8

1060 Glossary
Map Bias
On the right, increased map bias makes the dog appear to
float.
Map bias moves the shadow toward or away from
the shadow-casting object (or objects).
By default, this value is 1.0 world coordinate unit.
Increasing the bias moves the shadow away from
the object, and decreasing the bias moves t he
shadowclosertotheobject.TheMapBiasvalue
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 v aries with the angle of the spotlight
to the object. Extremely shallow spotlig ht ang les
usually require higher bias values.
Another purpose of bias is to avoid problems
withobjectsthatcastshadowsontothemselves.If
you see streaks or moiré patterns on the surface
of the object, the bias value is too low. If you
increase the bias so much that t he shadow becomes
disconnected f rom the object, reduce the bias and
increase the shadow map Size value instead.
Ma p Channel
Left: Scene uses different map channels to place different
copies of the same map s in different locations.
Right: The three maps used to create the streets and the
traffic markers painted on them
When you turn on Generate Mapping Coordinates
for an object, the coordinates use map channel
1. You can assign new map channels with new
mapping coordinates by applying a
UVW Map
modifier (page 1–905)
to the object. Map channel
values can r ange from 1 to 99.
A map channel associates a map with an object’s
mapping coordinates.
Tex t u re- b a ke d m a p s ( p a ge
3–139)
also use map channels.
For
NURBS (p age 1–1079) surface sub-objects,
you can assign a map channel without applying
UVW Map. The surface sub-object has a different
set of mapping coordinates for each map channel
you use.
A map’s map channel value identifies which of an
object’s mapping coordinates to use. Different
map channels a llow maps for the same object
to use different coordinates. For example, you
might use one channel for
diffuse mapping (page
2–1339)
and a different one for bump mapping
(page 2–1347)
. Map channels also let different
maps use different coordinates within a
compound
material (page 2–1396)
,a compositor map (page
2–1497)
,ora multi/sub-object (page 2–1403)
material.
Different map channels can have different U and
V ti ling v alues, different U and V offsets, and s o
on. In the UVW Map modifier , you can also set