Showing papers in "Computer Graphics Forum in 1993"

Abstract Interaction Objects

Abstract: The concept of an ‘interactor’ has been introduced by Faconti and Paterno' [6] as an abstraction of an entity in interactive graphics capable of both input and output. However the notion of interaction object need not be confined to graphics systems; it represents a useful structure for thinking and reasoning about the behaviour of interactive systems in general. As part of Esprit Basic Research Action 7040 (Amodeus-2) we are using the concept of interactor, and existing work on state-based processes and agents, to develop a model and theory of interactive systems. In this paper we describe two formal models for interaction objects and sketch how they can be used to build a small vocabulary of operators to support the rigorous specification of a graphics system. Our model differs from the approach of Faconti and Paterno' in that it abstracts away from any specific graphics framework and is thus suited to the level of abstraction demanded by formal approaches to system development.

An integrated system for modeling, animating and rendering hair

Abstract: There are basically four problems to solve in order to produce realistic animated synthetic actors with hair: hair modeling and creation, hair motion, collision detection and hair rendering. The authors describe a complete methodology to solve these basic four problems. They present how hair styles may be designed with a Hair Styler module. They survey the animation model and emphasize a method of collision processing. Finally, they explain how hair may be rendered using an extension of a standard ray-tracing program. Applications of synthetic actors with various hair styles and different styles of mustaches and beards are shown

A Survey of 3D Solid Reconstruction from 2D Projection Line Drawings

Abstract: The reconstruction of a 3D object from its 2D projection(s) and its corresponding problem of 3D object recognition are two of the important research areas in the field of computer vision and artificial intelligence. Reconstruction involves determining the geometric and topological relationship of an object's atomic parts whereas recognition involves identifying an object by some form of template matching. Nagendra and Gujar1 gave a survey of several papers on reconstruction of 3D object from its 2D views. In this paper we present a taxonomy of 3D object reconstruction from 2D projection line drawings. We base the classification on the number of 2D views of the 3D solid object, the degree of user interaction necessary for correct reconstruction, and the internal representation used in the reconstruction process. We discuss the basic issues associated with this problem, review the relevant literature and present topics for future research.

The Cone of Normals Technique for Fast Processing of Curved Patches

Abstract: The cone of normals technique for curved surface patches allows to perform various quick tests at the patch level such as front- or backfacing test, light influence test, and existence of silhouette edges test. For a given patch, a truncated cone of normals is constructed at creation time, which contains all points and all normal directions of the patch. At traversal time, a simple scalar product test determines whether the whole patch is backfacing or frontfacing, so that the costly step of tessellating the patch is avoided in case of patch level face culling. In addition, the technique quickly determines which light sources have no influence on a patch, and which patches have no silhouette edges. The technique can also be used for other surface primitives, such as triangular strips and quadrilateral meshes,

A Rendering Algorithm for Discrete Volume Density Objects

Abstract: : We present a new algorithm for simulating the effect of light travelling through volume objects. Such objects (haze, fog, clouds…) are usually modelized by voxel grids which define their density distribution in a discrete tridimensional space. The method we propose is a two-pass Monte-Carlo ray-tracing algorithm that does not make any restrictive assumptions neither about the characteristics of the objects (both arbitrary density distributions and phase functions are allowed) nor about the physical phenomena included in the rendering process (multiple scattering is accounted for). The driving idea of the algorithm is to use the phase function for Monte-Carlo sampling, in order to modify the direction of the ray during scattering.

Photo‐Realistic Imaging of Digital Terrains

Abstract: We present a method for the generation of photo-realistic images of views over terrain datasets by mapping a digital aerial photograph on a perspective projection of a digital elevation map. We use high resolution for both digital maps to increase the quality and the realism of the image at the cost of the overhead of processing very large data bases. In the core of this paper we present an accelerated ray casting technique based on a new algorithm of traversing a pyramidal data structure. Unlike other known traversal techniques, the cost of a single step of the algorithm consists of a few additions, shifts and comparisons only.

