Volume 10 (1991)

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Construction Techniques of Graphic, Direct-Manipulation User Interfaces
(Eurographics Association, 1991) Preea, Wolfgang; Pornberger, Gustav; Sikora, Hermann
This paper deals with human-computer interaction in several ways. On the one hand it presents the roots of interactive, graphic user interfaces and how such interfaces are implemented on the abstraction level of programming languages: The construction of graphic, direct-manipulation interfaces with conventional programming techniques is compared with an object-oriented approach based on powerful class libraries (called user interface application frameworks). Although application frameworks substantially ease the building of highly interactive applications the abstraction level is considered to be too low to support prototyping such interfaces in a comfortable way. Hence we portray DICE1 (Dynamic Interface Creation Environment), a tool for prototyping graphic user interfaces implemented itself in an object-oriented manner. In particular this paper discusses the question of how dynamic behavior can be added to a user interface prototype. It also presents a useful and powerful way to combine conventionally developed and object-oriented software systems.
Cartographic Algorithms: Problems of Implementation and Evaluation and the Impact of Digitising Errors
(Blackwell Science Ltd and the Eurographics Association, 1991) Visvalingam, M.; Whyatt, J.D.
Cartographic generalisation remains one of the outstanding challenges in digital cartography and Geographical Information Systems (GIS). It is generally assumed that computerisation will lead to the removal of spurious variability introduced by the subjective decisions of individual cartographers. This paper demonstrates through an in-depth study of a line simplification algorithm that computerisation introduces its own sources of variability. The algorithm, referred to as the Douglas-Peucker algorithm in cartographic literature, has been widely used in image processing, pattern recognition and GIS for some 20 years. An analysis of this algorithm and study of some implementations in wide use identify the presence of variability resulting from the subjective decisions of software implementors. Spurious variability in software complicates the processes of evaluation and comparison of alternative algorithms for cartographic tasks. No doubt, variability in implementation could be removed by rigorous study and specification of algorithms. Such future work must address the presence of digitising error in cartographic data. Our analysis suggests that it would be difficult to adapt the Douglas-Peucker algorithm to cope with digitising error without altering the method.
Combining Volume Rendering with Line and Surface Rendering
(Eurographics Association, 1991) Frühauf, Martin
Volume data is discrete sampled data in the three-dimensional space. Volume rendering is defined as volume visualization directly from volume primitives and not via surface primitives. Geometric objects are represented as a list of vertices and connecting lines or surface patches. Independent algorithms for rendering the two different categories of data are used. The system, proposed here, combines the results of the different algorithms in one image. A set of common parameters influencing both rendering algorithms and ensuring the consistency of the resulting merged image is identified. A volume rendering algorithm capable to produce the information to be merged is described in detail. The system is able to handle opaque and translucent objects by merging lists of image space elements. The independence of the both rendering modules allows to employ a wide range of algorithms for rendering of geometric objects, even rendering in hardware.
MOVE-X: A System for Combining Video Films and Computer Animation
(Eurographics Association, 1991) Ertl, Gerhard; Müller-Seelich, Heimo; Tabatabai, Behnam
The objective of the presented project was to implement a system for the visualization of buildings. The system is used to create movies of buildings and interior rooms before they are built. For a realistic impression of a building it is very important to show its actual environment in the film. The designed software solution permits to create films where real images of the environment are overlaid with computer generated images of the building. In order to overlay the video film with computer generated images, it is necessary to compute the exact position, the viewing direction and the adjustment of the zoom for every frame of the film. These parameters can be calculated from the video images. This paper describes the algorithms used to calculate the camera parameters and to track passpoints in a sequence of video images. Some problems resulting from interlaced video and low resolution are discussed in detail. The rendering techniques used to generate images of the new building are also discussed.
