Computer Graphics Laboratory
Computer Science Department
Stanford University

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We present Vispedia, a system that reduces the cost of data integration, enabling casual users to build ad hoc visualizations of Wikipedia data. Users can browse Wikipedia, select an interesting data table, then interactively discover, integrate, and visualize additional related data on-demand through a search interface and a query recommendation engine. This is accomplished through a fast path search algorithm over a semantic graph derived from Wikipedia. Vispedia also supports exporting the augmented data tables produced for use in more traditional visualization systems. We believe that these techniques begin to address the "long tail" of visualization by allowing a wider audience to visualize a broader class of data.

Both the Resource Description Framework (RDF), used in the semantic web, and Maya Viz u-forms represent data as a graph of objects connected by labeled edges. Existing systems for flexible visualization of this kind of data require manual specification of the possible visualization roles for each data attribute. When the schema is large and unfamiliar, this requirement inhibits exploratory visualization by requiring a costly up-front data integration step. To eliminate this step, we propose an automatic technique for mapping data attributes to visualization attributes. We formulate this as a schema matching problem, finding appropriate paths in the data model for each required visualization attribute in a visualization template.

The most widely used techniques for interactive rendering of hard shadows are shadow maps and shadow volumes. We observe that shadow map algorithms often suffice to determine shadowing for most areas of a rendered image. However, objectionable aliasing artifacts can arise in neighborhoods near the shadow silhouettes. In contrast, shadow volumes generate precise silhouettes, but perform significant amounts of extra work even for pixels far from shadow edges. In this paper, we propose the method of silhouette maps, in which a shadow depth map is augmented by storing the locations of representative points on the geometric silhouette. A piece-wise linear approximation to the true silhouette can then be found by applying dual-contouring techniques. The proposed method allows each silhouette map texel to represent a polygonal approximation to the actual shadowing geometry, in contrast to the jagged approximation provided by a conventional shadow map. We demonstrate an implementation of our approach running on programmable graphics hardware at interactive rates.

Light scattering from hair is normally simulated in computer graphics using Kajiya and Kay?s classic phenomenological model. We have made new measurements that exhibit visually significant effects not predicted by Kajiya and Kay?s model. Our measurements go beyond previous hair measurements by examining out-of-plane scattering, and together with this previous work they show a multiple specular highlight and variation in scattering with rotation about the fiber axis. We explain the sources of these effects using a model of a hair fiber as a transparent elliptical cylinder with an absorbing interior and a surface covered with tilted scales. Based on an analytical scattering function for a circular cylinder, we propose a practical shading model for hair that qualitatively matches the scattering behavior shown in the measurements. In a comparison between a photograph and rendered images, we demonstrate the new model?s ability to match the appearance of real hair.

Overview slides:				Older images

We present a modified photon mapping algorithm capable of running entirely on GPUs. Our implementation uses breadth-first photon tracing to distribute photons using the GPU. The photons are stored in a grid-based photon map that is constructed directly on the graphics hardware using one of two methods: the first method is a multipass technique that uses fragment programs to directly sort the photons into a compact grid. The second method uses a single rendering pass combining a vertex program and the stencil buffer to route photons to their respective grid cells, producing an approximate photon map. We also present an efficient method for locating the nearest photons in the grid, which makes it possible to compute an estimate of the radiance at any surface location in the scene. Finally, we describe a breadth-first stochastic ray tracer that uses the photon map to simulate full global illumination directly on the graphics hardware. Our implementation demonstrates that current graphics hardware is capable of fully simulating global illumination with progressive, interactive feedback to the user.

Movie:		(1.0 MB)

The photon map technique for global illumination does not specifically address animated scenes. In particular, prior work has not considered the problem of temporal sampling (motion blur) while using the photon map. In this paper we examine several approaches for simulating motion blur with the photon map. In particular we show that a distribution of photons in time combined with the standard photon map radiance estimate is incorrect, and we introduce a simple generalization that correctly handles photons distributed in both time and space. Our results demonstrate that this time dependent photon map extension allows fast and correct estimates of motion-blurred illumination including motion-blurred caustics.

Slides:		(1.8 MB)