An Integral Geometry Based Method for Fast Form‐Factor Computation

Abstract: Monte Carlo techniques have been widely used in rendering algorithms for local integration. For example, to compute the contribution of a patch to the luminance of another. In the present paper we propose an algorithm based on Integral geometry where Monte Carlo is applied globally. We give some results of the implementation to validate the proposition and we study the error of the technique, as well as its complexity.

Landscapes Synthesis Achieved through Erosion and Deposition Process Simulation

Abstract: This paper describes an original approach to terrain evolution in landscapes synthesis. In order to create some realistic landforms, we simulate geologically contrasted terrains and apply to them deterministic erosion processes. This allows us to relate the erosion on any point of the landsurface to local geological parameters. Any height field may be chosen as an initial topographic surface. Small perturbations may be introduced to avoid unpleasant regularities. A 3D model defines the geological parameters of each point according to its elevation. Our method is iterative: at each step, rock removal and possible alluvial deposition are computed at each point of the landsurface. The available erosion laws simulate mechanical erosion, chemical dissolution and alluvial deposition. At the end of each iteration, a new landsurface and the corresponding river network are created. Landsurfaces can be visualized at the final stage by two rendering algorithms including natural textures mapping. The stream network and the ridges may also be visualized.

The Potential Equation and Importance in Illumination Computations

Abstract: An equation adjoint to the luminance equation for describing the global illumination can be formulated using the notion of a surface potential to illuminate the region of interest. This adjoint equation which we shall call as the potential equation, is fundamental to the adjoint radiosity equation used to devise the importance driven radiosity algorithm. In this paper we first briefly derive the adjoint system of integral equations and then show that the adjoint linear equations used in the above algorithm are basically discrete formulations of the same. We also show that the importance entity of the linear equations is basically the potential function integrated over a patch. Further we prove that the linear operators in the two equations are indeed transposes of each other.

Modeling of Skylight and Rendering of Outdoor Scenes

Abstract: Photorealistic animated images are extremely effective for pre-evaluating visual impact of city renewal and construction of tall buildings. In order to generate a photorealistic image not only the direct sunlight but also skylight must be considered. This paper proposes a method of high-fidelity image generation for photorealistic outdoor scenes based on the following ideas: 1 The intensity distribution of skylight taking account of scattering and absorption due to particles in the atmosphere which coincides with CIE standard skylight luminance functions is sought, and realistic images considering about spectrum distribution of skylight for any altitude of the sun can be easily and accurately displayed. 2 A rectangular parallelepiped with a specialized distribution of intensity simulating the skylight is introduced for efficient calculation of illumination due to skylight, and by employing a graphics hardware calculation of the skylight illuminance taking into account shadow effects is obtained with high efficiency; these techniques can be used to generate sequences of images, making animations possible at far lower calculation cost than previous methods.

Verve. Voxel Engine for Real-time Visualization and Examination

Abstract: The design principles of a hardware accelerator for volume rendering are described. The architecture represents a voxel subsystem which interfaces easily to any existing workstation. Host requirements are low since it contains a multiport memory holding the complete data set and all arithmetic units needed to perform an effective visualization. Our approach aims at virtual reality by providing some “real-world’’ examination techniques. The user (e.g., a physician) is enabled to analyze the data set from an arbitrary viewpoint and, even more, to “walk through ” the volume model. For a realistic impression, the machine produces perspective projections, supports the illumination by non-parallel light coming from a freely movable point light source and provides depth cueing. The objects are Phong shaded at a rate of operations/s and can be displayed semitransparently. One unit achieves interactive speed: for real-time operation only a small number of units (typically 4-16) must be placed in parallel.

Automatic Reconstruction of Buildings from Stereoscopic Image Sequences

Abstract: A vision–based 3-D scene analysis system is described that is capable to model complex real–world scenes like streets and buildings automatically from stereoscopic image pairs. Input to the system is a sequence of stereoscopic images taken with two standard CCD Cameras and TV lenses. The relative orientation of both cameras to each other is known by calibration. The camerapair is then moved throughout the scene and a long sequence of closely spaced views is recorded. Each of the stereoscopic image pairs is rectified and a dense map of 3-D suface points is obtained by area correlation, object segmentation, interpolation, and triangulation. 3-D camera motion relative to the scene coordinate system is tracked directly from the image sequence which allows to fuse 3-D surface measurements from different viewpoints into a consistent 3-D model scene. The surface geometry of each scene object is approximated by a triangular surface mesh which stores the suface texture in a texture map. From the textured 3-D models, realistic looking image sequences from arbitrary view points can be synthesized using computer graphics.

