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Caustics and Global Illumination Rollout (mental ray Renderer) 109
map. When on, use the numeric field to specify the
distance threshold below which mental ray merges
photons. The result is a smoother, less-detailed
photon map that uses significantly less memory.
Default=off, 0.0.
Note: Loading a legacy file uses the default value
of 0.0. Also, using a value of 0.0 is equivalent to
turning the feature off.
Optimize for Final Gather (Slower GI)—If turned
on before you render the scene, the mental ray
renderer computes information to speed up the
regathering process. Specifically, each photon
stores additional information about how bright
its neighbors are. This is particularly useful when
combining Final Gather with Global Illumination,
in which case the additional information allows
Final Gather to quickly determine how many
photonsexistinaregion. Thefastlookup
computationcantakealongtime,butitcangreatly
reduce the total rendering time. Default=off.
The fast lookup computation can be can be stored
as additional data inside a photon map file (page
3–995), and then reused in subsequent renderings.
Note: In previous versions of the software, this
parameter was located on the Final Gather rollout
(page 3–111), and was named “Fast Lookup
(Slower GI).”
Volumes group
Thecontrolsinthisgroupandtheonesthatfollow
are for the photon maps (page 3–994) used to
calculate caustics and global illuminat ion. This
group controls volumetric caustics. Volumetric
caustics require a material to hav e a volume shader
assign to its Photon Volume component.
Ma xi mum Num. P hotons per S ample—Sets how
many photons are used to shade the volu me.
Default=100.
Max imum Sampling Ra dius—When the check box
is on, the spinner sets the size of photons. When
off, each photon is calculated to be one-tenth the
size of the scene extents (page 3–1007).Default:
check b ox=off; value=1.0.
This spinner is unavailable if the check box is
turned off.
Photon Map g roup
These controls tell mental ray how to ca lculate the
photon map for indirect illumination.
Note: If you’ve specified a photon map here, mental
ray continues to use that map instead of generating
a new one. To cause the photon map file to be
rebuilt, delete the existing file.
Read/Write File—When on, if the specified photon
map (PMAP (page 3–995))filedoesnotyet
exist, mental ray generates a n ew map file when
rendering. If the spe cified file does exist, mental
ray loads and uses the file.
This option becomes available after you click
Browse (“...”) and provide a name for the PMAP
file.
... [browse]—Click to display a file selector dialog,
which lets you specify a name and path for the
photon map (PMAP) file. This automatically tur ns
on Read/Write File.
[fi le na me]—When you have used the [...] button
to specify a photon map file, this field displays its
name and path.
Delete File—Click to delete the current PMAP
file.
Trace Depth group
The Trace Depth controls are similar to those for
calculating reflections and refractions, but they
refer to the photons used by c austics and g loba l
illumination, rather t han to r ays used in diffuse
reflection and refraction.