Graphics Reference
In-Depth Information
Light
source
random value
ξ ≤ Albedo value
Photon is scattered
Not stored in
the photon map
Stored in the
photon map
Photon is absorbed
ξ > Albedo value
A random value between
0 and 1 (excluding 0, 1)
log
ξ
s
d
d
= −
−
Scattering coefficient
σ
Figure 3.14
Volume photon simulation. At a collision, each particle is either absorbed or scattered ac-
cording to a random number ξ . A scattered photon travels distance
d
to the next collision.
Only secondary scattering events are stored, because the volume photon map is only used
for indirect lighting.
because it is possible that the values at the endpoints end up close purely by co-
incidence. The initial ray subdivision has to take that into account. Nonetheless,
adaptive ray marching has proven to be a useful technique for optimizing ray
marching performance, particularly in inhomogeneous media.
Accurate computation of the in-scattered light in each segment is critical to the
success of the ray marchingmethod. Another way efficiency can be improved is to
separate direct and indirect incident light in the in-scattering computation. Direct
light comes from a specific set of directions, those of the light sources; therefore
direct light can be sampled by tracing rays to the light source. In contrast, the
indirect light comes from all directions, so the entire sphere of directions has to
be sampled. In-scattering from indirect lighting is often small, so fewer samples
can be used and therein lies the optimization. However, more samples are needed
as the degree of scattering increases, and this increases the computational load.
HenrikWann Jensen and Per H. Christensen proposed an extension to the pho-
ton mapping method to improve the in-scattering computation efficiency [Jensen
and Christensen 98]. Instead of using sampled rays, their method uses parti-
the primary difference being that the simulation for participating media also in-
cludes scattering. Each photon is assumed to have a logarithmic probability of
colliding with a scattering particle along a free path in the medium. At a colli-
sion, a random number
ξ
in the range from 0 to 1 is chosen. If
ξ
does not exceed