Scale-Invariant Minimum-Cost Curves: Fair and Robust Design Implements

Abstract: Four functionals for the computation of minimum cost curves are compared. Minimization of these functionals result in the widely studied Minimum Energy Curve (MEC), the recently introduced Minimum Variation Curve (MVC), and their scale-invariant counterparts, (SI-MEC, SI-MVC). We compare the stability and fairness of these curves using a variety of simple interpolation problems. Previously, we have shown MVC to possess superior fairness. In this paper we show that while MVC have fairness and stability superior to MEC they are still not stable in all configurations. We introduce the SI-MVC as a stable alternative to the MVC. Like the MVC, circular and helical arcs are optimal shapes for the SI-MVC. Additionally, the application of scale invariance to functional design allows us to investigate locally optimal curves whose shapes are dictated solely by their topology, free of any external interpolation or arc length constraints.

The Generation of Straight Lines on Hexagonal Grids

Abstract: The best disposition of a discrete set of points on the plane can be reached if the points are on a hexagonal grid. This paper describes an algorithm for the generation of straight lines on hexagonal grids. It uses only integer arithmetic.

A New Approach for the Generation of Circles

Abstract: Two incremental algorithms for drawing circles using short line segments are proposed in this paper. The proposed algorithms can draw manypoints at one time. They use only addition, subtraction, and shift operations except in the initialization of the algorithm. Using this simplified approach we can greatly increase the speed of drawing circles, relative to other conventional algorithms.

A Dynamic Gesture Language and Graphical Feedback for Interaction in a 3D User Interface

Abstract: In user interfaces of modern systems, users get the impression of directly interacting with application objects. In 3D based user interfaces, novel input devices, like hand and force input devices, are being introduced. They aim at providing natural ways of interaction. The use of a hand input device allows the recognition of static poses and dynamic gestures performed by a user's hand. This paper describes the use of a hand input device for interacting with a 3D graphical application. A dynamic gesture language, which allows users to teach some hand gestures, is presented. Furthermore, a user interface integrating the recognition of these gestures and providing feedback for them, is introduced. Particular attention has been spent on implementing a tool for easy specification of dynamic gestures, and on strategies for providing graphical feedback to users' interactions. To demonstrate that the introduced 3D user interface features, and the way the system presents graphical feedback, are not restricted to a hand input device, a force input device has also been integrated into the user interface.

The Generation of Circular Arcs on Hexagonal Grids

Abstract: The best disposition of a discrete set of points on the plane can be reached if the points are on a hexagonal grid. This paper describes two algorithms for circular arc mesh point selection on hexagonal grids. They find the closest integer coordinates to the actual circular arc using only integer arithmetic. Error criteria are explicitly specified.

3D Computational Morphology

Abstract: Computational Morphology is the analysis of form by computational means. This discipline typically uses techniques from Computational Geometry and Computer Aided Geometric Design. The present paper is more specifically about the construction and manipulation of closed object boundaries through a set of scattered points in 3D. Original results are developed in three stages of computational morphology: impose a geometrical structure on the set of points; construct a polyhedral boundary surface from this geometrical structure; build a hierarchy of polyhedral approximations together with localization information; The economic advantage of this approach is that there is no dependency on any specific data source. It can be used for various types of data sources or when the source is unknown.

Direct 3D Manipulation Techniques for Interactive Constraint‐based Solid Modelling

Abstract: This paper presents a novel constraint-based 3D manipulation approach to interactive constraint-based solid modelling. This approach employs a constraint recognition process to automatically recognise assembly relationships and geometric constraints between entities from 3D manipulation. A technique referred to as allowable motion is used to achieve accurate 3D positioning of a solid model by automatically constraining its 3D manipulation without menu interaction. A set of virtual design tools, which can be used to construct constraint-based solid models within a virtual environment, are also supported. These tools have been implemented as functional 3D objects associated with several pre-defined modelling functions to simulate physical tools such as a drilling tool and T-square. They can be directly manipulated by the user, and precisely positioned relative to other solid models through the constraint-based 3D manipulation approach. Their modelling functions can be automatically triggered, depending upon their associated constraints and the user's manipulation manner. A prototype system has been implemented to demonstrate the feasibility of these techniques for model construction and assembly operations.

