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108 Chapter 17: Rendering
values, or by setting lo cal values (using multipliers
is the recommended method).
Average Caustic Photons per Light—Sets the
number of photons emitted by each light for use
in caustics. This is the number of photons in the
photon map (page 3–1088) used for caustics.
Increasing this value increases the accuracy of
caustics, but also increases the amount of mem ory
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.
Average G I Photons per Light—Sets the number
of photons emitted by each 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 leng th of render time.
Decreasing this va lue improves memory usage
and render time, and can b e useful for previewing
global-illumination effects. Default=10000.
Global Energy Multiplier—Sets the energy of each
light. Energy, or "flux," is the amount of lig ht
used in indirect illumination. Each photon car ries
a fraction of each light’s energy. This value is
independentofthelightintensitydeterminedby
the light’s color and Multiplier, so you can use
the Global Energy Multiplier value to fine-tune
indirect illumination effects without changing the
light’sothereffectsinascene(suchasproviding
diffuse illumination). Default=1.0.
Decay—Specifies how photon energy decays as it
movesawayfromeachlightsource.Thisvalueis
given by 1/(
distance
decay
), where distance is the
distance b etween the light s ource and an object,
and
decay is the value of this setting. Default=2.0.
The most common values are:
•
0.0—The energy doesn’t decay, and photons
can provide indirect illumination throughout
the scene.
•
1.0—The energy decays at a linear rate,
proportionally to its distance from the light.
That is, a photon’s energy is 1/
distance ,where
distance is the distance from the light source.
•
2.0—(The default.) The energy decays at an
inverse square rate. That is, a photon’s energy
istheinverseofthesquareofthedistancefrom
the light source: 1/
distance
2
.
In the real world, lig ht decays at an inverse square
rate (Decay=2.0), but this gives strictly realistic
results only if you provide a realistic value for the
energy of the light. Other values of Decay can help
you adjust indirect illumination without worrying
about physical accuracy.
Note: Decay values less than 1.0 are not
recommended, and can cause rendering artifacts.
Geometr y Properties group
All Objects G enera te & R eceive GI and
Caustics—
Whenon,atrenderingtime,all
objects in the scene can generate and receive
caustics and global illumination, regardless of
their local object properties settings. When off,
an object’s local object propert ies determine
whether it generates or receives caustics or g lobal
illumination. Turning this on is an easy way to
ensure that caustics and g lobal il lumination are
generated, though it can increase rendering time.
Default=off.
This setting does
not alter the object’s loca l object
properties settings for mental ray. When you
turn off All O bjects Generate & Receive G I And
Caustics, the prior object properties settings are in
effect once again.