Graphics Reference
In-Depth Information
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Figure 3.17
Forward ray marching.
contribution all the way back to the viewpoint are known at each segment. In
other words, the algorithm tracks how optically deep it is into the medium at each
step. As it goes deeper, a less precise approximation to in-scattering is needed,
which means that the segment size can be increased and fewer photons are needed
from the map. In fact, when the collected in-scattering and attenuation are large
enough, the rest of the ray can be ignored outright—light deep enough inside
dense smoke does not get out.
A year after the smoke simulation paper, Duc Q. Nguyen, Fedkiw, and Jensen
presented a paper entitled “Physically Based Modeling and Animation of Fire”
[Nguyen et al. 02]. The physical basis of the model is preserved through the
lifetime of the flame (ignition, radiation, and extinction). Figure 3.18 shows an
image of fire rendered using the methods described in the paper. Flame is an ex-
ample of a luminous participating medium—it emits light throughout its volume.
Although there is some scattering and attenuation, the basic appearance of flame
comes from emission of light within the flame. The light comes primarily from
two sources: a blue “core” caused by emission from heated ionized gas molecules,
and a yellowish envelope caused by glowing hot uncombusted soot particles.
When an object gets hot enough it begins to emit a visible glow in a phe-
nomenon known as
incandescence
. The emission spectrum of an incandescent
source is typically modeled as a
black body
, which is an object that absorbs all
incident irradiance. The absorbed energy is re-emitted in a spectral distribution
according to a
Plank curve
. The apparent color of a black body changes with
temperature: as an object is heated it first appears a dull reddish orange, then
shifts to a yellowish color and eventually to almost white (this is the origin of
color temperature
). Soot particles in flame are initially hot enough to appear yel-
lowish, but they cool as they rise, making the flame appear redder near the top.
Eventually the particles cool enough to stop emitting light, at which point the vis-
ible part of the flame ends and an observer sees a stream of smoke. This is the
third primary visible component of flame.
