C. W. A. M. van Overveld

The DenK-architecture : a fundamental approach to user-interfaces

In this paper we present the basic principles underlying the DenK-system, a generic cooperative interface combining linguistic and visual interaction. The system integrates results from fundamental research in knowledge representation, communication, natural language semantics and pragmatics, and object-oriented animation. Our design incorporates a cooperative and knowledgeable electronic assistant that communicates with a user in natural language, and an application domain, which is presented visually. The assistant, that we call thecooperator, has an information state that is represented in a rich form of Type Theory, a formalism that enables us to model the inherent cognitive dynamics of a dialogue participant. Pragmatic issues in man-machine interaction, concerning the use of natural language and knowledge in cooperative communication, are central to our approach.

An algorithm for polygon subdivision based on vertex normals

In order to achieve the impression of a smooth surface while rendering a polygon mesh, normal vector vectors may be provided in the vertices of the mesh that are the average of the surface normals of the adjacent polygons. Interpolation of these normal vectors while rendering of the polygons in the mesh, and using the interpolated normal vectors in the shading computations, yields a smoothly varying intensity distribution. There is an inherent mismatch, however, between the smoothness of the shading thus achieved and the non-smoothness of the geometry which is particularly visible at silhouettes, showing as straight edges and non-smooth edge junctions at the silhouette vertices. A remedy for these artefacts is suggested. The remedy consists of subdividing each input polygon into a mesh of polygons prior to rendering. The shape of this resulting polygon mesh is controlled by the normal vectors that are provided in the vertices of the original polygon, unlike other subdivision schemes that make use of adjacent polygons. With the method, polygons equipped with vertex normal vectors can therefore be processed without further knowledge of neighbour polygons. This makes the method well-suited in the context of graphics libraries, such as OpenGl, that treat polygons typically on a per-polygon basis. So the proposed computation of the mesh which replaces the original polygon can be viewed as a filter which may operate as a process in front of a traditional polygon rendering pipeline.

Small steps for mankind: Towards a kinematically driven dynamic simulation of curved path walking

A simple approach for the simulation of bipedal locomotion is presented. It is based on a kinematically driven rigid body dynamics simulation. As inputs, our system uses a 14-DOF simplified human body model, and the instantaneous walking direction and the gait length. From these parameters, the trajectories of the left and right feet are computed over time, using a purely kinematical recipe. Optionally, trajectories of other body parts (head, shoulders, hips, hands, knees, etc.) may be derived from a simple kinematic body model. Using these additional trajectories provides for increased control over the motion, adding extra kinematic constraints to the dynamical system. The trajectories, together with optional force fields (gravity etc.), serve to drive the motion of the articulated human body model. This means that for those points in the body that have not been specified kinematically, a rigid body dynamics calculation is used to describe their motion over time. An implementation of these ideas turns out to allow a frame update rate of about 20 Hz on a personal IRISTM workstation, which is sufficiently fast for real-time interaction.

Scattering of 22.3 MeV polarized protons from the even-mass cadmium isotopes 106-116Cd

Measurements have been done on polarized-proton scattering from the even-mass cadmium isotopes 106-116Cd, at a beam energy of 22.3 MeV. The experimental data were analysed using first- and second-order vibrational models in a coupled-channels formalism. The 01+, 21+ and 31- levels are represented satisfactorily by theory. In 106-116Cd the 22+ and 41+ levels were resolved; for the 22+ levels the theory systematically underestimates the first diffraction maximum of the cross section at 30 degrees while the 41+ levels are reproduced reasonably well. In 112-114Cd the 02+ level could also be studied; there are indications for a strong coupling between the two-phonon 0+ state in the ground-state band and the 0+ head of the intruder band. In 114Cd we also investigated the 23+ level, probably the first excited state of the intruder system, which showed an angular pattern with more pronounced details than the 21+ states. Finally in 112Cd the 11

The generalized display processors as an approach to real‐time interactive 3‐D computer animation

and 51- levels were studied which have a strong direct coupling to the ground state.

A simple approximation to rigid body dynamics for computer animation

The concept of the display processor unit (DPU; see Reference 1, p. 94) is extended to arrive at a generalized display processor (GDP) which is able not only to produce a static screen image given the descriptions of a list of graphics elements but which allows for these graphics elements to show an autonomous kinematic (motion-)behaviour. A language to program such a GDP is presented, and a software implementation of an interpreter/processor for this language is discussed. The application of a GDP both in computer animation and in interactive process control applications is sketched.

A Recursive Subdivision Algorithm for Piecewise Circular Spline

Recently, the dynamics of linked articulated rigid bodies has become a valuable tool for making realistic three-dimensional computer animations. An exact treatment of rigid body dynamics, however, is based on rather non-intuitive results from classical mechanics (e.g. the Euler equations for rotating bodies) and it relies heavily on sophisticated numerical schemes to solve (large) sets of coupled non-linear algebraic and differential equations. As a result, articulated rigid bodies are not yet supported by most real-time animation systems. This paper discusses an approach to rigid body dynamics which is based on (both conceptually and algorithmically much simpler) point mechanics; this gives rise to an asymptotically exact numerical scheme (NSI) which is useful in the context of real-time animation, provided that the number of degrees of freedom of the simulated system is not too large. Based on NSI, a second scheme (NS2) is derived which is useful for approximating the motions of linked articulated rigid bodies; NS2 turns out to be sufficiently fast to give at least qualitative results in real-time simulation. In general, the algorithm NS2 is not necessarily (asymptotically) exact, but a quantitative analysis shows that in the absence of reaction forces it conserves angular momentum.

Sticky splines: definition and manipulation of spline structures with maintained topological relations

We present an algorithm for generating a piecewise G1 circular spline curve from an arbitrary given control polygon. For every corner, a circular biarc is generated with each piece being parameterized by its arc length. This is the first subdivision scheme that produces a piecewise biarc curve that can interpolate an arbitrary set of points. It is easily adopted in a recursive subdivision surface scheme to generate surfaces with circular boundaries with pieces parameterized by arc length, a property not previously available. As an application, a modified version of Doo–Sabin subdivision algorithm is outlined making it possible to blend a subdivision surface with other surfaces having circular boundaries such as cylinders.

Hanging cloths and dangling rods: A unified approach to constraints in computer animation

This paper describes an augmentation to the spline concept to account for topological relations between different spline curves. These topological relations include incidence relations, constraining the extremes of spline curves to other spline curves, and also more general geometric relations, for example, involving the tangents of spline curves in their extremes. To maintain these incidence relations, some spline curves may have to be transformed (translated, rotated, scaled), or even deformed (i.e., the shape of the curve may change) as a result of modifying other spline curves. A data structure and algorithms are given to implement the propagation of these transformations and deformations. Based on the augmented spline concept, to be called sticky splines, both a script system to represent spline structures and an interactive system for editing drawings while automatically, maintaining their topological structure are presented.

A family of algorithms for generating discrete embeddings of continuous objects

A simple unified approach for dealing with constraints on real-valued parameters in computer simulations and computer animation is described. The main characteristics of the approach are its flexibility (it works for a variety of constraint-types and it can be implemented to handle both under- and over-constrained systems in a stable way) and its ease for distributed computation. The method is based on the idea of genetic algorithms. The mathematical merits of the method for one constraint-type are discussed in some detail and some examples of applications are given.