Shading with Area Light Sources
(Eurographics Association, 1991) Tanaka, Toshimitsu; Takahashi, Tokiichiro
This paper derives a shading model for area light sources which covers both diffuse and specular reflection. The shading model assumes ideally diffuse polygonal light sources and uses Phong’s reflection model. The model can accurately integrate the intensities of diffuse and specular reflection without simulating an area light source as an array of point light sources. To simplify the reflection integration, each light source is transformed from the Cartesian coordinate system into the polar system. The light source is projected onto a unit sphere and then triangulated along great circles of the unit sphere. Finally, the integration value is calculated by polynomial approximation. Since our method can accurately integrate both diffuse and specular reflection, it can generate images that are more photorealistic than conventional methods. Because point light sources are not employed, highlight roughness is completely suppressed. Several images are presented that show the advantages of our method.
Block Iterations in the Complex Plane
(Blackwell Science Ltd and the Eurographics Association, 1991) Szyszkowicz, Mieczyslaw
In this note we propose a new technique for execution of the feedback process z ? z2+ c. The presented method is used to discover a family of sets in the complex plane.
The Shortest Way to Draw a Connected Picture
(Blackwell Science Ltd and the Eurographics Association, 1991) Seebold, Patrice; Slowinski, Karine
With any word over the alphabet ?=r, r?, u, u, we associate a connected picture in the following manner: the reading of each letter of this word induces a unit line: r (r?, u, u respectively) stands for a right (left, up, down respectively) move. We present a rewriting system which can yield, from any word over ?, all the words describing the same picture. Particularly, we give an algorithm to find a minimal word describing a given picture: this word represents the shortest way to draw this picture without penup .
Computer Art from Numerical Methods
(Blackwell Science Ltd and the Eurographics Association, 1991) Szyszkowicz, Mieczyslaw
Zero-finding methods, Euler s method and the numerical algorithms elaborated as package subroutines are studied by using a computer graphics technique. The patterns obtained show chaotic behaviour (science) and beauty of numerical algorithms (art).
Continuation Methods for Approximating Isovalued Complex Surfaces
(Eurographics Association, 1991) Zahlten, Cornelia; Jürgens, Hartmut
Basically there are two different approaches for rendering isovalued surfaces in 3D space: projection methods and surface reconstruction. We are discussing two algorithms of the second kind. Both use continuation methods for efficiently scanning an isovalued surface. A simplicial pivoting algorithm by Ralf Widmann which continues earlier work of E. L. Allgower et al., is compared to an approach which is based on subdividing space into cubes. The algorithms determine all simplices or cubes intersecting the surface and then generate an oriented polygonal approximation. For demonstration and comparison we use several fractal and some smooth surfaces. These surfaces are implicitely defined by a function, but it is also possible to apply both methods to 3D volume data.
A Testbed for Image Synthesis
(Eurographics Association, 1991) Trumbore, Ben; Lytle, Wayne; Greenberg, Donald P.
Image Synthesis research combines new ideas with existing techniques. A collection of software modules that provide such techniques is extremely useful for simplifying the development process. We describe the design and implementation of a new Testbed for Image Synthesis that provides such support. This Testbed differs from previous Testbeds in both its goals and its design decisions. The Testbed design addresses the problems of high model complexity, complicated global illumination algorithms and coarse grain parallel processing environments. The implementation is modular, portable and extensible. It allows for statistical comparison of algorithms and measurement of incremental image improvements, as well as quantitative comparison of Testbed images and light reflectance measured from physical models. The Testbed is designed to interface with any available modeling system. This compatibility was achieved through careful design of the data format that represents environments. The software modules of the Testbed are organized in a hierarchical fashion, simplifying application programming.
Sampling and Anti-Aliasing of Discrete 3-D Volume Density Textures
(Eurographics Association, 1991) Sakas, Georgios; Gerth, Matthias
In recent years, a number of techniques have been developed for rendering volume effects (haze, fog, smoke, clouds, etc.) in order to enhance reality in computer-generated imagery as well as to improve the performance of flying, ship, and driving optical simulators. For modeling such effects, volume 'density' objects are used, which are defined by their density distribution in 3-D space. For such a description a three-dimensional voxel field (solid texture) is usually used. Since we deal with 3-D textures, the methods used for sampling 2-D pixel fields cannot always be employed. In this paper, we propose two variants of a new technique for sampling and anti-aliasing 3-D density voxel fields. First, we point out the problems which occur when such 3-D textures are sampled, especially when the point sampling Monte-Carlo method is used. 'Distance sampling' and 'pyramidal-volume sampling' are then introduced. The first ,technique samples the texture along a straight line defined by the eye position and the pixel midpoint, whereas the pyramidalvolume technique approximately samples the volume of the pyramid defined by the eye and the four pixel comers. In comparison to other existing methods, both methods greatly reduce aliasing and calculation time. Especially the second one provides a constant-time filtering, whereby minimizing the number of texture evaluations. In the last paper section we demonstrate the applicability of the proposed methods for animation as well as for visualization purposes.
