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

2.1 Technical Visualization
dye in a flowing fluid or air. The first type of line uses evenly spaced points for
a persistent injection of particles and the second approach defines a continuous
line of seed points with evenly spaced injection times. Hence streak lines can be
compared with the commonly known technique of smoke in a wind tunnel and
time lines with a permanent dunking of a dye plate in a streaming fluid. Following
[Kalkusch2005] they are defined for a varying τ only.
• Stream ribbons, stream tubes and stream balls can be generated by additional
considerations of the rotational effect of a flow field along a trajectory.
Stream ribbons will behave similar to completely elastic strips brought into an
unsteady flow. Stream tubes are the same as stream lines but with a constant
diameter or an elliptic diameter to get information of the rotation component
as well and balls are geometries placed at supporting point positions with variable
defined elongation depending on the flow’s direction and strength at these
positions.
For the sake of completeness, another non-particle trace based approach for spare
flow field representations can be mentioned. With the principles described in section
2.1.1.1, iso-surfaces can be used to show flow characteristics as well. Surfaces may
be extracted, for example from positions with the same velocity magnitude or related
parameters as shown in figure 2.14(e).
Even illumination of structures like massless lines or particles can be very important
for depth perception as compared in figure 2.14(b) . Stream ribbons, stream tubes and
stream balls consist of connected geometry parts, which allows a classical Gouraud
shading or Phong shading due to well defined normal vectors of the surfaces. In the
case of massless particles and lines illumination is not as trivial because of arbitrary
direction of the normal vector within a tangential perpendicular plane according to the
line. These inconveniences have been resolved by [Zockler1996] with a reformulation of
the Phong lightning model. The light intensity at a point on a surface is given by
I = k a + k d ( L ⃗ · ⃗N) + k s ( V ⃗ · ⃗R) n (2.6)
with the ambient, diffuse and specular coefficients k a , k d , k s and n. The unit length
vectors N ⃗ for the normal of a surface, L ⃗ and V ⃗ pointing toward the light source and the
eye-position and R ⃗ referring to the reflection of L ⃗ and N. ⃗ These are all defined except
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