EG1988 Proceedings (Technical Papers)

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Tools for Efficient Photo-Realistic Computer Animation
(Eurographics Association, 1988) Stößer, Achim; Schmitt, Alfred; Neidecker, Burkhard; Müller, Heinrich; Maus, Thomas; Leister, Wolfgang
The goal of the project “Occursus Cum Novo” was to generate a complex photo-realistic animation of nontrivial length in reasonable time at reasonable cost. Photographic realism comprises complex geometric models as well as the implementation of several optical effects. Both can be achieved by simulation. Simulations guaranteeing high quality, as ray tracing does for rendering, are known to be very time consuming. They require the design of powerful data structures and algorithms taking the abilities of todays hardware into account. The first part of this paper is devoted to rendering, here carried out by the raytracing method. An organizing scheme for a network of work stations is described which enabled us to generate 5-minutes of raytraced animation within 2 months without affecting any regular user of the work stations. The second part is devoted to the simulative modeling tools applied in “Occursus Cum Novo”. The recursive generator WAXI and a motion simulator METAMORPHOSIS are described to some detail. The results of the project are of general interest since they show a way leading to efficient high quality photo-realistic animation synthesis in the future.
Animation of Stochastic Model-Based 3-D Textures
(Eurographics Association, 1988) Gagalowicz, Andre; Ma, Song D.
We have previously shown the validity of a statistical model proposed by us for realistic textures. We have also shown how to use this model to map realistic textures onto a still surface of any shape and extent, maintaining consistency and scaling without producing repeating patterns.The problem we have to solve in this paper is how to use this statistical model in order to produce animated objects while keeping temporal consistency. This problem a priori difficult to solve as our mapping technique is statistical.We propose two different techniques. The first method called the image space method creates texture directly on the image planes. The second one called the object space method performs the texture synthesis on the object surface itself which has to be sampled finelly beforehand.
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.
IXPHIGS: A Portable Implementation of the International PHIGS Standard
(Eurographics Association, 1988) Gorog, Jenö; Krammer, Gergely; Vincze, Arpad
This paper describes a "portable" object-oriented implementation of the international standard PHIGS. A rapid prototype has been achieved by making use of earlier developments : the IX-CGI module for graphical input, output and interaction and IX-TAM for the creation of and access to objects of different classes, and thus the creation, manipulation and traversal of graph structures with different classes of leaf objects, The “post” function of PHIGS by definition is applied to structure node objects. For each node, it goes. through the element list of the node and, for each element in the list, it invoker the post function of the appropriate element class. Standard extensions are incorporated into this IX-POST module. The limited capabilities provided by the PHIGS input functions are enhanced by an additional interaction interface module IX-IF. A STRUCture e d i TOR program built on top of the PHIGS application interface serves not only as a demonstration of PHIGS capabilities but also as a turnkey program for the wide class of applicaticans with the task of creating and editing symbolic drawings. IXPHIGS is written in the language C and it is a "full software implementation" of all PHIGS capabilities, even for parts which may he enhanced later by using advanced hardware on some target configurations.
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.
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.
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.
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.
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.
A Evalution of Some Three-Color Tiling Patterns
(Eurographics Association, 1988) Alt, Paul; Cordonnier, Vincent
Some 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.
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.
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.
Experimenting with a Parallel Ray-Tracing Algorithm on a Hypercube Machine
(Eurographics Association, 1988) Priol, Thierry; Bouatouch, Kadi
A parallel space tracing algorithm is presented. It subdivides the scene into regions. These latter are distributed among the processors of an iPSC hypercube machine designed by Intel company. Each processor subdivides its own region into cells to accelerate the ray tracing algorithm. Processors communicate by means of messages. The pyramidal shape of the regions allows the deletion of the primary ray messages. A method of performing a roughly uniform load distribution is proposed.
A Method for Providing Full Interactive Control of the Shape of 3-D Curves and Surfaces
(Eurographics Association, 1988) Cros, Frederic; Brock, Philip J.
This paper presents a new method for providing an interactive local control on 3-D curves and surfaces. The possibility of interacting directly on the vector tangent to the curve at each control point (e.g. changing its magnitude and direction) gives the user a more intuitive control for shaping a curve than if he has to manipulate "abstract" parameters (like bias or tension). This technique is based on the use of Cardinal-splines and can also provide a means to obtain local control on surfaces.
An Evaluation of CSG Trees Based on Polyhedral Solids
(Eurographics Association, 1988) Badouel, Didier; Hegron, Gerard
Set operation on polyhedra is an important component of Geometric Modeling System (GMS) when a Constructive Solid Geometry (CSG) representation with polyhedral solid primitives is used. Output data will be the unique resulting polyhedron which provides an efficient data structure for displaying objects. With no use of spatial coherency, computational complexity of a set operation is quadratic. The new evaluation scheme called Boolean Octree limits set operation evaluation in a ‘minimal space of calculation’ where primitive boundaries intersect each other and where resulting evaluation participates in the construction of the final resulting object. Boolean Octree computes set operations in a local level providing a linear complexity for geometric calculations. During space subdivision, Boolean Octree has a global view on local CSG tree (projection of the CSG tree in local space) taking into account simplifications of the boolean expression. Set evaluation is done in the local volumes containing only two operands the configurations of which are ‘simple’, that is to say for a local description of an object there is only one vertex with any face number, one edge, or one face.
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.

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