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118 Chapter 17: Rendering
Autovolume also enables a camera to move
through the nested or overlapping volumes.
Default=off.
To use Autovolume, Ray Trace must be tur ned on,
Scanline must be turned off, and the shadow mode
must be set to Segments. (You set the shadow
mode on the Shadows And Displacement rollout
(page 3–114) .) If these conditions aren’t met when
you click to turn on Autovolume, an alert warns
you about this, and gives you the option of making
theappropriatesettingchanges.
Raytrace Acceleration Group
Method—The drop-down list sets which algorithm
to use for raytrace acceleration (page 3–1000).
The other controls in this group box change,
depending on which acceleration method you
choose. These are the alternatives:
•BSP(thedefault)
The BSP method has Size and D epth controls.
See Ray-Trace Acceleration: Parameters for the
BSP Methods (page 3–129).
This metho d is the fastest on a single-processor
system. Use it for small-to-medium size scenes
on a single processor. BSP is also the best
method to use when ray t racing is turned off.
•Grid
TheGridmethodhasSize,Depth,and
Resolution controls. See Ray-Trace
A cceleration: Parameters for the Grid Method
(page 3–129).
This method uses less memory than BSP. It is
also faster than BSP on multiprocessor systems.
Note: If you attempt to render motion blur with
the Grid method active, the mental ray renderer
automatically switches to the BSP method. This
happensintheMIfilebutisnotreflectedinthe
3ds Max interface.
•LargeBSP
TheLargeBSPmethodhasthesamecontrolsas
BSP. See Ray-Trace Acceleration: Parameters for
the BSP Methods (page 3–129).
This method is a variant of the BSP method.
Portions of the partitioning tree it uses can
be swapped in and out of memory. This
enables mental to render very large scenes, at
a cost of ray -tracing time. Use this method
for very large ray-traced scenes, and also
when Use Placeholder Objects is turned on
(see Translator Options Rollout (mental ray
Renderer) (page 3–119)). Use Placeholder
Objects is recommended when you are doing
distributed rendering (page 3–124).
Trace Depth group
Trace depth controls the number of times a light
ray can be reflected or refracted. At 0, no reflection
or refraction occurs. Increasing these values can
increase the complexity and realism of a scene, at
a cost of greater rendering time.
Tip: In some cases, you m ight want to set Max.
Refractions high and Max. Reflections low. For
example, you mig ht have the camera looking
through several glasses that are lined up, so they’re
overlapping from the camera’s point of view. In
this situation, you might want the light rays to
refract twice for each glass (once for each layer),
so you’d set Max. Refractions to 2 x [number
of glasses]. However, to save rendering time,
you could set Max. Reflections to 1, resulting in
accurate multi-layer refraction with a relatively
fast rendering time.
Max. Depth—Limits the combination of reflection
and refract ion. Tracing of a r ay stops w hen the
total number of reflections and refractions reaches
the Max Depth. For example, if Max Depth equals
3 and the two trace depths each equal the default
value of 2, a ray can be reflected twice and refracted
once,orviceversa,butitcan’tbereflectedand
refracted four times. Default=6.