Volume 14 (1995)

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3D Interaction with the Desktop Bat
(Blackwell Science Ltd and the Eurographics Association, 1995) Steed, Anthony; Slater, Mel
Many applications now demand interaction with visualizations of 3D scenes and data sets. Current flat 2D displays are limited in their capacity to provide this not only by the display technology but the interaction metaphors and devices used. The Desktop Bat is a device that has 5 degrees of freedom whilst retaining the simplicity of use o fa mouse. To use it for general 3D interaction several metaphors were created for the tasks of navigation and cursor manipulation and a set of experiments were conducted to determine which metaphors were the most efficient in use. Of these metaphors, a velocity control metaphor was the best for navigation and a metaphor that applied rotations and translations relative to the eyepoint coordinate system was best for object control.
An Adaptive Spatial Subdivision of the Object Space for Fast Collision Detection of Animated Rigid Bodies
(Blackwell Science Ltd and the Eurographics Association, 1995) Bandi, Srikanth; Thalmann, Daniel
Collision detection tests between objects dominate run time simulation of rigid body animation. Traditionally, hierarchical bounding box tests are used to minimize collision detection time. But the bounding boxes do not take shapes of the objects into account which results in a large number of collision detection tests. We propose an adaptive spatial subdivision of the object space based on octree structure to rectify this problem. We also present a technique for efficiently updating this structure periodically during the simulation.
An Algorithm for Perspective Viewing of Objects Represented by Octrees
(Blackwell Science Ltd and the Eurographics Association, 1995) Aref, Walid G.; Samet, Hanan
A new algorithm is presented for viewing three-dimensional objects, represented by an octree, from an arbitrary location. The algorithm generates aperspective view of the objects while eliminating hidden surfaces. The viewer can be located anywhere inside or outside the objects. The algorithm presented in this short notefixes an artifact that is generated by a previously published algorithm due to Meagher when the viewer is located in certain regions in space. The new algorithm traverses the octree in a back-to-front order and recursively chooses correct orders for visiting the sons of non-leaf nodes.
Algorithms for Extracting Correct Critical Points and Constructing Topological Graphs from Discrete Geographical Elevation Data
(Blackwell Science Ltd and the Eurographics Association, 1995) Takahashi, Shigeo; Ikeda, Tetsuya; Shinagawa, Yoshihisa; Kunii, Tosiyasu L.; Ueda, Minoru
Researchers in the fields of computer graphics and geographical information systems (GISs) have extensively studied the methods of extracting terrain features such as peaks, pits, passes, ridges, and ravines from discrete elevation data. The existing techniques, however, do not guarantee the topological integrity of the extracted features because of their heuristic operations, which results in spurious features. Furthermore, there have been no algorithms for constructing topological graphs such as the surface network and the Reeb graph from the extracted peaks, pits, and passes. This paper presents new algorithms for extracting features and constructing the topological graphs using the features. Our algorithms enable us to extract correct terrain features; i.e., our method extracts the critical points that satisfy the Euler formula, which represents the topological invariant of smooth surfaces. This paper also provides an algorithm that converts the surface network to the Reeb graph for representing contour changes with respect to the height. The discrete elevation data used in this paper is a set of sample points on a terrain surface. Examples are presented to show that the algorithms also appeal to our visual cognition.
Automatic Reconstruction of Unstructured 3D Data: Combining a Medial Axis and Implicit Surfaces
(Blackwell Science Ltd and the Eurographics Association, 1995) Bittar, Eric; Tsingos, Nicolas; Gascuel, Marie-Paule
This paper presents a new method that combines a medial axis and implicit surfaces in order to reconstruct a 3D solid from an unstructured set of points scattered on the object s surface. The representation produced is based on iso-surfaces generated by skeletons, and is a particularly compact way of defining a smooth free-form solid. The method is based on the minimisation of an energy representing a"distance" between the set of data points and the iso-surface, resembling previous reserach19. Initialisation, however, is more robust and efficient since there is computation of the medial axis of the set of points. Instead of subdividing existing skeletons in order to refine the object s surface, a new reconstruction algorithm progressively selects skeleton-points from the pre- computed medial axis using an heuristic principle based on a"local energy" criterion. This drastically speeds up the reconstruction process. Moreover, using the medial axis allows reconstruction of objects with complex topology and geometry, like objects that have holes and branches or that are composed of several connected components. This process is fully automatic. The method has been successfully applied to both synthetic and real data.
