Master's Thesis - Studierstube Augmented Reality Project - Graz ...

5.3 Visualization nodes and subgraphs
18 // between different ones .
19 // This color can now be illuminated per fragment with I.
20 gl_FragColor = gl_Color * I;
21 }
22 // End of fragment shader
5.3.3.4 Path lines
Calculation of path lines is built upon the calculation of streamlines. This line type
has to consider the fourth dimension as well since they represent a particle’s trajectory
over time. The simplest way to generate a path line is to assemble it with parts of
streamlines from each volume. This method is error-prone but efficient to implement
by simple changing the volumes over time during the calculation of streamlines as
described in section 5.3.3.3. Assuming that a given four dimensional dataset consists of
t volumes and the position buffer texture is of the size n × m the volume number has to
be incremented each floor( m t
) frame. Since these lines are now constant when the user
for example changes the image volume, an additional indicator should be available for
convenience. Coloring parts of the lines with a signal color has proved to show the user
in which volume which part of the line was generated. Figure 5.12 shows that behavior
on an example.
Certainly all illumination and further coloring techniques can be used with these
lines in the same way as with streamlines. The remaining thing with this algorithm
is to evaluate whether an interpolation in temporal direction - which means an about
quadratic increase of computational costs - would produce a smaller error or not. These
considerations are intended for future work.
5.3.3.5 Stream tubes
When path lines and stream lines calculations are done even a more realistic visualization,
so called stream tubes are possible. The basic idea is described in detail in
section 2.1.1.2 in chapter 2.
Generating a real mesh of vertices around an actual calculated line would result
in a vast amount of vertex calculations and consequently in a very low performance.
To handle this problem a so called impostor rendering technique was used. This trick
allows to render reasonably realistic tubes with only twice as many vertices as used with
simple line visualizations. Instead of a line strip a quad strip 1 is stored in a display list
and then subsequently displaced to the correct positions. The visualization algorithm
can be performed in one vertex shader and one fragment shader which is illustrated in
figure 5.13.
The main task for the vertex shader is to displace the two(!) vertices corresponding
to one supporting point position always perpendicular to the viewing direction. The
points of the quad strip itself can be distinguished by different initial positions so
1 GL QUAD ST RIP vertex definition for OpenGL)
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