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110 Chapter 17: Rendering
Max. Depth—
Limits the combination of reflection
and refraction. Reflection and refraction of a
photon stop w hen the total number of both equals
the Maximum Depth setting. For example, if
Maximum Depth equals 3 and the trace depths
each equal 2, a photon can be reflected twice
and refracted once, or v ice versa, but it can’t b e
reflected and refracted four times. Default=5.
Max. Reflections—Sets the number of times a
photon can be reflected. At 0, no reflection occurs.
A t 1, the photon can be reflected once only. At
2, the photon can be reflected twice, and so on.
Default=5.
Max. Refractions—Sets the number of times a
photon can be refracted. At 0, no refraction
occurs. At 1, the photon can be refracted once
only. At 2, the photon can be refracted twice, and
so on. Default=5.
Light Properties g roup
Controls in this group affect how lights behave
when calculating indirect illumination. By default,
the energy a nd photon settings apply to all lights
in a scene. Use the mental ray Indirect Illumination
rollout (page 2–1343) for light objects to adjust an
individual light either by multiplying the global
values, or by setting local values (using multipliers
is the recommended method).
Average Caustic Photons per Light—Sets the
number of photons emitted by each light for use
incaustics. Thisisthenumberofphotonsin
the photon map (page 3–994) u sed for c austics.
Increasing this value increases the accuracy of
caustics, but a ls o increases the amount of memory
used and the length of render time. Decreasing
this value improves memor y usage and render
time, and can b e useful for previewing caustic
effects. Default=10000.
Average G I P hotons per L ight—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 i l lumination, but also increases the amount
of memory used and the length of render time.
Decreasing this value improves memor y usage
and render time, and can be useful for previewing
global-illumination effects. Default=10000.
Decay—Specifies how photon energy decays as it
moves away from each lig ht source. This value is
given by 1/(distance
decay
), w here distance is the
distance b etween the lig ht source and an object,
and decay is the va lue 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 r ate,
proportionally to its distance from the light.
That is, a photon’s energy is 1/distance,where
distance isthedistancefromthelightsource.
•
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, light decays at an inverse square
rate (Decay=2.0), but this gives strictly realistic
results only if you prov ide a realistic value for the
energy of the light. Other values of Dec ay can help
you adjust indirect illumination without worrying
about physical accuracy.
Note: Decayvaluesoflessthan1.0arenot
recommended, and can cause rendering artifacts.
Geometry Properties group
All Obj ects Gener ate & R eceive GI and
Caust ics—
Whenon,atrenderingtime,all
objects in the scene can generate and receive
caustics and global illumination, regard less of
their local object properties settings. When off,
an object’s local object properties determine