Interactive Neural Network Texture Analysis and Visualization for Surface Reconstruction in Medical Imaging

Abstract: The following paper describes a new approach for the automatic segmentation and tissue classification of anatomical objects such as brain tumors from magnetic resonance imaging (MRI) data sets using artificial neural networks. These segmentations serve as an input for 3D–reconstruction algorithms. Since MR images require a careful interpretation of the underlying physics and parameters, we first give the reader a tutorial style introduction to the physical basics of MR technology. Secondly, we describe our approach that is based on a two–pass method including non–supervised cluster analysis, dimensionality reduction and visualization of the texture features by means of nonlinear topographic mappings. An additional classification of the MR data set can be obtained using a post–processing technique to approximate the Bayes decision boundaries. Interactions between the user and the network allow an optimization of the results. For fast 3D–reconstructions, we use a modified marching cubes algorithm but our scheme can easily serve as a preprocessor for any kind of volume renderer. The applications we present in our paper aim at the automatic extraction and fast reconstruction of brain tumors for surgery and therapy planning. We use the neural networks on pathological data sets and show how the method generalizes to physically comparable data sets.

Ray Tracing an Octree: Numerical Evaluation of the First Intersection

Abstract: An algorithm is presented which finds the first intersection of a directed semi-infinite straight-line (called ray) with an octree, without resorting to the evaluation of neighbouring nodes Given a pointer-based region octree, intersections of the ray with a node's bisecting planes are first evaluated to determine in which sub-octants the ray-node intersections may lie; a local ordering then determines the sequence in which these sub-octants should be examined so that the intersection closest to the ray's origin can be selected This idea is applied recursively starting from the root of the octree; the novelty of the approach consists of the fact that the choice of each node's child in the octree is directly derived from the intersection of the ray-segment within the tested sub-octant, thus avoiding searching for any neighbouring nodes The problem of selecting the correct octant while using the commonly available floating-point arithmetic is also addressed in a ray tracing environment Comparisons of this approach with the method given by Samet17 are carried out in a variety of cases involving millions of voxels The improvement ranged from 32% to 62% in terms of execution time

An Object Model for Multimedia Programming.

Abstract: The development of multimedia applications is a complex task. Much of this complexity stems from requirements associated with programming multiple media objects and the control of dependences and inter-relationships between these media objects and the user(s). It is clearly necessary to have a basic framework on which to build multimedia applications in the face of such complexities. Such a conceptual model is what we have called an object model and it is the main subject of this paper. The MADE object model represents a novel approach to multimedia application programming that is founded on the two principal concepts of active objects and delegation. Although these concepts are not novel in themselves, we believe that their combined use in a multimedia development environment represents a substantial enhancement to more traditional approaches to programming in this area.

Accurate and Consistent Reconstruction of Illumination Functions Using Structured Sampling

Abstract: The study of common classes of diffuse emitters, such as planar convex polygons, reveals several interesting properties of the functions of illumination these emitters cast on receiver surfaces. Some properties, such as the position of the maximum and the curvature are of particular interest for sampling and reconstruction of illumination across receivers. A computationally efficient approach is presented that identifies these properties, and uses them to select samples of illurnination. In addition these properties are used to determine upper bounds on the error due to linear and quadratic interpolants. These bounds are then used to adaptively subdivide the non-uniform sampling grid, resulting in accurate reconstruction. Results show that the method reduces the error compared to uniform approaches, and produces more consistent animated sequences.

