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1344 Chapter 15: Lights and Cameras
photonsusedtogeneratecausticsbythisparticular
light. Default=1.0.
GI Photons—MultipliestheglobalGIPhotonsvalue
to increase or decrease the count of photons used
to generate global illumination by this particular
light. Default=1.0.
Manual Settings group
When Automatically Calculate is off, the Global
Multipliersgroupbecomesunavailable,andthe
manual settings for indirect illumination become
available.
On—When on, the light can generate indirect
illumination effects. Default=off.
Filter color—Click to display a ColorSelector(page
1–161) and choose a color that filters the light
energy. Default=white.
Energy—Sets the e nergy of the light. Energ y, or
"flux," is the amount of light used in indirect
illumination.Eachphotoncarriesafractionof
the lig ht’s energy. T his value is independent of
the light intensity determined by the light’s color
and Multiplier, so you can use the Energy value
to fi ne-tune indirect illumination e ffects without
changing the light’s other effects in a scene (such as
providing diffuse illumination). Default=50000.0.
Decay—Specifies how photon energy decays as
it moves away from the light source. Range=0.0
to 100.0. Default=2.0 (inverse square; physically
correct falloff).
This value is applied as an exponent to the distance
(r) between the light source and each indirectly
illuminated point to determine the energy at that
point, in the formula 1/r
[Decay]
), and is intended
for tweaking the GI solution. The most commonly
used va lues are:
•
0.0 (no decay)—The energy doesn’t decay, and
photons provide equal indirect illumination
throughout the scene.
•
1.0 (inverse)—The energy decays proport ionally
to its distance f rom the light, w ith linear falloff.
That is, a photon’s energy is 1/r,wherer is the
distance from the light source.
•
2.0 (inverse square)—The energy decays at an
inverse square rate. That is, a photon’s energy
is the inverse of the square of the distance (r)
from the light source (1/r
2
).
In the real world, light decays at the inverse square
rate, but this gives stric tly realistic results only
if you provide a realistic value for the energy of
the light. You can use other values to help adjust
indirect illumination without worrying about
physical accuracy.
Caustic Photons—Sets the number of photons
emitted by the light for use in caust ics. This is the
number of photons in the photon map (page 3–994)
used for caustics. Increasing this value increases
the a ccur acy of caustics, but also increases the
amount of memory used and the length of render
time. Decreasing this value improves memory
usage and render time, and can be useful for
previewing caustic effects. Default=10000.
GI Photons—Sets the number of photons emitted
by the light for use in global illumination. This is
the number of photons in the photon map used
for global illumination. Increasing this value
increases the accuracy of global illumination, but
also increases the amount of memory used and
the length of render time. D ecreasing this value
improves memory usage and render t ime, and
can be useful for previewing globa l-illumination
effects. Default=10000.