An Algorithm of hidden Surface Removal based on Frame-To- Frame Coherence
(Eurographics Association, 1991) Tost, Daniele
Frame-to-frame coherence capitalizes on the continuity existing between successive frames in order to predict the visibility of the scene at a given instant on the basis of its visibility at the previous instant. It is shown that all the algorithms based on frame-to-frame coherence share a common model and involve similar data structures. This model is used as a framework for a discussion of possible frame-to-frame coherence algorithms. A specific solution based on a partition of the image space into several 2-D regions is then presented. A temporal graph of the depth priority relationship of the objects is first computed in a pre-process along with a temporal BSP tree of each object. In the first frame a total traversal of both data structures is required in order to establish the priority list of the faces of the scenes. In successive frames, the priority list is easily updated with partial traversals and with no geometrical computations. The complexity of the proposed algorithm is discussed. Some examples are presented with comments on the results of their implementation.
A Survey of Several Root-Finding Methods in the Complex Plane
(Blackwell Science Ltd and the Eurographics Association, 1991) Szyszkowicz, Mieczyslaw
In this paper a few classes of root-finding iterative methods are presented and used in the complex plane to solve the equation z4? 1 = 0. These methods are studied using computer graphics techniques.
Fat curves
(Blackwell Science Ltd and the Eurographics Association, 1991) Yao, C.; Rokne, J.
Fat curves in two-dimensional Euclidean space are discussed. Previous work on fat curves is reviewed and a new definition is given for a fat curve having a smooth axis. The joining of two fat curves is discussed and a technique for scan-converting fat curves is presented.
Solid-Interpolating Deformations: Construction and animation of PIPs
(Eurographics Association, 1991) Kaul, Anil; Rossignac, Jarek
Computer programs that simulate the deformations of geometric shapes have played a key role in the increasing popularity of software tools for artistic animation. Previously published techniques for specifying and animating deformations are either limited in their domain or ill suited for interactive editing and visualization. This is because the effects of alterations performed by the animator on the model's parameters may not always be anticipated, and because realtime animation may only be produced by visualizing pre-computed sequences of 3D frames, which are obtained by a slow process and require vast amounts of storage. To support an interactive environment for animation design, we have developed a new, simple, and efficient animation primitive: a Parameterized Interpolating Polyhedron, or PIP for short. PIPs are easily specified and edited by providing their initial and final shapes, which may be any polyhedra, and need not have corresponding boundary elements. PIPs may be efficiently animated on standard graphic hardware because a PIP is a smoothly varying family of polyhedra bounded by faces that evolve with time. The faces have constant orientations and vertices that each move on a straight line between a vertex of the initial shape and a vertex of the final one. The cost of recalculating the time dependant information of a PIP is small in comparison to the display cost. We provide simple and efficient algorithms, based on Minkowski sum operations, for computing PIPs. When both the initial and final shapes are convex, the resulting faces are the true boundary of the deforming object, otherwise subsets of the resulting faces may lie inside the object. In both cases, correct images are automatically generated using standard depth-buffer hardware. The tools we have developed are convenient for interactively designing animation sequences that show the metamorphosis of 3D shapes. They may also be used to simulate the geometric effect of a variety of manufacturing operations, and for interactively selecting the optimal compromise between two or more shapes. They are being integrated in the LAMBADA design and inspection environment for animated assemblies, where deformations and rigid-body motions may be easily combined and synchronized using a hierarchical representation.