Bresenham's Line Generation Algorithm with Built-in Clipping
(Blackwell Science Ltd and the Eurographics Association, 1995) Kuzmin, Yevgeny P.
One of the most important operations in many graphical systems is the generation of a line segment. This process consists of two stages: clipping and drawing. These two stages are separated in current graphical applications. In this paper a new approach to line generation is proposed, which unifies these stages. The proposed algorithm is based on Bresenham s line generation algorithm to include necessary line clipping. The line clipping stage is an operation-reduced, integer arithmetic only algorithm. The notion of correctness of line clipping is introduced and correctness of the proposed algorithm is shown. Complete C-notation of the algorithm is included.
Collision Detection for Animation using Sphere-Trees
(Blackwell Science Ltd and the Eurographics Association, 1995) Palmer, I. J.; Grimsdale, R. L.
The detection of collisions between moving polyhedral objects is one of the most computationally intensive tasks in the computer animation process. The use of object-oriented techniques to encapsulate data within the objects structures compounds this problem through the requirement for inter-object message passing in order to obtain geometric information for collision detection. The REALISM system decreases the time for collision detection by using a three stage process. The first stage identifies objects in the same locality using a global bounding volume table. The second stage locates regions of possible collision using a sphere-tree data structure (a hierarchical tree of spheres based on octree-type spatial subdivision). The final stage finds intersections between polygonal faces of the objects that are contained within the intersecting pairs of leaf nodes. Hence the algorithm uses a spherical geometry approximation rapidly to locate regions of potential collisions and then uses a local intersection test with actual object geometry information. The system is therefore fast and accurate. Tests for various geometric objects support this and show performance improvements of jive times over traditional polyhedral intersection tests.
Colouration Issues in Computer Generated Facial Animation
(Blackwell Science Ltd and the Eurographics Association, 1995) Patel, M.
In everyday interactions with one another we use the face for recognising people and for communicating with them. Despite the considerable amount of research into computer generated facial animation, one particular aspect, that of the colouration of the face appears to have been neglected. In this paper we address issues pertinent to the use of colour for both modelling the appearance of the face and for enhancing communication during facial expression and animation. Colouration is an integral part of the face, which helps in the recognition of faces as well as in the interpretation of the often subtle signals emitted by the human face.
The Compositing Buffer: A Flexible Method for Image Generation and Image Editing
(Blackwell Science Ltd and the Eurographics Association, 1995) Lau, Wing Hung; Wiseman, Neil
In this paper, we describe a new buffer architecture called the Compositing Buffer. Although it is based on the A-buffer architecture described by Carpenter1, the new architecture introduces two important ideas which make image generation more flexible. The first idea is the storing of bitmask index instead of the bitmask. This allows accurate images to be generated while at the same time, minimising memory usage. The second idea is the introduction of the concept of dynamic object. This allows images to be edited interactively. Applications of the two ideas will also be discussed in the paper.
A Direct Manipulation Interface for 3D Computer Animation
(Blackwell Science Ltd and the Eurographics Association, 1995) Snibbe, Scott Sona
We present a new set of interface techniques for visualizing and editing animation directly in a single three-dimensional scene. Motion is edited using direct-manipulation tools which satisfy high-level goals such as"reach this point at this time" or"go faster at this moment". These tools can be applied over an arbitrary temporal range and maintain arbitrary degrees of spatial and temporal continuity.We separate spatial and temporal control of position by using two curves for each animated object: the motion path which describes the 3D spatial path along which an object travels, and the motion graph, a function describing the distance traveled along this curve over time. Our direct-manipulation tools are implemented using displacement functions, a straightforward and scalable technique for satisfying motion constraints by composition of the displacement function with the motion graph or motion path. This paper will focus on applying displacement functions to positional change. However, the techniques presented are applicable to the animation of orientation, color, or any other attribute that varies over time.
Discrete Ray-Tracing of Huge Voxel Spaces
(Blackwell Science Ltd and the Eurographics Association, 1995) Stolte, Nilo; Caubet, Rene
The quality of images produced by Discrete Ray-Tracing voxel spaces is highly dependent on 3d grid resolution. The huge amount of memory needed to store such grids often discards discrete Ray-Tracing as a practical visualization algorithm. The use of an octree can drastically change this when most of space is empty, as such is the case in most scenes.Although the memory problem can be bypassed using the octree, the performance problem still remains. A known fact is that the performance of discrete traversal is optimal for quite low resolutions. This problem can be easily solved by dividing the task in two steps, working in two low resolutions instead of just one high resolution, thus taking advantage of optimal times in both steps. This is possible thanks to the octree property of representing the same scene in several different resolutions. This article presents a two step Discrete Ray-Tracing method using an octree and shows, by comparing it with the single step version, that a substantial gain in performance is achieved.
