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

5.3 Visualization nodes and subgraphs
67 // End of vertex shader
Such an efficient particle effect can be easily applied for the visualization of streak
lines and time lines as they are described in section 2.1.1.2 in chapter 2. The only
things to modify would be in the first case the initialization of the initial positions to
evenly spaced seed points placed within a plane or line and in the latter case the initial
ages in the attribute textures so that all particles are always being released at the same
time.
5.3.3.3 Stream lines
Streamlines show - as described in section 2.1.1.2 in chapter 2 - the trajectories of a
particle influenced by a static velocity field. Consequently these lines are only valid for
one separate temporal volume and will completely change when changing to another
field in time. However, local structures of the flow may be explored very well by these
lines.
Two approaches have been implemented to calculate particle trajectories. First a
single pass shader was evaluated which subsequently updates to the lines in the first
render pass with a for-loop. Since only with the GeForce 8x Series, a shader may be
used recursively in one render pass with its output as input. This solution was chosen to
guarantee a compatibility to reference systems as defined in section 6.1. The one-pass
implementation on other architectures is extremely inefficient for dynamic manipulation
of the scene due to a necessary recalculation of already calculated positions. In this case
the access to the output texture is not possible in one pass! However, this rudimentary
algorithm is not suitable for a dynamic definition of the seed region, for example by
a dragger since every (complete) recalculation of the trajectory texture takes about
one second up to 5 seconds. Nevertheless, this approach is for randomly but fixed
distributed seed points in the volume as feasible as the multi-pass approach.
Due the focus on interactive usable flow visualization, the further line based algorithms
uses a multi-pass algorithm.
The multi-pass approach utilizes a double buffered texture for the calculation of
results and a vertex displacement for the actual line rendering. The main differences
are that in the sample points position texture each column corresponds to the supporting
points of one line and that the displaced vertices in the vertex list belong to
line strips 1 . Consequently, a position initialization texture would be reduced to one
dimension defining the first row of a vertex position texture. Considering that not all
lines will be of an equal length and that some parts of the supporting position texture
will remain empty, several other memory management approaches for exploiting the
available texture memory have been considered. Finally the simplest approach was
used because of the fact that each other, even texture reorganizing algorithms need as
much time for the memory management as the calculation itself. Additionally, the used
GF 8x architecture provides plenty of texture memory, so the position texture can be
1 in OpenGL: GL LINE ST RIP vertex order.
106