A streamline is a smooth curve that exists at an instantaneous time, along which the tangent at an arbitrary point coincides with the local flow direction. Here is an AVI movie (129MB --- note that the dynamic frame rates shown in this movie are lower than those of the original performance due to the animation capturer that consumed a significant amount of CPU resources) made for an exhibition at ACM / IEEE Super-Computing'07 (Reno-Sparks Convention Center, Reno, Nevada, Nov 10~16, 2007). It demonstrates real-time user-driven illuminated streamlines for explorative visualization of a wind volume (360 x 181 x 13) in the sky through the use of ActiveFLOVE (FLOw Visualization Environment, developed by Zhanping Liu at MSU / HPC / VAIL, Fig. 1). Besides this application, ActiveFLOVE has been employed for explorative visualization of the Northeast Pacific Ocean layers by means of flow lines (streamlines are turned on while pathlines are off in Fig. 3) and Line Integral Convolution.
Figure 1. Two snapshots of ActiveFLOVE for wind volume visualization.
In fact, user-driven streamline-based flow exploration is achieved by means of a dynamic seed distribution scheme called "seeder" (Fig. 2). As the user navigates through the flow field, the seeder moves with the center being "at" the user's position. Actually the center is always "attached" to a regular grid point nearest to the user while the resolution of the virtual grid is the same as the seeder's resolution. The seeder's capacity determines how many seeds are placed schematically at any time to obtain streamlines, of which only a few are indeed generated from scratch, whereas the rest are readily available via streamline reuse. This underlying working mechanism is effective enough to visually cheat the user as if all streamlines were produced on the fly. Thus the seeder scheme is well suited for real-time flow investigation to identify regions of interest (Figures 3 and 4).
Figure 2. The evolution of a 2D seeder with 25-seed capacity (© Zhanping Liu).
Figure 3. A snapshot of ActiveFLOVE for seeder-based exploration of the layered Northeast Pacific ocean flow.
Figure 4. A snapshot of ActiveFLOVE for seeder-based investigation of a 144 x 73 x 81 wind volume where a vortex core can be revealed.
One problem with the use of streamlines, even through seeder-based streamline distribution, is that an incomplete view or a cluttered display of the flow tends to be obtained unless an effective streamline placement method is adopted. A layout of evenly spaced (or uniform) streamlines provides an aesthetic as well as informative pattern to facilitate mental reconstruction of the flow. Our ADVESS (ADVanced Evenly Spaced Streamline placement) solution is so far the fastest of this kind for interactive high-quality placement generation coupled with robust loop detection. Our IVDESS (Interactive View-Driven Evenly Spaced Streamline placement) algorithm allows for coherent explorative visualization of surface flows at interactive frame rates on a low-end WinTel PC.
Flow Visualization
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