Distributed Augmented Reality for Collaborative Design Applications
(Blackwell Science Ltd and the Eurographics Association, 1995) Ahlers, Klaus H.; Kramer, Andre; Breen, David E.; Chevalier, Pierre-Yves; Crampton, Chris; Rose, Eric; Tuceryan, Mihran; Whitaker, Ross T.; Greer, Douglas
This paper presents a system for constructing collaborative design applications based on distributed augmented reality. Augmented reality interfaces are a natural method for presenting computer-based design by merging graphics with a view of the real world. Distribution enables users at remote sites to collaborate on design tasks. The users interactively control their local view, try out design options, and communicate design proposals. They share virtual graphical objects that substitute for real objects which are not yet physically created or are not yet placed into the real design environment.We describe the underlying augmented reality system and in particular how it has been extended in order to support multi-user collaboration. The construction of distributed augmented reality applications is made easier by a separation of interface, interaction and distribution issues. An interior design application is used as an example to demonstrate the advantages of our approach.
Domain Extension of Isothetic Polyhedra with Minimal CSG Representation
(Blackwell Science Ltd and the Eurographics Association, 1995) Arinyo, Robert Juan
We consider the problem of converting boundary representations of isothetic polyhedra into constructive solid geometry (CSG) representations. The CSG representation is a boolean formula based on the half-spaces supporting the faces of the polyhedron. This boolean formula exhibits two important features: no term is complemented (it is monotone) and each supporting half-space appears in the formula once and only once. It is known that such formulas do not always exist for general polyhedra in the three-dimensional space. In this work first we give a procedure that extends the domain of polyhedra for which such a nice representation can be computed. Then we prove that not all cyclic isothetic polyhedra have a CSG representation of the style given above.
Efficient Parallel Gouraud Shading and Linear Interpolation over Triangles
(Blackwell Science Ltd and the Eurographics Association, 1995) Narayanaswami, Chandrasekhar
A parallel raster algorithm to draw Gouraud shaded triangles is presented. At the heart of the algorithm is a new constrained parallel edge-traversal technique. This parallel traversal represents an increased level of parallelism compared to the existing solutions. Next, traditional algorithms take different amounts of time to advance from one horizontal span to another for the left edge and the right edge of the triangle when the slope of one of the edges is more than one and that of the other edge is less than one. This causes one processor to wait for another processor. The parallel constrained edge traversal technique removes this problem by directly jumping from one span to the next. It also ensures that adjacent triangles that share an edge do not share any pixels. Moreover, no cracks occur between adjacent polygons. Unlike some existing algorithms whose complexity depends on the size of the bounding box of the triangle, the complexity of our algorithm is solely dependenton the perimeter and area of the triangle.Due to the above features, the algorithm presented here exposes a greater degree of parallelism at considerably lesser cost and achieves better processor utilization, compared to existing algorithms for this problem1, 2, 3, 4, 5, 6. The algorithm is well suited for hardware implementation.
Fair Surface Reconstruction Using Quadratic Functionals
(Blackwell Science Ltd and the Eurographics Association, 1995) Kolb, Andreas; Pottmann, Helmut; Seidel, Hans-Peter
An algorithm for surface reconstruction from a polyhedron with arbitrary topology consisting of triangular faces is presented. The first variant of the algorithm constructs a curve network consisting of cubic Bezier curves meeting with tangent plane continuity at the vertices. This curve network is extended to a smooth surface by replacing each of the networks facets with a split patch consisting of three triangular Bezier patches. The remaining degrees of freedom of the curve network and the split patches are determined by minimizing a quadratic functional. This optimization process works either for the curve network and the split patches separately or in one simultaneous step. The second variant of our algorithm is based on the construction of an optimized curve network with higher continuity. Examples demonstrate the quality of the different methods.
Fast Generation of Leakproof Surfaces from Well-Defined Objects by a Modified Marching Cubes Algorithm
(Blackwell Science Ltd and the Eurographics Association, 1995) Roll, Stefan; Haase, Axel; von Kienlin, Markus
Local surface reconstruction by the Marching Cubes algorithm and its derivatives has a well known ambiguity, which prevents constructed surfaces from being closed and simple. We investigate this ambiguity assuming that a 3D image samples well-defined objects. In this case it is justified to aim at tiling of extracted object voxels rather than at reconstructing iso surfaces. Compared to iso surface reconstruction, our algorithm provides essentially the same level of confidence with respect to surface location at a lower computational cost. We present a leak detection and mending scheme which resolves the Marching Cubes ambiguity and guarantees a well-defined behaviour with respect to which objects are covered by which surface. We detail how to implement our leak mending method within a completely tabulated Marching Cubes algorithm. We finally give an example of how the adapted algorithm is of benefit to a recently developed 3D MR spectroscopy technique.
