Michael V. Capps

Surface reconstruction with anisotropic density-scaled alpha shapes

Generation of a three-dimensional model from an unorganized set of points is an active area of research in computer graphics. Alpha shapes can be employed to construct a surface which most closely reflects the object described by the points. However, no /spl alpha/-shape, for any value of /spl alpha/, can properly detail discontinuous regions of a model. We introduce herein two methods of improving the results of reconstruction using /spl alpha/-shapes: density-scaling, which modulates the value of a depending on the density of points in a region; and anisotropic shaping, which modulates the form of the /spl alpha/-ball based on point normals. We give experimental results that show the successes and limitations of our method.

Enhanced graph models in the Web: multi-client, multi-head, multi-tail browsing

One key to the wide and rapid acceptance of the World Wide Web is the simplicity of its model. We see this in its short-lived connections and generally stateless servers, as well as in the relationship between nodes (HTML pages) and embedded links. Though these factors contribute to the Webs success, they also come at a cost: precise control of how documents are presented to the user is beyond this basic model. Richer graph models permit authors to “program” the browsing behavior they want readers to see by turning the hypertext into a hyperprogram with specific semantics. Multiple browsing streams can be started under the authors control and then kept in step through the synchronization mechanisms provided by the graph model. Our current work adds a Semantic Web Graph Layer (SWGL) which allows dynamic interpretation of link and node structures according to graph models. As a motivating example of the utility of the SWGL, we have chosen to implement the graph model for Colored Petri Nets (CPNs). The previous MMM project[6] implemented a limited subset of the Petri net model to give Web authors the ability to control concurrency and synchronization in a single readers browsing session. CPNs extend this protocol to give control of multiple readers in a like fashion. This paper details the SWGL and its architecture, some sample protocol implementations, and the latest extensions to MHTML[6] that were necessary to support these enhancements.

Communication visibility in shared virtual worlds

Though the service of shared virtual worlds is an active area of research, little effort has been made to optimize such systems for urban world spaces. Tracking the motion, action, and communication of thousands of users in a city requires an application of visibility for spatial and logical subdivision of updates. We propose the City-Level Optimizations for Virtual Environments (CLOVES) substrate for the MIT City Scan (automated urban geometry acquisition) project. CLOVES includes a generalized spatial subdivision optimized for visibility; a Graduated Visibility Set (GVS) generator; associated interest management techniques; and model service to distributed heterogeneous clients.

Educational applications of multi-client synchronization through improved Web graph semantics

The MMM (Multi-head, Multi-tail, Multi-client) browsing project is a continuing effort to bring stronger graph semantics to the World Wide Web, thereby increasing the Webs usefulness for collaboration and education. The first modified browser provided facilities for links with multiple heads and tails, thereby giving the content author the ability to direct concurrent and synchronized browsing streams. The Web author can direct the paths of his readers and ensure that they visit pages in the correct context and order desired. Our most recent effort was the construction of a layer that filters content between the reader and the Web. This layer allows the ready composition of graph protocols so that Web content can be interpreted according to a variety of graph models. The model of the first project was expanded to be a full colored Petri net, thereby allowing synchronization of multiple browsing streams; the application of the MMM project to education now extends to a full collaborative classroom.

The World Wide Web: what cost simplicity?

The ubiquity of the World Wide Web owes much to the simplicity of its graph model. Unfortunately that graph model omits powerful features found in traditional hypertext systems: concurrency and synchronization. These shortcomings are addressed in an extensible manner as part of the Multi-head, Multi-head, Multi-client Browsing Project; our research is focused on extending the Web Web through the use of the more powerful link semantics.

Distributed interoperable virtual environments

The paper exhibits the use of existing software bus technology in interconnecting virtual-reality environment (VE) software. Interoperability and application construction from heterogeneous modules are well-explored topics of distributed systems. A joint project using the Polylith software bus from the University of Maryland and VE software from the UNC graphics lab has shown the utility of composing existing applications as opposed to making extensive individual modifications. The paper claims only a unique application of these methods to a new client area. Multi-user VE walkthroughs (software navigators) are an exciting new area in graphics software but we see that with the rapid development of graphics technology, next-generation applications (including multiuser systems) are commonly redesigned from the ground up. Here we see an excellent opportunity to examine module reusability, with proven software, in a new application area. We hope our experiments will likely lead to conclusions about VE programming abstractions and produce development methods for making easily interoperable next-generation VE applications.

Software engineering of distributed simulation environments

Shared design environments, distributed virtual environments, networked gaming software, cooperative work environments, and even multi user dungeons are all applications representative of the new enthusiasm for distributed simulation software. Yet despite this enthusiasm, there has been little effort made to provide software engineering support for these unique kinds of programs. We lay out the requirements that any such software engineering techniques will have to meet, then suggest a formalism and a software interconnection tool that will allow the building of reusable shared components of distributed simulation programs.

Geometric reconstruction with anisotropic alpha-shapes

of a three-dimensional model from an unorganized set of points is an active area of research in computer graphics. Such point sets come from a number of common sources, such as range data from three-dimensional scanning hardware, implicit surface, and medical imaging. The notion of ␣-shapes provides an elegant mathematical framework for extracting the geometric structure of a set of points in three dimensions. 2 Briefly, the ␣-shape is a set of triangles and tetrahedra that is a subset of the Delaunay tri-angulation of the input point set. The theory of ␣-shapes provides a method for obtaining a surface by selecting a subset of the triangles in the triangulation. A triangle is in the ␣-shape if one of the radii of the following spheres is at most ␣: the spheres circumscribing the trian-gles vertices and the two nearest neighboring points, and the sphere circumscribing the triangle. While this definition gives good results for point sets of roughly uniform density with large separation between surfaces, this definition is clearly not optimal for non-uniform point sets, or for surfaces that are separated by a distance less than their sampling density. For these, there exists no value of ␣ that includes all desired triangles and deletes all undesired triangles. See the first image in the figure for an example of the best possible surface obtained using ␣-shapes. We propose two extensions to alleviate these problems in the case where normal information is available (or estimated as in Hoppe et al. 3) at each point. These extensions allow reconstruction from a larger class of point sets: 1 Anisotropic scaling: we allow the spheres to vary in shape, and change the triangulation accordingly. A fundamental contribution of this work is incremental retriangulation based on a user-specified factor ␶, for the influence of the anisotropy. The spherical forbidden region, which depends on three or four points, is locally deformed along the local average normal direction d. It is compressed along d if the point normal and the normals of the triangle align well. It is stretched otherwise, decreasing the likelihood that this triangle will be selected for the ␣-shape. This in effect varies the local metric tensor. 1 The amount of deformation depends on the normal correlation and is multiplied by an interactively adjustable parameter ␶. The local normal direction d is multiplied by ␶, so the user has direct control of the anisotropy; ␶ = …