8

1094 Glossary
Ray-Trace Acceleration (mental
ray Renderer)
The mental ray renderer (page 3–77) provides
three different ray-tracing methods of accelerating
the process of ray tracing. The methods are:
• BSP (Binary S pace Partitioning). This method
(the default) performs best for most purposes.
• Grid. This method can perform better on
multiprocessor s ystems.
• Large BSP. This method can perform better
with large scenes and with distributed bucket
rendering.
Yousettheraytraceaccelerationmethodon
the Render Scene Dialog > Renderer panel >
Rendering Algorithms rollout (page 3–113) .
Ray-Trace Bias
On the right, increased map bias makes the dog appear to
float.
A single p arameter, Ray-Trace Bias, affects the
generation of
ray-traced shadows (page 3–1094) .
You set this parameter in the Shadow Parameters
rollout.
The Ray-Traced bias control in the Shadow
Parameters rollout moves the shadow toward or
away from the shadow-casting object (or objects).
By default, this va lue is 1 unit.
Increasing the bias moves the shadow away from
the object, and decreasing the bias moves t he
shadow closer to the object. The Ray -Trace Bias
value can be any positive floating-point number.
For example, if a shadow-casting object intersects
another object but its shadow doesn’t meet
properly at the intersection, the bias is too high .
This effect varies with the angle of the spotlight
to the object. Extremely shallow spotlight angles
usually require higher bias values.
Another purpose of bias is to avoid problems with
objects that cast shadows onto themselves. If you
see streaks or moiré patterns on the surface of t he
object, the bias value is too low.
Ray-Traced Shadows
Example of ray-trace d shadow s
Ray-traced shadows are generated by tracing
the path of rays sampled from a light source.
Ray-traced shadows are more accurate than
shadow-mapped shadows (p age 3–1104) .They
always produce a hard edge.
Ray-traced shadows are more realistic for
transparent and translucent objects. Also, only
ray-tracing can generate shadows for wireframe
objects.
Because ray-traced shadows are calculated without
amap,youdon’thavetoadjustresolutionasyou
do for shadow-mapped shadows. The parameters
for ray-traced shadows adjust the shadow’s
position (known as
ray-trace bias (page 3–1094) )