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Next: Combining CHARDIRS and SPHERETUFTS Up: Visualization of critical points Previous: CHARDIRS - visualizing eigen-manifolds


SPHERETUFTS - using many streamlets

The second technique presented here, i.e., SPHERETUFTS, is based on direct visualization of flow rather than on topological or geometrical analysis. A bunch of streamlets (many short stream lines) is used near the critical points of a dynamical system to directly represent the flow characteristics. The seed points of all the streamlets are stochastically chosen on a small sphere around the critical point. Forward integration as well as backward integration in time is performed with a fixed length in time. Thereby the spatial length of the streamlets directly encodes velocity. Forward streamlets are colored differently from backward integrated ones to distinguish between saddles from attractors/repellors.
  
Figure 6.3: Visualizing a linear system near the critical point with a pair of conjugated complex eigenvalues:  (a) a repellor and  (b,c) two saddles. [left image] [center image] [right image]
\framebox[\textwidth]{
\begin{tabular*}{.93\linewidth}{@{}@{\extracolsep{\fill}...
...plex.2.ps}
\\ {\small{}(a)}
& {\small{}(b)}
& {\small{}(c)}
\end{tabular*} }

In Fig. 6.3 three examples are given. Fig. 6.3(a) shows a linear repellor focus, whereas Fig. 6.3(b) depicts the saddle focus of a linear dynamical system. Fig. 6.3(c) shows also a linear saddle focus as in Fig. 6.3(b) with different eigenvalues. Using this direct visualization technique subtle differences in the flow characteristics become visible. By the use of SPHERETUFTS the differences between Fig. 6.3(b) and Fig. 6.3(c) become visible, although the flow geometry is identical in both cases: the flow component related to the real eigenvalue is much stronger in Fig. 6.3(c) than in Fig. 6.3(b).

In Fig. 6.2(b) the Lorenz system was visualized by placing bunches of streamlets near the critical points. The most important flow characteristics are intuitively depicted. See Fig. 6.4 for a visualization of the Lorenz system by the use of SPHERETUFTS and CHARDIRS.

  
Figure 6.4: Visualizing the Lorenz system using CHARDIRS and SPHERETUFTS.
\framebox[\textwidth]{
\includegraphics[width=.93\linewidth]{pics/Lorenz-old.ps}
}

Although the results gained with this method are quite expressive, the uniform distribution of streamlet seed points over a sphere enclosing the critical point might not be the optimal choice. Using a seed value distribution, which reflects the distance to the characteristic directions, i.e., the trajectories which coincide with the eigenvectors of the critical point, the visualization can be improved.


next up previous contents
Next: Combining CHARDIRS and SPHERETUFTS Up: Visualization of critical points Previous: CHARDIRS - visualizing eigen-manifolds
Helwig Löffelmann, November 1998,
mailto:helwig@cg.tuwien.ac.at.