Timewarps: A Temporal Reparameterization Paradigm for Parametric Animation
(Eurographics Association, 1991) Smith, Jeff; Drewery, Karin
The ability to reparameterize the temporal axis in an animated sequence is a powerful and underexplored tool. Timewarps are described as extensions to previous work and the advantages of this tool are outlined. Timewarps allow an animator to progressively refine the timing of all or part of an animation. Problems inherent to this new concept were brought to light by the authors' implementation and are described in terms of both user interface and development issues. Solutions are discussed and directions for further study are outlined.
Interactive Conformance Testing for PHIGS
(Eurographics Association, 1991) Cugini, John V.
Conformance testing for the Programmer’s Hierarchical Interactive Graphics System (PHIGS) standard presents certain novel difficulties, especially the indirect effect of many functions, and the inaccessibility to the program of visual effects. The PHIGS Validation Tests (PVT) incorporate several innovative design features in order to address these difficulties. The model of deductive inference suggests ways to organize a system as logically complex as the PVT. This complexity makes the use of certain database concepts quite valuable in allowing users to navigate within the system. The problem of inaccessible effects can be addressed by careful design of the user interface, so as to minimize the subjectivity and operational difficulty inherent in testing such features. Subjectivity is minimized by posing short simple questions to the operator, in which the expected answer is randomized. Several design features enhance ease of use, including a customizable interface, self-explanatory displays, and automatic capture of results.
FACES: Facial Animation, Construction and Editing System
(Eurographics Association, 1991) Patel, Manjula; Willis, Philip J.
The aim of the Facial Animation, Construction and Editing System (FACES) is to provide a software simulation of the human face. Attention has focused on the face as an important means of non-verbal communication. The interactive composition and modification of the human head and its subsequent animation, have been identified as being of particular interest. The novelty of FACES is that it integrates the modelling and animation of faces using a three-layer anatomical model. FACES consists of four sub-systems: CONSTRUCT, MODIFY, ANIMATE and RENDER. The CONSTRUCT and MODIFY sub-systems enable changes to be made to the structure of the head, at both global and local levels, enabling specific faces to be created. The ANIMATE sub-system caters for motion specification and control, so that real and exaggerated facial expressions can be animated. The RENDER part of the system facilitates the generation of realistic images and their real-time playback. In this paper we consider the system from the user's point of view, examining the facilities which are provided, their appropriateness and practicality.
A constraint-based UIMS using graph unification
(Eurographics Association, 1991) Samuel, J. F.
This paper describes an experimental UIMS developed to investigate methods of combining and reusing objects within a constraint-based system. A novel approach based on graph-unification facilitated the definition of composite objects and allowed behaviours to be transferred easily from one object to another. A very simple form of local propagation was used as the constraint satisfaction mechanism, which, as well as being fast, could also cope with cyclic dependencies. This method used a time-stamping method which made it possible to express time-dependency in the constraints. The system has features from both class-based and prototype-based object-oriented languages. Dynamic modification of objects can be performed rapidly enough for interactive control. Interfaces built with the system include a variety of 'standard' user interface objects such as sliders as well as a number of physical simulations utilising time-dependent constraints.
Inverse Displacement Mapping
(Blackwell Science Ltd and the Eurographics Association, 1991) Patterson, J.W.; Hoggar, S.G.; Logie, J.R.
Inverse displacement mapping is a variant of displacement mapping which does not actually perturb the geometry of the surface being mapped. It is thus a true texture mapping technique which can be applied during rendering without breaking viewing pipeline discipline. The method works by first projecting probing rays into texture space and solving for a ray-texture intersection there. Shadows can also be determined by mapping a probe from the intersection point towards the light source into texture space and seeing if an intersection results. Our implementation uses as much knowledge about the base surface as possible to speed up the ray-surface intersection calculation. We have limited our treatment to spheres, cones, cylinders and planes, and our rendering method to ray casting, in order to contain the scope of this work up to the present. The inverse displacement mapping technique can, however, be applied more widely, for example as part of a full ray-tracer, and also as part of the rendering pipeline for a wider class of smooth surfaces.

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