Sky models are quantitative representations of natural luminance of the sky under various atmospheric conditions. They have been used extensively in studies of architectural design for nearly a century, and more recently for rendering objects in the field of computer graphics. The objectives of this paper are to (1) describe sky models, (2) demonstrate how map designers can render terrain under various sky models in a typical geographic information system (GIS), (3) illustrate potential enhancements to terrain renderings using sky models, and (4) discuss how sky models, with their well-established standards from a different discipline, might contribute to a virtual geographic environment (VGE).Current GIS hill-shading tools use the Lambertian assumption which can be related to a simple point light source at an infinite distance to render terrain. General sky models allow the map designer to choose from a gamut of sky models standardized by the International Commission on Illumination (CIE). We present a computer application that allows the map designer to select a general sky model and to use existing GIS tools to illuminate any terrain under that model. The application determines the orientations and weights of many discrete point light sources that, in the aggregate, approximate the illumination provided by the chosen sky model. We discuss specific enhancements to terrains that are shaded and shadowed with these general sky models, including additional detail of secondary landforms with soft shadows and more realistic shading contrasts. We also illustrate how non-directional illumination models result in renderings that lack the perceptual relief effect. Additionally, we argue that this process of creating hill-shaded visualizations of terrain with sky models shows parallels to other geo-simulations, and that basing such work on standards from the computer graphics industry shows potential for its use in VGE.

We attached a small LCD display and video camera to a surgical drill. The LCD shows the tool position with respect to a planned trajectory, overlaid on video captured by the camera. We performed a user study to determine whether such a tool-mounted guidance display yields faster and more accurate tool placement than the conventional guidance display on a separate computer monitor. Our study showed that the tool-mounted display provides better positional and angular accuracy than the conventional display but that the video camera provides no significant improvement in error.

We present an algorithm to triangulate a multi-resolution hierarchical hexagon mesh. The triangulation provides good triangle strips, which result in efficient rendering of the hexagon mesh, and well proportioned triangles, which avoid rendering artifacts.

Sphere packing arrangements are frequently found in nature, exhibiting efficient space-filling and energy minimization properties. Close sphere packings provide a tight, uniform, and highly symmetric spatial sampling at a single resolution. We introduce the Multiresolution Sphere Packing Tree (MSP-tree): a hierarchical spatial data structure based on sphere packing arrangements suitable for 3D space representation and selective refinement. Compared to the commonly used octree, MSP-tree offers three advantages: a lower fanout (a factor of four compared to eight), denser packing (about 24% denser), and persistence (sphere centers at coarse resolutions persist at finer resolutions). We present MSP-tree both as a region-based approach that describes the refinement mechanism succintly and intuitively, and as a lattice-based approach better suited for implementation. The MSP-tree offers a robust, highly symmetric tessellation of 3D space with favorable image processing properties.

We present ECSGlasses: wearable eye contact sensing glasses that detect human eye contact. ECSGlasses report eye contact to digital devices, appliances and EyePliances in the user's attention space. Devices use this attentional cue to engage in a more sociable process of turn taking with users. This has the potential to reduce inappropriate intrusions, and limit their disruptiveness. We describe new prototype systems, including the Attentive Messaging Service (AMS), the Attentive Hit Counter, the first person attentive camcorder eyeBlog, and an updated Attentive Cell Phone. We also discuss the potential of these devices to open new windows of interaction using attention as a communication modality. Further, we present a novel signal-encoding scheme to uniquely identify EyePliances and users wearing ECSGlasses in multiparty scenarios.

This paper presents a shading model for volumetric data which enhances the perception of surfaces within the volume. The model incorporates uniform diffuse illumination, which arrives equally from all directions at each surface point in the volume. This illumination is attenuated by occlusions in the local vicinity of the surface point, resulting in shadows in depressions and crevices. Experiments by other authors have shown that perception of a surface is superior under uniform diffuse lighting, compared to illumination from point source lighting.

A terrain is most often represented with a digital elevation map consisting of a set of sample points from the terrain surface. This paper presents a fast and practical algorithm to compute the horizon, or skyline, at all sample points of a terrain. The horizons are useful in a number of applications, including the rendering of self-shadowing displacement maps, visibility culling for faster flight simulation, and rendering of cartographic data. Experimental and theoretical results are presented which show that the algorithm is more accurate that previous algorithms and is faster than previous algorithms in terrains of more than 100,000 sample points.

A new surface radiance model for diffuse lighting is presented which incorporates shadows, interreflections, and surface orientation. An algorithm is presented that uses this model to compute shape-from-shading under diffuse lighting. The algorithm is tested on both synthetic and real images, and is found to perform more accurately than the only previous algorithm for this problem.

No abstract available.

We present a surface radiance model for diffuse lighting that incorporates shadows, interreflections, and surface orientation. We show that, for smooth surfaces, the model is an excellent approximation of the radiosity equation. We present a new data structure and algorithm that uses this model to compute shape-from-shading under diffuse lighting. The algorithm was tested on both synthetic and real images, and performs more accurately than the only previous algorithm for this problem. Various causes of error are discussed, including approximation errors in image modelling, poor local constraints at the image boundary, and ill-conditioning of the problem itself.

This paper describes a fast, practical algorithm to compute the shadow boundaries in a polyhedral scene illuminated by a polygonal light source. The shadow boundaries divide the faces of the scene into regions such that the structure or “aspect” of the visible area of the light source is constant within each region. The paper also describes a fast, practical algorithm to compute the structure of the visible light source in each region. Both algorithms exploit spatial coherence and are the most efficient yet developed.

The architecture of Newton, a general-purpose system for simulating the dynamics of complex physical objects, is described. The system automatically formulates and analyzes equations of motion, and performs automatic modification of this system of equations when necessitated by changes in kinematic relationships between objects. Impact and temporary contact are handled, although only using simple models. User-directed influence of simulations is achieved using Newton's module, which can be used to experiment with the control of many-degree-of-freedom articulated objects. >

The authors suggest that there are two components required in most control schemes: an algorithmic component which takes a high-level goal and generates joint trajectories, and a dynamics component which takes these joint trajectories and generates the required joint torques. The joint torques are then sent to the mechanism, which (it is hoped) achieves the high-level goal. An approach is presented which simplifies the programming of the algorithmic component for high-degree-of-freedom objects. Instead of supplying a complete set of joint trajectories as a function of time, the algorithm controls other, more intuitive, degrees of freedom. The degrees of freedom are automatically converted to the more conventional joint trajectories. The algorithm can underconstrain the object, in which case constrained optimization is used in converting to joint trajectories. This approach is applied to generate joint trajectories for a walking biped. The walking algorithm, is presented along with the results from testing with the authors' Newton simulation system (ibid., vol.3, p.1806-11).>