Data exploration and interactive presentation with low demands on computational and/or networking resources have been the driving factors for the development of the RTVR (Real Time Volume Rendering) library [39]. It unifies the techniques described in the previous chapters into a flexible framework which can be used to provide volume visualization on PC-hardware and can be easily extended to include new approaches and techniques. In this chapter some of the distinguishing design issues of RTVR are described which are responsible for its high efficiency with respect to real-time volume rendering and flexibility.
In contrast to established volume visualization toolkits like VolVis [2] or VTK [53], which cover a very broad range of data representations and applications, RTVR is focused on providing interactive visualization for rectilinear volumes on desktop hardware by relying on preprocessing, voxel extraction, and the RenderList as a flexible internal data structure which is well-suited for fast rendering [42,43,44]. The data structure itself and also the fast shear-warp rendering which is used by RTVR are not suited to perform high-quality rendering. On the other hand, z-buffer output from the method could be used to accelerate high-quality rendering which is based on ray-casting.
The memory-efficient way of handling volumetric data makes RTVR well-suited for remote rendering of volumetric data even over low-bandwidth networks [40], either for interactive presentation of data which has been generated offline, or within a split client/server approach for online visualization. The amount of data which actually is transmitted to the client for visualization is very low (about the size of several images, see chapter 5). The user is not restricted to pre-computed views and has full control over visualization parameters. The only restriction for rendering is that just those parts of the volume which have been pre-selected for presentation and transmission can be rendered.
When used in a distributed client/server scenario, the software-only rendering approach of RTVR provides much more flexibility in terms of rendering parameters than volume previewing using texture mapping hardware, still at comparable or even lower costs in terms of bandwidth requirements.
In the following, visualization capabilities and the internal structure of the RTVR library are presented. Section 6.1 gives a short overview over the rendering features and visualization techniques which are implemented by RTVR. Section 6.2 presents RTVR's internal data structure, its handling of user interactions, and the rendering algorithms used. Timings for typical application scenarios are given in section 6.3.