9

966 Glossary
Map Bias
On the r ig ht, 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 va lue 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 hi gh.
This effect varies with the angle of the sp otlight
to the object. Extremely shallow spotl ight angles
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 the shadow becomes
disconnected from 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
copiesofthesamemapsindifferentlocations.
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 1–922) to the object. Map channel
values can range from 1 to 99.
A map channel associates a map with an object’s
mapping coordinates. Te x ture -baked m aps (pa ge
3–144) also use map channels.
For NURBS (page 1–1078) 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 allow maps for the same object to
use different coordinates. For example, you m ight
use one channel for diffuse mapping (page 2–1498)
and a different one for bump mapping (page
2–1506). Map channels als o let different maps use
different coordinates within a compound material
(page 2–1587),acompositor map (page 2–1687),or
a multi/sub-object (page 2–1594) 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