Symmetrical Patterns from Dynamics

Abstract: Automatic generation of symmetrical patterns belonging to all seventeen wallpaper groups is considered from a dynamical system's point of view A pseudo-code of the algorithm is provided to generate the computer graphics The method is shown to be able to create unlimited varieties of patterns and demonstrates the possibility of relaxing the crystallographic restriction to produce aperiodic patterns

Graphic Objects: A Mathematical Abstract Model for Computer Graphics

Abstract: This paper defines an abstract mathematical model for graphic systems which incorporates modelling operations. The model is based on the “graphic object” concept. A graphic object is an abstraction of a graphic entity. Graphic objects are a generalization of the Fiume graphic object concept, supporting modelling operations, which give the set of graphic objects an algebraic structure of vector space and booelan algebra. Fiume visualization formalism can be applied to graphic objects. Further, product operations of graphic objects are defined, which are a generalization of solid sweep operations. Graphic object products may be used to build common 2D and 3D objects such as triangles, squares, cubes, circles and spheres from lines and arcs.

Cooperative Visualization of Computational Fluid Dynamics

Abstract: Tempus Fugit/Interview is a computational fluid dynamics visualization application for which processing is distributed between high performance graphics workstations and supercomputers. Facilities are provided in the application for more than one user to view shared images creating a cooperative visualization environment. The way in which the computation is partitioned between the super computer and the workstations is critical to the capability of the application to present simultaneous, identical, animated images of fluid dynamics to more than one user.

A Prototype of a Cooperative Visualization Workplace for the Aerodynamicist

Abstract: Several feasibility studies for cooperative visualization were performed based on an existing modular visualization system. Two different architectures for the implementation of a cooperative system were identified, implemented and tested. One architectural model, the broadcast model, assumes either one single visualization system running on a central facility driving multiple displays or in case of cooperative work it assumes that each visualization module drives multiple displays. The second model, the synchronized visualization systems, features distributed visualization tools with synchronization mechanisms. The synchronization ensures that all partners create identical images from identical data. This model requires special control features to avoid conflicts created by user-interaction. Both architectures are complete by adding bi-directional video connection and audio communication over the network plus offering remote cursor functionality. Two prototype implementations were produced to compare the two architectures and to evaluate the feasibility of cooperative visualization on existing hardware. Because of the poor access to wide-area networks, the prototypes were tested in a local area network. Visualization protocols, data communication, video and audio ran on a standard Ethernet based on TCP/IP communications. The tests were done using typical data from aerodynamics or numerical flow simulations. In spite of the limits due to low network bandwidth and slow performance of some hardware components the second architecture using the synchronized systems seems to be useful and feasible even under today's circumstances. There are no additional features identified which would be needed to run the cooperative visualization on a wide area network but a higher network throughput would be required.

Extracting contour lines from a hierarchical surface model

Abstract: The Hierarchical Triangulated Irregular Network (HTIN) is a structure for representing 2½-dimensional surfaces at different levels of detail through piecewise-linear approximations based on triangulations of the surface domain. In this paper, we present two algorithms that allow extracting a representation of the surface and contour lines at a given level of detail, directly from the HTIN.

A Declarative Design Method for 3D Scene Sketch Modeling

Abstract: In this paper, we present a dynamic model associated with an intelligent CAD system aiming at the modeling of an architectural scene sketch. Our design methodology has been developed to simulate the process of a user who tries to give a description of a scene from a set of mental images. The scene creation is based on a script which describes the environment from the point of view of an observer who moves across the scene. The system is based on a declarative method viewed as a stepwise refinement process. For the scene representation, a qualitative model is used to describe the objects in terms of attributes, functions, methods and components. The links between objects and their components are expressed by a hierarchical structure, and a description of spatial configurations is given by using locative relations. The set of solutions consistent with the description is usually infinite. So, either one scene consistent with this description is calculated and visualized, or reasons of inconsistency are notified to the user. The resolution process consists of two steps: firstly a logical inference checks the consistency of the topological description, and secondly an optimization algorithm deals with the global description and provides a solution. Two examples illustrate our design methodology and the calculation of a scene model.

A Multiprocessor Implementation of Radiosity

Abstract: This paper describes a multi-processor implementation of form factor computation in the radiosity method. Form factors are computed using the ray casting method and the algorithm is enhanced with Binary Space Partition (BSP) Trees and bounding boxes. Experimental results are presented together with a discussion of load balance efficiency.