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Anisotropic spot noise

Another part of the proposed approach is the use of spot noise as an additional stream surface texture. Spot noise (see Fig. 4.10) has been introduced to the computer graphics community by Jarke van Wijk in 1991 [87]. It is a powerful technique to generate various random textures that are suitable for different purposes. By constructing a spot noise texture, which emphasizes stream lines and time lines on a stream surface, the visualization of flow using stream surfaces can be improved. See Fig. 4.11 for a typical image. To find an appropriate spot is facilitated by the fact that spot noise textures directly reflect the geometric characteristics of the spot used for its construction. As we want to use the texture to emphasize stream lines and time lines, we parameterize the stream surface such that parameter lines directly correspond to stream lines and time lines. This parameterization is easily achieved, since the construction of stream surfaces is based on stream line integration. Using this type of stream surface parameterization a spot emphasizing horizontal and vertical directions in texture space, e.g., a cross or a hash, should be used. In Fig. 4.10 the spot we use and the resulting spot noise texture is shown. Fig. 4.11 shows a textured stream surface of the mixed-mode oscillations model.
  
Figure 4.10: spot (enlarged) and the resulting spot noise texture.
\framebox[\textwidth]{
\begin{tabular*}{.93\linewidth}{@{}@{\extracolsep{\fill}...
... & \includegraphics[width=.82\textwidth]{pics/spot-texture.ps}
\end{tabular*} }

  
Figure 4.11: Stream surface with anisotropic spot noise texture.
\framebox[\textwidth]{
\includegraphics[width=.93\textwidth]{pics/texture.ps}
}

We also thought of using line integral convolution [14] as an alternative to spot noise, but mainly two reasons induced us to use spot noise instead. First, the use of spot noise allows to emphasize both stream lines and time lines simultaneously. Using line integral convolution on the other hand, just stream lines or time lines could be emphasized. Another reason for the use of spot noise is, that it is less costly than line integral convolution.

Combining spot noise texture and stream arrows produces an expressive stream surface visualization technique. While the use of the stream arrows texture might raise a problem at ill-behaved areas of the stream surface, e.g., regions of large divergence or convergence, spot noise textures suffer from less problems in this situations.


next up previous contents
Next: Selective cuts Up: Stream arrows Previous: Hierarchical stream arrows
Helwig Löffelmann, November 1998,
mailto:helwig@cg.tuwien.ac.at.