This algorithm Is based on the transformation of Cartesian coordinates of voxels in the object universe to octant addresses or to node addresses of octree. The surface coherence, edge coherence, voxel coherence are used to speed up the transformation. In order to save memory space, Iinear octree Is used and the Intermediate resulting octant addresses are condensed as early as possible. The resolution of octree structure may also be controled by defining the number of levels of octree. This Is the bottom-up algorithm for converting boundary representation of 3D object to octree structure.
Conic Beta-Splines with Local Tension Control for Interactive Curve Fitting
(Eurographics Association, 1988) Pham, Binh
Polynomial Beta-splines were introduced by Barsky as an extension of polynomial B-splines with bias and tension parameters which allow more flexibility in controlling shape in curve fitting. It is possible to show that a quadratic Beta-spline segment is equivalent to a quadratic B-spline segment with suitably modified control vertices. This provides a simple method for evaluating quadratic Beta-splines using De Boor's algorithm for calculating polynomial B-splines. A representation for conic Beta-splines with one tension parameter is introduced and some properties are derived. They form a basis for an efficient algorithm for interactive curve fitting with conic Beta-splines. The results are extended further to cover the case of conic Beta-splines with varying tension where the tension parameter is an interpolating function between the tension values at each end of a segment.
Monte-Carlo Integration Applied to an Illumination Model
(Eurographics Association, 1988) Bouville, C.; Dubois, J.L.; Marchal, I.; Viaud, M.L.
The use of Monte-Carlo integration together with stochastic sampling is very useful for dealing with the scattering phenomena that occur in the propagation and reflection of light. In this paper, these techniques have been applied to the implementation of a physicsbased global illumination model. The theoretical basis of this approach is presented briefly and various applications to realistic image rendering are then described. This concerns the rendering of penumbra and scattered reflection effects, antialiasing and accurate color modelling through spectral integration. For all these applications, both theoretical and implementation aspects are developed and it is shown that stochastic techniques can provide very simple and efficient algorithms.
Efficient Adaptive Subdivision of Bezier Surfaces
(Eurographics Association, 1988) Clay, Reed D.; Moreton, Henry P.
The support of free-form surfaces by modem solid modeling systems has necessitated the development of hardware and algorithms for the interactive display of surface patches. This paper describes an algorithm for the display of multiple surface patches at frame rates capable of providing smooth motion. The algorithm runs on the Silicon Graphics IRIS 4D GT workstation. Real-time motion is achieved by exploiting a microcodable pipeline of floating point processors and existing support for the display of Gouraud shaded Z-buffered polygons. The paper presents tradeoffs in the development of a high performance rendering algorithm. We discuss issues related to hardware support, surface representation, adaptive display, and the minimization of display artifacts such as faceting and cracking. More detailed discussion addresses the choice of Bezier surface patches and subdivision criteria.
Outline Phase Control for Character Rasterization
(Eurographics Association, 1988) Hersch, R. D.
Character rasterization on middle-resolution output devices (screens, laser printers) is one of the most difficult tasks in the domain of resolution-independent raster imaging. Low sampling leads to unacceptable low-quality discrete character shapes Better looking shapes can be generated by adapting parts of the shape outline to the sampling grid Grid adaptation of character parts is realized by several outline phase control mechanisms Algorithms have been developed to adapt horizontal and vertical bars as well as curvilinear character parts to the grid. Special phase control mechanisms are used to ensure a uniform appearance of characters over the baseline.
Using Algebraic Constraints in Interactive Text and Graphics Editing
(Eurographics Association, 1988) Li, Jiarong
In many graphical application areas such as architectural and mechanical design, spatial relationships are vastly used to construct and manipulate complex objects. They are used to describe dependencies between components in objects or between objects and their contexts. These relationships, especially the precise ones, can be represented with algebraic relations and appropriately processed by using constraint programming techniques. Rather than embedding relationships in pre-defined objects, this approach offers possibility to treat relationships explicitly. This paper presents an editing program for presentation materials. The program provides facilities to interactively specify and modify relationships such as alignment, connection and enclosure. The normal editing operations are combined with constraint satisfaction so that an operation on one object may also change the related objects in a chain reaction fashion. A constraint satisfaction algorithm has been implemented in the program. This algorithm is able to solve linear equations.
Boundary to Constructive Solid Geometry: A Step Towards 3D Conversion
(Eurographics Association, 1988) Juan, R.
Several solid representation schemes exist today in solid modeling. As none of them has properties that are uniformly better than any other, the need for modeling systems working on two or more different schemes which perform every operation in the most suitable scheme, became evident. Then it is necessary to provide geometric modeling systems with the ability of converting from one scheme into another scheme. As most solid modellers use either, a boundary representation (BR) or a constructive solid geometry representation (CSG), conversion from one into the other appears to be of capital importance. Conversion from CSG into BR has been carefully studied but a few attention has been deserved to the conversion from BR to CSG. This paper presents an algorithm which generates CSG binary trees from BR representations over a restricted class of polyhedra. The tree is optimal in the output size. Unfortunately, the algorithm does not in general warrant a correct conversion and, consequently some open questions and ideas for further research are presented.
An Expert System for Polyhedra Modelling
(Eurographics Association, 1988) Martin, Philippe; Martin, Dominique
Very 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.
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.
Form-Factors for General Environments
(Eurographics Association, 1988) Shao, Ping-Pine; Peng, Qun-Sheng; Liang, You-Done
A 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.
A Simple Spectral Approach to Stochastic Modelling for Natural Objects
(Eurographics Association, 1988) Anjyo, Ken-ichi
Stochastic 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.
A Formal Specification of a Boundary Representation
(Eurographics Association, 1988) Baumann, Peter
Geometric modellers are used in a wide spectrum of applications in computer graphics. As many other applications use the generated internal representation of solids, semantic correctness is of special importance. On the other hand, the methodology of formal specification offers, in particular, the advantage of ensuring correctness by mathematical means. It is therefore a good idea to formally specify a geometric modeller. This paper describes an abstract data type defining a boundary representation. Special attention is paid to the operations for manipulating solids, i.e. the so-called Euler operations. The operation mfe (make face & edge) is used to show in detail how pre- and post-condition can be derived. Finally, mappings of this abstract data type onto implementations with different data models are discussed.
An Object-Oriented Approach to Process Monitoring
(Eurographics Association, 1988) Hütter, Ronald
Current and future user interface requirements of process monitoring applications are summarized. An object-oriented approach to meet these requirements is described. A special displayfile structure, the socalled object-oriented displayfile as well as some important object types are highligted, to explain the benefits of object orientation in the area of process monitoring user interfaces.
The Calculus of the Non-Exact Perspective Projection - Scene-Shifting for Computer Animation
(Eurographics Association, 1988) Hofmann, Georg Rainer
This 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.
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.
Towards a System for Exploring the Universe of Polyhedral Shapes
(Eurographics Association, 1988) Colin, Christian
It is often easier to model a shape by giving its properties rather than describing it explicitly. Thus, we are interested in methods which allow us to create shapes automatically by only giving a set of properties. We propose a certain number of thoughts on the creation of objects achieved by the gluing together of elementary polyhedra. The formal approach that we develop is based on an exploration of the universe of polyhedral shapes. We also discuss techniques to automatically compute viewpoints, which emphasize the properties of the structures which have been created. These methods are one of the advantages of such a modeling.
Liberation from Rectangles: A Tiling Method for Dynamic Modification of Objects on Raster Displays
(Eurographics Association, 1988) Slater, Mel; Davison, Allan; Smith, Mark
When 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.

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