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Compared to the nineties where fast 3D graphics was the domain of
expensive workstations, in the last few years the development of
ever faster 3D graphics hardware was mainly driven by the gaming
industry. The upcoming of programmable PC graphics hardware has
opened the field for new graphics algorithms which allow
unprecedented realism in real time applications. Nevertheless, one
area of application is persistently resisting most efforts to
achieve sufficient rendering performance: This is the area of volume
rendering. Because of the huge amounts of data that have to be
processed to obtain a three-dimensional visualization, it is very
challenging to achieve real time performance for large volumetric
data sets.
In this thesis we try to tackle this problem in one specific
application field. We devise algorithms that are suitable for the
real time display of natural gaseous phenomena. In particular our
goal is to render clouds and fog in real time. In principle, the
problem reduces to solving the so called ray integral. A common
technique for solving this ray integral is ray casting which
collects the incoming light on each viewing ray by sampling the
volume. On the one hand ray casting achieves very good rendering
quality, but on the other hand it becomes very slow at high screen
resolutions. Many improvements have been presented to accelerate the
original approach, but despite all efforts ray casting is still only
beginning to be an option for high-quality real-time rendering. Very
recent advances in graphics hardware have lead to the implementation
of hardware-accelerated ray casters, but this approach still suffers
from a variety of limitations of the graphics hardware.
The main technique developed in this thesis is the so called
pre-integrated cell-projection method which offloads as much
computation of the ray integral as possible into a preprocessing
step. This is the first step toward real-time rendering of natural
gaseous phenomena. In a second step we develop a hierarchical
approximation scheme which decimates the huge amount of data in a
view-dependent way. For this purpose we borrow ideas from the area
of terrain rendering and apply the so-called continuous level of
detail method to the three-dimensional case, that is fog and cloud
volumes. In combination with the pre-integrated cell-projection
method this permits real-time flights through natural looking clouds
and ground fog. In comparison to previous methods image quality is
also improved significantly.
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