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

5.3 Visualization nodes and subgraphs
more complex operations are performed for the plane. Such operations may be required
since with a simple mapping onto a 2D plane a third dimension and its information
gets lost. For example, the magnitude of the flow through a certain vessel can be
investigated but not the directional effects.
With a mesh-plane it is possible to displace its vertices in direction of the sampled
velocity value. The result will be a height profile, indicating the strength and direction
of the flow. A subsequent assignment of the direction of the flow to the vertex as normal
vector will even provide a Phong illumination of that crinkly plane.
The usage of the velocity direction as normal vector can also be applied to a new
approach for quad-planes. The drawback of the former vertex displacing algorithm is
that one the one hand the plane will occlude itself and on the other hand the vertex
processing of a mesh is more expensive than the processing of only four. Consequently,
a related algorithm but implemented with a fragment shader can use the flow directions
within the plane as normal map for the illumination. For computer games this technique
is called ”bump mapping” because it lets actual flat surface appear with a profile. If the
user knows that the velocity direction is used as normal vector, he can easily interpret a
three dimensional information in a two dimensional plane. The crib for example a headlighted
scene will then be: ”The more it reflects, the more the flow indicates towards
me”. The drawback of this method is that it requires this crib as meta knowledge.
Certainly, with planes much more sophistical visualizations may be implemented
and some such ideas are outlined in chapter 7. However, the goal of them would go
farther than the focus of this work since most of them would suggest a plane as kind of
sensor array placed in a hemodynamic system. The results of such approaches would
rather all provide a measured quantity than an instructive visualization.
5.3.3.7 Concurrent Combinations
The benefit of a free definable view as presented is obvious. All visualization nodes can
be combined, even with different seed regions. Eventually the buffer sizes have to be
reduced due to performance. This may be also necessary to preserve the overview of the
visualizations since for example a million particles with ten-thousands of arbitrary lines
would only produce visual clutter. It was empirically found that at most two different
visualizations per seed region are meaningful. More than one dragger in a scene also
appears distracting. A combination of side by side views will be more satisfying for such
a concurrent visualization. More promising control approaches are therefore presented
in section 7.4 in chapter 7.
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