Fast Shadowing Algorithm for Linear Light Sources
(Blackwell Science Ltd and the Eurographics Association, 1995) Tanaka, Toshimitsu; Takahashi, Tokiichiro
This paper presents a fast shadowing algorithm for linear light sources that uses a ray-oriented buffer. Space segmentation by the buffer guarantees that if a point is included in a subspace, all light rays toward the point are also contained in the subspace. Each cell of the buffer stores a list of objects that lie within or intersect the subspace allocated to the cell. Therefore, candidate objects, those that may cast shadows onto a point, are determined by referring to the cell where the point is mapped. In addition, whether each candidate object actually casts shadows or not is tested with the bounding-volume of the shadow space to reduce the number of objects subjected to expensive light clipping. The bounding-volumes are also stored in the buffer. For efficiently generating the ray-oriented buffer, we present the cylindrical scan-conversion algorithm. The algorithm preconverts objects surfaces to trapezia to decrease the light clipping cost, then connects the trapezia to the buffer cells.Due to the above improvements, our algorithm achieves over 10 times faster shadow generation compared to the conventional methods. Experimental results confirm that our method can generate realistic images with soft shadows in a few minutes.
Fast Wavelet Based Volume Rendering by Accumulation of Transparent Texture Maps
(Blackwell Science Ltd and the Eurographics Association, 1995) Lippert, L.; Gross, M. H.
In the following paper, a new method for fast and accurate volume intensity and color integration is elaborated, which employs wavelet decompositions and texture mapping. At this point, it comprises and unifies the advantages of recently introduced Fourier domain volume rendering techniques and wavelet based volume rendering. Specifically, the method computes analytic solutions of the ray intensity integral through a single wavelet by slicing its Fourier transform and by backprojecting it into the spatial domain. The resulting slices can be considered as RGB textures where R, G and B account for the decomposed volume color function. Due to the similarity of the basis functions, the computation of the texture map has to be figured out only once for each 3D mother wavelet. Hence, the final volume rendering procedure turns out to be a superposition of self-similar, transparent and colored textures, which is supported by modern hardware accumulation buffers. Linear shading and attenuation can be introduced by modifications of the wavelet s Fourier transform.The main advantages of this method are the provision of accurate solutions and quantification of error bounds, the absence of any expensive prefiltering and the independence of the computational costs from the image resolution. Furthermore, any required discretization, such as the resolution of the basis textures is defined within the computational framework of the wavelet transform. The method is not restricted to a specific type of wavelet unless is provides an analytic Fourier description, such as any B-spline wavelets do.
Filtering, Clustering and Hierarchy Construction: a New Solution for Ray-Tracing Complex Scenes
(Blackwell Science Ltd and the Eurographics Association, 1995) Cazals, Frederic; Drettakis, George; Puech, Claude
Data structures that handle very complex scenes (hundreds of thousands of objects) have in the past either been laboriously built by hand, or have required the determination of unintuitive parameter values by the user. It is often the case that an incorrect choice of these parameters can result in greedy memory requirements or severely degraded performance. As a remedy to this problem we propose a new data structure which is fully automatic since it does not require the user to determine any input parameters. The structure is built by first filtering the input objects by size, subsequently applying a clustering step to objects of the same size and finally building a hierarchy of uniform grids . We then show that this data structure can be efficiently constructed. The implementation of the shows that the new structure is stable since it s memory requirements grow linearly with the size of the scene, and that it presents a satisfactory compromise between memory usage and computational efficiency. A detailed comparison with previous data structures is also presented in the results.
Fractals and Quasi-Affine Transformations
(Blackwell Science Ltd and the Eurographics Association, 1995) Nehlig, P. W.; Reveilles, J.-P.
In the continuum , contracting affine transformations have a unique fixed point. It is well known that this property is not preserved by dicretization and that the dynamics of discretized functions are very complicated. Discrete geometry allows us to start a theory for these dynamics and to illustrate some of their features by pictures. These pictures, rendered by a simple algorithm, reveal a very large spectrum of fractal structures, from the simplest to the intricatest.

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