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108 Chapter 17: Rendering
Filter—
Sets the filter to u se for sharpening caustics.
Can equal B ox, C one, or Gauss. The Box option
requires less rendering time. The Cone option
makescausticsappearsharper.Default=Box.
The Gauss fi lter uses a Gauss (bel l) curve, and can
be smoother than the Cone filter.
Filter Size—Controls the sharpness of caustics
whenyouchooseConeasthecausticfilter.This
valuemustbegreaterthan1.0. Increasingthe
value makes caustics more blurry. Decreasing
thevaluemakescausticssharper,butalsoslightly
more noisy. Default=1.1.
Note: In previous versions of the software, this
parameter was named “Kernel.”
Opaque Shadows when Caustics Are
Enabled—
When on, shadows are opaque.
When off, shadows can be partially transparent.
Default=on.
Opaque shadows render more quickly than
transparent shadows.
Global Illumination (GI) group
These settings let you control the usage of photons
by mental ray for generating global illumination
(page 3–93). By default, all objec ts generate and
receive global illumination. The settings for
generating and receiving GI a re lo cated on the
Object Properties dialog > mental ray Panel (page
1–126).
Note: In order to render global il lumination in
mental ray, the photons must be able to bounce
among two or more surfaces. This can be
accomplished by having a single object with some
concavity in its surface that’s exp osed to the light
source, or at least two objects, and at least one
object must be set to receive globa l illumination
(see mental ray P anel (Object Properties Dialog)
(page 1–126)). O therwise you’ll receive error
messages and no photons will be stored.
Enable—When on, the mental ray renderer
calculates global illumination. Default=off.
Multiplier/color swatch—Use these to control
the intensity and color of the indirect light
accumulated by global illumination. T he defaults,
1.0 and white, produce physically correct
rendering.
This is useful for adjusting the contribution of the
GI effect, thus improving the quality of an image.
Ma xi mum Num. P hotons per S a mple—Sets how
man y photons are used to compute the intensity
of the global illumination. Increasing this value
makes global illumination less noisy but also
more blurry. Decreasing this value makes global
illuminat ion more noisy but less blurry. The larger
the Samples value, the greater the rendering time.
Default=500.
Tip: To preview global illumination, set Samples to
100, then increase the va lue for a final rendering.
Maximum Sampling Ra dius—When on, the spinner
value sets the size of photons. When off, each
photon is calculated to be 1/10 of the radius of the
full scene. Default=off, 1.0.
In many cases, the default photon size (Maximum
Sampling Radius= off) of one-tenth the scene size
gives useful results. In other cases, the default
photon size might be too large or too small.
When photons overlap, the mental ray renderer
uses sampling to smo oth them together. Increasing
the number of samples increases the amount of
smoothingandcancreatemorenatural-looking
caustics. When photons have a small radius and
don’t overlap, the Samples setting has no effect.
For global illumination, photons should overlap.
To get good results, you might need to turn on
MaximumSamplingRadiusandincreasethe
photon size.
Merge Near by P hotons (sav es memor y )—Enables
reductionofthememoryfootprintofthephoton