دانلود رایگان مقاله eBits: جریان فشرده اصلاحات شبکه برای تجسم از راه دور

Abstract

We focus on applications where a remote client needs to visualize or process a complex, manifold triangle mesh, M, but only in a relatively small, user controlled, Region of Interest (RoI) at a time. The client first downloads a coarse base mesh , pre-computed on the server via a series of simplification passes on M, one per Level of Detail (LoD), each pass identifying an independent set of triangles, collapsing them, and, for each collapse, storing, in a Vertex Expansion Record (VER), the information needed to reverse the collapse. On each client initiated RoI modification request, the server pushes to the client a selected subset of these VERs, which, when decoded and applied to refine the mesh locally, ensure that the portion in the RoI is always at full resolution. The eBits approach proposed here offers state of the art compression ratios (using less than 2.5 bits per new full resolution RoI triangle when the RoI has more than 2000 vertices to transmit the connectivity for the selective refinements) and fine-grain control (allowing the user to adjust the RoI by small increments). The effectiveness of eBits results from several novel ideas and novel variations of previous solutions. We represent the VERs using persistent labels so that they can be applied in different orders within a given LoD. The server maintains a shadow copy of the client’s mesh. To avoid sending IDs identifying which vertices should be expanded, we either transmit, for each new vertex, a compact encoding of its death tag –the LoD at which it will be expanded if it lies in the RoI–or transmit vertex masks for the RoI and its neighboring vertices. We also propose a three-step simplification that reduces the overall transmission cost by increasing both the simplification effectiveness and the regularity of the valences in the resulting meshes.

Conclusion

We have presented a compact format, called eBits, for transmitting the local connectivity of the subset of a triangle mesh that lies inside the Region of Interest (RoI) as the RoI is moved in a user-controlled manner. When the RoI includes the entire mesh, eBits transmission cost is only 2.08 bpt, including the cost of transmitting the base mesh. When the RoI is smaller the, Effective Transmission Cost approaches 2.08 bpt—for example, if the RoI has 2000 vertices, the ETC is about 2.5 bpt. There are numerous avenues for future exploration. As it stands, eBits provides a block level random accessible compressed triangle mesh format for remote visualization. By transmitting mesh connectivity and geometry instead of server rendered images, eBits allows for client side geometry processing. While multiple clients can be trivially served by maintaining a single copy of the VER table on the server, saving client side edits of local geometry and connectivity on the server is an interesting topic of future research. Isosurfaces from structured grids are often ordered along a spatial axis. An easy option for random access of such meshes is to transmit the entire slices stabbed by the RoI. eBits sends only portions of these slices inside the RoI (plus balancing rings). However, eBits does not exploit the spatial ordering itself, which could be used to devise efficient out of core simplification algorithms for the server side pre-processing. Finally, the current implementation of eBits does not support non-manifold meshes or meshes with boundaries. Meshes with boundaries can be supported by extending the encoding of tcollapses to also account for border c-triangles or by preventing the collapse of border triangles. A watertight non-manifold mesh (when each edge has an even number of incident faces) could be converted to a manifold representation using MatchMaker [31]. Handling more general non-manifold meshes requires further research.